LIGHT.

SECT. I. HISTORY OF THE SUBJECT.

The nature and properties of the agent on which vision depends have been objects of philosophical disquisition from ancient times. The earliest speculations which have reached us on this subject are those of Pythagoras, who considered vision as produced by particles continually emanating from the surfaces of bodies, and entering the pupil of the eye. The Platonists, on the other hand, conceived that vision was the consequence of the emission of something from the eye meeting with certain emanations from the surfaces of things; yet, with this very gratuitous hypothesis, the Platonists appear to have detected several properties of Light, especially its propagation in right lines, and the equality of the angles of incidence and reflection when it falls on bright and polished surfaces. The effects of the concentration of the sun’s rays by concave specula were certainly known to the ancients; and antiquaries have supposed, that, in this manner, the Romans kindled their sacred fire ; and thus also it has been alleged that Archimedes destroyed the Roman fleet at the siege of Syracuse.

Aristotle regarded light but as a mere quality of matter; he has some ingenious speculations on the rainbow, and on other luminous meteors. Ptolemy the geographer wrote a treatise on optics, which has perished; but, from some fragments preserved by other authors, he appears to have had distinct ideas on the subject of atmospherical refraction.

A long interval of scientific darkness succeeded his era, until the Arabians began to cultivate the learning of the Greeks, and several of their philosophers treated of optics; but the earliest Arabian work which has reached our times, is the celebrated treatise of Alhazen. In it we find a description of the eye, and the uses of its different parts. The author details many experiments on refraction, both as exhibited in the atmosphere, and as light is modified in passing from one medium to another of different density. He also notices the magnifying power of segments of spheres of glass; a hint from which it has been supposed that the important invention of spectacles originated. We also owe to him the idea that single vision, with two eyes, is produced by images painted on corresponding points in each retina; and that stars may be seen by refraction, when they are actually below the horizon: remarkable speculations for the twelfth century.

The work of Alhazen was in 1270 commented on by Vitellio, a native of Poland, who added a considerable number of observations on the refractive power of air, water, and glass, which he reduced into a tabular form. He made some ingenious attempts to explain the phenomena of refraction; and he seems to have conceived the true idea of burning lenses.

Roger Bacon, the contemporary of Vitellio, was undoubtedly acquainted with the magnifying property of segments of spheres, and recommended that small segments should be preferred for such purpose; adding, et idea hoc instrumentum est utile senibus et habentibus oculos debiles. This so plainly indicates the invention of spectacles, that we cannot doubt that it had then been made. We know that they became common in the thirteenth century, and are described by Spina of Pisa in 1313, although we have no absolute certainty of their first constructor.

After the revival of letters, one of the earliest cultivators of mathematics was Maurolycus of Messina, who made optics his study. He proved that the crystalline lens of the eyes of animals converges the rays of light which enter that organ, and transmits them to the retina, in or near which the foci of the lenses are situated. Hence also he inferred, that in persons who are short sighted, the defect is owing to the too sudden convergence of the pencil of rays before the retina; and that in those who are long sighted, the foci are placed behind that expansion of the optic nerve. Maurolycus, however, did not discover that the images of objects are painted on the retina.

Baptista Porta, the author of Magia Naturalis, a Neapolitan of rank, was much addicted to philosophic research; to him we owe the first description of the camera obscura, and its application to the delineation of objects. His work contains many observations on light, some of which are accurate; and though some are now found to be erroneous, his remarks are always ingenious. This subject also engaged the attention of Lord Bacon, who complained that the/òzvnand origin of light had been too much neglected; but his labours in other branches of philosophy diverted his powerful mind to different objects.

The true theory of the rainbow was first given by Antonio, bishop of Spalatro, although he could not satisfactorily explain the cause of the colours.

The next optical discovery of importance was the telescope, for which we are indebted to Zacchias Jansen, a spectacle-maker of Middleburg, in Walcheren, in 1590; and this important invention was quickly applied by Galileo to physical astronomy, with brilliant success, crowned by the discovery of the satellites of Jupiter, the structure of the Via Lactea, the phases of Venus, the ring of Saturn, the spots on the sun’s disk, and a knowledge of numerous stars unknown to former observers.

The study of light was improved under the auspices of Kepler, who gave an explication of the effect of lenses on the rays, and suggested the form of the telescope now called astronomical. He treated of refraction, and discovered, that when light falls within glass at an angle a little above 42°, it is wholly reflected; but his theory of vision is much more important; and he showed that images of external objects are painted on the retina, and appear there inverted, a fact also ably illustrated by Schemer. The invention of the compound microscope seems also due to Jansen, and dates from the same period.

Perspective was first scientifically treated by Pietro del Borgo, Baldassare Perussi, and Guido Ubaldi. To the second of these is due the detection of the distance points, to which all lines making an angle of 45° with the ground line are drawn; to the third, the convergence of all parallel lines inclined to the ground line, in a point in the horizontal line ; and also that a line drawn from the eye, parallel to them, will pass through this point. These principles are the foundation of perspective, which afterwards received improvements from ’ sG>avesande, and were completed by Brooke Taylor. The true law of refraction is undoubtedly due to Willebrod Snell, or Snellius, professor of mathematics at Leyden. He experimentally showed that the co-secants of the angles of incidence and refraction are always in the same ratio. This discovery, we are assured, on the authority of Huygens, was appropriated by Descartes, who had consulted the papers of Snell, but gave it as his own, under a somewhat different form. The successive labours of Descartes, Kircher, Grimaldi, De la Hire, Hooke, and Huygens, gave to the study of optics a profound scientific character; and the interesting discoveries of that century were crowned by the important researches of our immortal [7:13:330]Newton into the optical properties of light. During the last century, our knowledge of this subject has been steadily progressive, by the labours of a multitude of philosophers, so numerous, that we can afford space for little more than to record the names of some of the most successful inquirers into the mysteries of this subtile agent. Among these, Mairan, Dufay, Mariotte, Boscovich, Euler, Mitchell, Melville, Canton, Bennet, and Lagrange, stand conspicuous; nor must we omit the important fact, illustrated by the labours of Bradley and Roemer, that the velocity of light, from whatever source derived, whether from the sun, the fixed stars, the planets or their satellites, is equal, or that its velocity before and after reflection is the same: a formidable objection to the theory of emission. During the present century, the progress of discovery in this field has been no less brilliant. Very early in it, Dr Thomas Young illustrated the principle of the interference of the rays of light, founded on some facts observed by Grimaldi, but first distinctly stated in Dr Young’s Memoir in the Philosophical Transactions for 1803, entitled Experiments and Calculations Relative to Physical Optics ; and his conclusions have been demonstrated beyond all doubt by the researches of Fresnel and of Sir John Herschel. The splendid talents of Laplace, of Poisson, Biot, Arago, Pouillet, Cauchy, Ampère, and Fresnel, among continental philosophers, have especially illustrated the phenomena and theory of light: and in our own country, Sir William Herschel, Young, Brewster, the younger Herschel, Airy, Whewell, have pursued these delicate investigations with singular ability and success. Above all, we must record, as among the most signal triumphs of modern science, the detection and explanation of the polarization of light, and the singular confirmation thereby afforded of the theory of its propagation by undulations.

SECT. II. NATURE OF LIGHT.

Notwithstanding this long list of splendid discoveries, the nature of light is still in some degree enigmatical. It is admitted, that the phenomena of vision depend upon the agency of a subtile, extremely attenuated matter, set in motion by the sun and other luminous bodies. Its materiality is inferred from its deflection from its rectilinear course, in passing near various bodies; by its being arrested by certain substances, though it passes freely through others; by its reflection from polished surfaces; by its capability of condensation and dispersion, in passing through certain media; by its producing chemical changes in some compounds; and by its apparently entering into the composition of some bodies, from which it may be again extricated.

Thus far the majority of philosophers are agreed; but two opposite theories have been advanced respecting its propagation, and the mode in which it manifests itself to our senses.

Some maintain that light is a peculiar matter, which is projected in all directions from luminous bodies in a rapid succession of material particles. This theory is sustained by the illustrious name of Newton, and has been very generally received; but of late, certain difficulties in the explanation of the recently-discovered properties of light, especially its polarization, have tended to revive the doctrine maintained by Descartes, Huygens, and Euler, viz. that all the phenomena of light depend on the undulations of a highly attenuated fluid or ether, universally diffused throughout space, which, while at rest, is inappreciable by our senses, but, when acted on by luminous bodies, is thrown into a succession of waves. Luminous bodies are thus supposed to act on the universally diffused fluid somewhat in the manner that sonorous bodies do on air in the production of sound.

It is true, that all the known facts regarding light may be explained upon either hypothesis. It must be owned, however, that the remarkable coincidence of fact with theory, and the facility of explanation, favour the theory of undulations, while it scarcely can be said to involve any greater assumption than the doctrine of direct transmission. Both assume the existence of a subtile fluid; both admit the influence of luminous bodies; and it does not seem more difficult to conceive them acting, by causing an undulation in the matter of light, than by projecting it in a rapid succession of particles, the minuteness and velocity of which almost elude the grasp of our imagination.

Whichever hypothesis we adopt, the propagation of light is a process of astonishing rapidity. Astronomers have found, from observations on the eclipses of Jupiter’s satellites, that planetary light requires about fourteen minutes to cross the earth’s orbit; or, if we adopt the more recent and probably more accurate determination of Bradley, that the light of the sun requires about eight minutes to reach our earth; and, if we reckon the mean distance of the sun to be 94,879,956 English miles, it follows, whether we regard it as an emanation or an undulation, that light must travel with a velocity of about 200,000 miles per second.

It is difficult to form any adequate idea of such enormous velocity; but we may approximate it, by comparing this with the ascertained velocity of a cannon ball. A twenty-four pounder, with the common charge of powder, according to Robins, discharges its ball with an initial velocity equal to 1600 feet per second; yet, if such a ball were to continue this velocity un diminished, it would require about ten years to traverse a space which the light of the heavenly bodies pervades in eight minutes.

This prodigious rate, and the ease with which light can penetrate many solid bodies, have been adduced as arguments against the doctrine of the successive emanation of particles from luminous surfaces; but did not the undulatory theory afford an easier solution of certain recently-discovered properties of light, we should not regard such arguments as conclusive. Now, however, the undulatory theory has been shown to correspond so exactly with known facts, and has even enabled us to predict so exactly what experiment has since confirmed, that it has received the sanction of the greatest names in modern science.

SECT. Ill PROPERTIES OF LIGHT.

Whichever hypothesis be adopted, light must be considered as a material substance, possessed of certain properties, detected by observation and experiment.

1. Light is given out by luminous bodies in all directions, and from every point of the luminous surface. This is proved by its being equally seen from every point of observation.

2. It is divisible into homogeneous, independent portions, like air or water; the smallest portion we can separate is termed a ray ; and several rays form a pencil of light.

3. Light appears to be absorbed by certain bodies, and is again given out by them spontaneously. This property is well seen in the diamond, which, after being exposed to the sun’s rays, continues for a short time to shine in the dark. Various artificial phosphori do the same, as the Bolognian stone, calcined oyster shells, and the like.

4. All solids give out light when heated between 700° and 800° of Fahrenheit’s thermometer, and are then said to be incandescent. All liquids that can be heated to that point are luminous, as melted metals; and, if the elasticity of their vapours be repressed, other liquids appear capable of this emission. Gases do not, however, seem to be capable of incandescence, yet the phenomena attending their sudden condensation, when they enter into chemical union, [7:13:331]shows that they contain light. Thus, when a mixture of oxygen and hydrogen is suddenly and strongly compressed, the gases unite to form water, and both light and heat are extricated.

5. Some bodies have the property of arresting the progress of light, and are termed opake ; others transmit it, and are said to be transparent. Yet probably no substance is either perfectly opake or perfectly transparent. Thus, gold is one of the most condensed and opake bodies in nature; yet if we enclose gold leaf between two plates of glass, and examine it by transmitted light, it appears of a decidedly greenish hue; showing the transmission of some light through the metal. On the other hand, the most transparent glass, when viewed in a thick plate, and the most limpid water, when in a deep column, appear greenish.

6. When the rays of light fall obliquely on the surface of •all bodies, whether transparent or opake, solid or fluid, they are more or less reflected. Smooth and shining surfaces reflect most light, but the degree also depends on the nature of the reflecting substance: the reflected rays are returned from the surface at an angle equal to the angle of incidence: if the reflecting surface be a plane, the parallel rays that fall on it are reflected parallel to each other.

When the rays are reflected from a concave surface, the reflected rays are more inclined to each other than the incident rays; and if that concave surface be the segment of a sphere, the parallel incident rays will converge to a point in the axis of the mirror half way between its surface and the centre of the sphere of which it is a segment; and this point is termed its principal focus, or focus of the parallel rays: if the incident rays be converging, they will meet the axis between the principal focus and that centre; if the incident rays be diverging, they will meet the axis in a point between the principal focus and the surface of the mirror. From the immense distance of the sun, all the incident rays may be considered as parallel; and therefore his rays will be condensed into the principal focus of such a mirror, which will in this instance also be that of greatest heat. This is the principle of burning mirrors. When the incident rays fall on a convex mirror, they are all reflected more divergingly, or are dispersed.

The properties of reflected light form the object of the science of Catoptrics. See Optics.

We are not, however, to imagine that all the light incident on bright surfaces is reflected. Many curious experiments were made on this subject by Bouguer. The quantity of light returned differs with the inclination of the rays to the reflecting surface. It is generally strongest at small angles of incidence; and the difference becomes excessive when the rays impinge on the surface of transparent fluids with different degrees of obliquity. Metals, from their opacity and splendour, form the best reflecting surfaces; but even pure mercury, perhaps the most perfect of reflectors, does not reflect more than three fourths of the whole incident light.

7. When the rays of light fall on transparent bodies, they* are differently affected, according to the angle of incidence. When a ray passes from one transparent medium to another, in a direction perpendicular to their touching surfaces, that ray will pass through them in a straight line; but when the ray passes in a direction oblique to their touching surfaces, that ray will be bent, or will form an angle at their junction.

When the density of a medium is uniform, the rays of light traverse it in straight lines; but in a medium varying in density, like columns of liquids, or the atmosphere, in which density increases with the superincumbent pressure, the passage of the rays will form curves.

When a ray passes obliquely from a dense to a rarer medium, it is bent or deflected from a line perpendicular to their touching surfaces. When passing obliquely from a rare to a denser medium, they are bent toward the perpendicular. In such instances the light is said to be refracted. In the first instance, the angle of refraction is always greater than the angle of incidence; in the latter it is always less. The study of the properties of refracted light constitutes the science of Dioptrics. (See Optics).

8. The refractive power of different media is unequal; and when the rays of light pass from one medium to another, it may be measured by the ratio between the sines of the angles of incidence and refraction ; and the number expressing the ratio between the first and the last is the exponent or index of the refractive power of that substance.

The refractive power of different substances appears to be nearly in the ratio of their density; but with inflammable bodies, or those containing an inflammable principle, the refractive power is in a ratio greater than their density. It was this law which led Newton to his happy conjecture that water and the diamond might contain an inflammable principle; speculations which have been verified by modern chemistry. The same opinion as regards the diamond was long before maintained by Boetius de Boodt. He says that, unctuous and fiery bodies are easily united, but will not mix with watery substances; and because the diamond readily adheres to resins, which are of a. fiery nature, and because, like amber, another fiery body, the diamond, when rubbed, attracts light bodies, the diamond itself must be of an inflammable or sulphureous nature: an argument which he considers as confirmed by that gem being “produced in a hot, sulphureous climate.” It is obvious that the deduction of Newton differs widely in its principles from the hypothesis of De Boodt; but the latter must be regarded as a curious instance of a true conclusion derived from unsound premises.

9. If a pencil of light be admitted by a small hole in the window-shutter of a darkened room, through a triangular prism of any transparent substance, the white light will be found to undergo a remarkable change. The rays will be separated in the prism, their image will be enlarged; and, if received on a white screen, will be seen variously coloured. The colours will assume a certain determinate order of juxtaposition; and this appearance is termed the prismatic spectrum. This coloured spectrum will then be seen divided into colours, of which Newton enumerates seven; red, orange, yellow, green, blue, indigo, violet. These, it is evident, may be resolved into red, yellow, and blue; for the boundaries of the colours are not well defined, and the compound colours which lie between these may be considered as made up of the intermixture of contiguous rays. These rays are not in equal proportions in the spectrum. If we consider it. divided into 360°, the red occupies 45°, the orange 27°, the yellow 48°, the green 60°, the blue 60°, the indigo 40°, the violet 80°; and it is worthy of remark, that this division of the scale of colour is a striking approximation to the divisions of a chord that would give the musical intervals of the octave. But as these colours are not bounded by defined lines, but graduate into each other, it is very difficult to determine their relative extent with tolerable precision. The cause of their separation is the difference of their refrangibility by the prism; the red being the least, the violet the most refrangible; that is, turned from the line of the incident pencil of light. The green ray will be found in the centre of the prismatic spectrum; and hence its index of refraction is considered as the mean refraction of the substance of which the prism consists. Newton employed in his experiments prisms of different substances; but he seems to have taken it for granted, that when the mean refraction was the same, the length of the spectrum was also equal, or that the dispersive power of the bodies in that case was equal. He considered that prisms and lenses of all kinds of glass, [7:13:332]and of all bodies, whether solid or fluid, with the same mean refraction, possessed also the same dispersive power, or formed spectra proportional to their mean refraction. Hence he was led to conclude that “the improvement of the refracting telescope was desperate.” This error has, since his day, been detected; and this principle forms the basis of Dollond’s admirable invention of the achromatic telescope, in which the error of refraction in one species of glass is ingeniously remedied by a correction derived from the different dispersive power of another kind of glass, so adapted to the first as to form with it one object-glass. The difference in dispersive power has now been ascertained in a considerable number of diaphanous bodies; and tables of this difference have been formed from the observations of many philosophers, particularly of Sir David Brewster.

The illuminating power of the different rays of the spectrum is different. Sir William Herschel found, that with a prism of flint-glass, the greatest illumination is towards the middle of the spectrum; the yellow rays affording most light, while the illuminating power, diminishing towards each end of the spectrum, is least in the violet ray. A series of experiments on this subject by Frauenhofer, a late celebrated instrument-maker of Munich, showed, that with the best made prisms, when ether light is carefully excluded, the most luminous point is nearer the red than the Holet end of the spectrum, in the proportion of one to four; and he states the mean refrangibility to be between the blue and indigo rays. But one of the most curious discoveries of this ingenious inquirer is, that the solar spectrum is traversed by numerous dark lines of unequal thickness, perpendicular to the length of the spectrum, and parallel to one another. These lines require a fine, prism for their exhibition, a microscope for their detection, and the exclusion of light, except that of the coloured ray under examination. He counted 590 of these lines in the spectrum; the greatest number of them being towards the most refrangible end of the spectrum.

It is well known that the rays of the sun communicate heat as well as light; but the heating power of the coloured rays is very different. Herschel discovered that the red ray raises the thermometer most; and that the effect diminishes as we approach the other end of the solar spectrum. This is sufficiently striking; but in pursuing his investigations he made another singular discovery, that the point of greatest heat is fully half an inch beyond the red end of the prismatic spectrum. Delicate thermometers were placed in the different rays, and gave the following results :—
[table]

Ihese curious experiments were confirmed by Sir Henry Englefield and Sir H. Davy. The inference from them is, that light and heat are unequally refracted.

The prisms used by these philosophers appear to have been of flint-glass; but Dr Seebeck has since found that the position of the point of greatest heat varies with the nature of the refracting prism. Seebeck found, as Herschel did, that with flint-glass the greatest heat was beyond the red; with plate-glass, in the middle of the red; with sulphuric acid in a hollow thin glass prism, in the orange; with water, in the yellow. The sun’s rays would appear to be still more complex. Early in this century, Ritter of Jena found that the rays of the solar spectrum possessed different chemical powers. He found that the salts of silver became soonest black a little beyond the violet end of the spectrum, a little less so in the violet, and still less so in the blue; and Seebeck, in repeating the experiments, found that beyond the violet ray muriate of silver became reddish brown; in the blue, bluish gray; and in the yellow it retained its white colour, or at most had a yellowish tint; it became reddish in the red ray, and even when placed beyond it. These changes might have been attributed to the influence of heating or illuminating power, had not the greatest deoxidating effect been observed where the heat and illumination are the least. Dr Wollaston, who observed these facts about the same time with Ritter, considers the sunbeams as compounded of calorific and deoxidating as well as luminous rays, all with different degrees of refrangibility. But if the experiments of Morichini are confirmed, the sunbeams have also the property of magnetizing steel. About twenty-five years ago he announced this discovery; it was repeated by several persons without success, but Mrs Somervile appears to have succeeded. She covered one half of small sewing needles with paper, in Morichini’s method, and expused the naked half to the violet rays for two hours, when she found that the ι needle had thus acquired a north pole. The indigo ray produced nearly the same effect, but the effect was feeble in the blue and the green; while, though exposed in the orange, yellow, and red, for two successive days, no magnetism was induced. Similar effects followed when one half of the needle was enveloped in white paper, and the other half, exposed to the rays, was covered with blue or green glass, or with silk of those colours.

10. The facts already noticed respecting the bendingof the rays toward the perpendicular, when they pass from a rare to a denser medium, lead to the inference that the disposition of the surfaces of the refracting medium must materially influence the direction of the rays of light which enter and pass through. Accordingly, it is found that if one or both surfaces be convex, the rays are bent toward the axis of the medium. If the medium be spherical, or a segment of a sphere, the ray, falling perpendicularly on its centre, will pass straight through; but all those that fall obliquely on the spherical surface will emerge from the medium in a direction inclined to the central ray (which may be considered as the axis of the medium), and will cut this axis in some point, which is termed the refracted focus of those rays. When the medium has its sunaces forming segments of spheres, it is called a lens ; and lenses are divided into convex and concave, plano-convex and plano-concave, double convex and double concave, according to the form of their surfaces.

When a lens is nearly or really spherical, optical principles will show, that all the emergent rays will not meet in the same point; those farthest from the axis will meet first. But if the lens be a thin segment of a sphere, with one or both of its surfaces convex, this error will not be very conspicuous, and the emergent rays will meet nearly in one focus.

When both surfaces of a lens are concave, or when one is concave and the other plane, the emergent rays will be bentj⅛w the axis. The mathematical demonstration of these facts, their application to practical purposes, and to the explanation of natural appearances, belong to Optics ; to which article attention is directed.

11. Some crystallized boches have the property of dividing the rays of light which permeate them into two distinct portions, one of which passes in the ordinary direction, while the other pencil undergoes an extraordinary refraction, passing at some distance from the other. Hence, when any body is viewed through such a crystal in a certain direction, both sets of rays become apparent by giving a double image of the object. This curious property was first detected by Erasmus Bartholin, in calcareous spar brought from Iceland. But this subject was fiι⅛t philosophically investigated with his usual sagacity by Huygens, [7:13:333]who proved that the property of double refraction was not confined to calcareous spar; and it has, since his time, been shown, that all crystals, the primitive form of which is neither a cube nor a regular octahedron, possess this property. Newton attempted to explain this double refraction; but his explanation was not happy. He ascribed it to an original difference in the rays of light, by which some are refracted in the usual manner, while others undergo unusual refraction. Huygens discovered, that when the ray of light was received through the Iceland crystal in any direction but one, it was always divided into two rays of equal intensity; but he remarked with surprise, that when he received the divided rays through a second crystal of Iceland spar, the two portions into which each of them was now subdivided were no longer equally intense; that their relative brightness depended on the position of the second rhomb with regard to the first; and that there were two positions of the second, in which one of the rays vanished altogether. This Newton supposed to depend on the rays having different sides, possessed of different properties, each of which “answers to or sympathizes with that virtue or disposition of the crystal, as the poles of two magnets answer to one another.”

This idea was followed up by Malus. He conceived that the molecules of this polarized light have all their homologous sides in the same direction; and he expressed this modification of light by the term polarization, as he compared the effect produced to the influence of a magnet, which directs the poles of a series of needles all to the same side—an hypothesis which Biot modified by supposing that each molecule of light had one axis, similarly placed in each, and all turned in one direction, in a polarized ray; while the molecules were conceived to have a free motion round such axes, by which they could assume different positions according to the attractions and repulsions they experience at the surface of each new medium they traverse. The term polarization is not certainly very happy, and it is to be regretted that one more appropriate and less hypothetical had not been employed.

If the rays, thus divided into two pencils by calc spar, be received by a rhomb of the same substance, while the axes of both crystals are in the same direction, no new division of the rays takes place; but if, while the first crystal remains at rest, the second be turned round, by the time it has made one eighth of a revolution the rays will be again subdivided, and four images will be produced. By continuing the motion until the crystal has described one fourth of a revolution, the subdivision will again disappear.

Malus discovered that an analogous effect was produced by reflection. If a pencil of rays fall on a polished surface of glass at an angle of 35° 25', it is reflected at any angle equal to the angle of incidence. If we now place another plate of glass in such a position that the rays reflected from the first shall fall on the second also at an angle of 35° 25', or when the plane of both reflections coincide, the rays will also be reflected from the second plate: but if the second plate be turned round one quarter of a revolution, so as to make the plane of the second reflection perpendicular to the plane of the first, the whole of the rays will now be transmitted through the second plate: when this plate has described half a revolution, the rays will be reflected as at first; and when it has made three quarters of a revolution, they will again be transmitted, i. e. when the planes of reflection are parallel, light is reflected, but when they are perpendicular, it is transmitted—or light in such circumstances can permeate glass in one direction, but not in another. Sir David Brewster, soon after Malus, began a vast series of experiments to determine the angles of polarization of different media, and to investigate the general law which regulates polarization by reflection from transparent bodies, which was crowned with the beautiful discovery that “the tangent of the angle of polarization is equal to the refractive index,” or that when a ray is entirely polarized by reflection, “the angles of incidence and refraction arc complimentary ” In this sketch it would be impossible to do justice to the investigations and beautiful theoretic deductions of Fresnel, which have combined the whole into an inductive science. We must direct the reader to the article Polarization for his important labours, as well as for the profound researches of Airy, Poisson, Biot, Arago, and Cauchy.

Newton rejected the explanation of double refraction offered by Huygens, because he considered the apparent polarization of the rays of light inconsistent with motions “propagated through a fluid medium ;” but this arose from his limiting his ideas of luminous vibrations, as entirely analogous to those of producing sound in air, in which they are propagated in the direction of the advance of the undulations. We owe to the late Dr Thomas Young the first idea of the vibrations being transverse to the direction of the luminous wave; an hypothesis which he illustrated by the propagation of the vibrations of a stretched cord put in motion at one of its ends. This happy idea has been shown to be a necessary consequence of the phenomena of the interference of polarized light, if we admit the theory of luminous waves. The subsequent investigations of Arago and of Fresnel have confirmed the speculations of the English philosopher, which have connected and elucidated those brilliant discoveries that have conferred lustre on the names of Malus, Fresnel, and Brewster. See Optics and Polarization.

, SECT. IV.— colour of objects.

The discovery by Newton of the colours produced by the decomposition of the sun’s rays, naturally turned the attention of that profound philosopher to the cause of colour in different objects; and he has delivered a theory of colours, of which we shall now exhibit an outline.

1. Newton regards the colour of natural objects, not as produced by any modification which light undergoes from refraction or reflection at their surfaces, but as something inherent in the rays according to their different degrees of refrangibility. The same degree of refrangibility invariably gives the same colour; and when the rays are fully separated from each other by the prism, he found it impossible to change the colour. Thus he refracted the red ray with prisms, but found its tint unaltered; he reflected it from bodies which in day light had other colours, but still it remained red ; he transmitted it through coloured media of different tints, he passed it through the coloured rings produced by pressing together plates of glass, but he was unable to convert it into another colour. By condensation or dispersion he could render it stronger or fainter, but still it remained red. Similar experiments on the other rays were attended with similar results.

2. He found, however, that by mingling the different rays of the coloured spectrum, he could produce a sort of intermediate tint: thus the intermixture of the yellow and red rays formed an orange, and that of the yellow and blue, a green. But this effect was only distinctly produced by the intermixture of contiguous rays; if they were far removed from each other in the spectrum, no such effect was produced: thus the orange and indigo rays do not produce an intermediate green.

3. The intermixture of all the rays reproduced white light. Newton reflected a pencil of rays through a prism into a dark room, and then interposing a lens of three íëet radius, about four or five feet distance from the aperture admitting the light, he collected the convergent rays upon a paper screen, and obtained an intense spot of white light. By [7:13:334]moving the paper he could easily find the point of perfect whiteness; and, by drawing it farther from the lens, he could reproduce the coloured spectrum in an inverted order, as the crossing rays diverged farther from each other.

If any of the coloured rays were cut off before their convergence by the lens, the image on the paper exhibited colour; and if either of them were made to predominate, that tint was rendered perceptible. Newton endeavoured to show the same with mixtures of coloured powders; and though this method presents mechanical difficulties not easily overcome, and the mixtures only afford a gray shade, yet when these were strongly illuminated by concentrated solar light, they became of a dazzling white: and all coloured objects appeared most splendid in the prismatic rays of their own colour. From these facts he considered the colour of objects to depend on the predominance of the coloured rays they reflect. Thus minium, or red lead, appears red, because it reflects principally the least refrangible rays; a violet appears of the colour so denominated, because it chiefly reflects the most refrangible rays; and what we denominate the colour of an object is merely the hue of the rays which it most copiously returns to the eye.

On the other hand, transparent bodies which have colour, when held between the eye and the light, appear soby transmitting most copiously that ray. Thus, too, we see why a body not quite transparent may sometimes appear of different colours by transmitted and reflected light. Such a body may transmit most copiously the blue rays, and reflect the green ones; as we often find in coloured liquids, and sometimes observe in the crystals of fluor spar, and other mineral substances. This fact is a confirmation of the Newtonian theory of colour; for, were the colour inherent in the substance itself, it ought to appear equally by either mode of viewing it.

In transparent coloured liquids the shade often varies with the thickness of the column through which the light is transmitted. Thus a clear red liquid in a conical wine glass appears below of a pale yellowish hue; higher up it seems orange, and only has its full red hue when the column is of considerable thickness. This is owing to the most refrangible rays never being able to penetrate the liquid at all. The remaining part of these rays gives the yellowish colour to the thin film at the bottom of the glass; the separation of part of the yellow rays gives an orange tint to the next film of liquid; and, when the yellow rays are wholly stopped by the thicker column, the red, or least refrangible, come undilâted to the eye of the observer, and give their colour to the body of the liquid in the glass.

SECT. V. RELATION OF LIGHT AND HEAT.

It is well known that light and heat are intimately mixed in the beams of the sun—that some bodies give out both light and heat during combustion—and that a high temperature causes the extrication of light in all bodies, the gases excepted. This intimate relation between light and heat has induced some philosophers to consider them as mere modifications of each other. Certain it is that they have many properties in common. They are capable of reflection, of refraction, of concentration, of dispersion, and of polarization—they radiate between distant objects with great celerity, they penetrate solid bodies very readily, they are absorbed by dark and rough surfaces, are generally reflected by smooth surfaces, and they are capable of subverting some chemical combinations.

These properties show very striking analogies; and the phenomena of the polarization of heat, so well illustrated by Professor Forbes of Edinburgh, have undoubtedly rendered this analogy still more apparent: yet in the present state of our knowledge, it would be rash to pronounce their absolute identity. Their total separation, at least as far as our means of detection extend, in some instances,—the very different substances which permit or retard their progress,— the different manner in which they affect our sensations,— have led some inquirers to the opposite conclusion; and though it may still be true that they are modifications of the same kind of matter, it is safest to content ourselves with pointing out those circumstances in which they differ, as well as their general agreements.

1. Light and caloric are not intercepted by the same substances. If we interpose a plate of thin transparent glass between the face and a bright blazing fire, the intensity of the light has no apparent diminution, but the calorific rays seem immediately arrested; and if we make the experiment with a thin diaphanous plate of ice, they seem absolutely intercepted. Some recent experiments of Professor Forbes, with thin plates of ice, afforded an almost microscopic effect on the galvanometer of Melloni’s apparatus; but supposing there was no minute hole in the ice, the difference of the transmission of light and heat through diaphanous ice is sufficiently striking. The same takes place with all other species of terrestrial light, though, as we shall presently see, the calorific rays of the sun instantaneously pervade ice.

2. The rays of caloric are more powerfully reflected from a metallic mirror, even of an imperfect shape, than from the best glass mirror; whereas the latter very powerfully reflects light. Dark and dense solids are very readily penetrated by caloric, though they are totally impervious to light.

3. Light affects the organs of vision in a peculiar manner, without producing inconvenience to that most delicate organ, the eye; but a radiation of heat without light, as from a vessel of boiling fluid, though the rays entering the eye may be so powerful as painfully to affect the eye, does not produce any thing analogous to vision.

In the sun’s rays, however, heat and light are so intimately 'blended, that we cannot entirely separate them. The difference in their refractive power causes a partial separation in the coloured spectrum; but both the heat and light of the sun’s rays seem to pervade glass or ice with equal facility. Leslie’s photometer, placed behind a sheet of diaphanous ice, is immediately affected by the direct rays of the sun, and a lens of transparent ice will concentrate them, so as to fire combustibles; as was long ago observed by Jan Metius and Descartes, and has more recently been proved by Scoresby. This difference between the calorific influence of the sun and of artificial fires has been attributed to the different initial velocity imparted to the calorific emanations in both cases. This is not improbable; but some have considered the calorific influence of the sun’s rays as an effect of the condensation or fixation of light. This was the idea of the late celebrated Leslie, and is the principle of his elegant photometer.

SECT. VI. MEASURES OF LIGHT.

Various methods have been proposed for affording comparative measures of light. The principles of these depend either on the illumination, as ascertained by the distance at which we can distinctly perceive small objects, such as printed letters of a certain size; the comparative depths of the shadows of an opake object; or the heat excited by the luminous emanations of the bodies compared.

1. The distance at which the same eye can read a particular printed paper forms certainly a good criterion of the comparative degree of light given out by two or more luminous bodies, at the moment of comparison; but as it must greatly vary with the goodness of eye, it obviously cannot afford the basis of a general scale of illumination, [7:13:335]by which the same individual can compare his observations at distant periods, or render the experiments of one person comparable with those of another. Still it is a convenient method, and requires but a very simple apparatus; a tube to admit the light in an uniform manner to the paper, and a graduated sliding rule to ascertain with ease the distance of the paper from the eye.

2. The comparison of shadows, which appears first to have been employed by Bouguer among several other ingenious contrivances, was the mode recommended by Count Rumford, who, in 1794, read a paper on this subject to the Royal Society of London. It is reprinted in a volume of his Philosophical Papers, published in 1802. This instrument, though well suited to the object in view, is cumbrous, and somewhat complicated. The photometric part a is a box eight inches wide; its back a plate of glass, covered by tissue paper, on which the shadows are projected. It is supported at a convenient height by a tripodstand. The table consists of two narrow arms c d, resting on b at one end, and kept horizontal by feet at the other, intended to support the moveable brackets e e, on which are placed the lights to be compared. The arms are divided into decimals of an inch; and are here represented on a smaller scale than the rest of the instrument.

Similar results may be obtained by the following contrivance, proposed, we believe, by Dr W. Ritchie. It consists of a rectangular box of brass, α, b, c, el, three inches long, and 2∙2 inches wide. In its centre are two plane glass mirrors, g g, two inches square, cut from the same plate, and placed accurately at angles of 45° to the base of the box, as in the diagram. Each end e f of the box is open, and has, at equal distances from the mirrors, two . cylindrical wires of brass, h, h, 0∙2 inch in diameter, fixed vertically in the centre of the box. The top of the box consists of two thin plates of glass, i i, on which is pasted tissue paper. The inside of the box and the wires are blackened. When the two lights to be compared are placed before the ends of the box, and in its axis, the shadows of the wires will be reflected from the inclined mirrors on the tissue paper. The adjustments of the lights to the machine may be conveniently made by sliding brackets, placed on a long and steady table. If one of the lights be fixed, the other is to be moved backwards or forwards, in the line of the axis of the machine, until both shadows of the wires shall be of equal intensity. Thus, as the intensity of the light, in such cases, is inversely as the square of the distance of the luminous body, the difference between the position of each light, ascertained either by a graduated fillet on the table, or by a common Gunter’s scale, will afford a numerical value of the comparative intensity of each light. The method appears sufficiently accurate for such experiments, but, like the former mode, is not susceptible of a fixed scale, unless we could find some uniform unvarying light to be considered as a standard.

3. The calorific influence of luminous matter was proposed as the measure of the light by Lambert, and was adopted by Sir John Leslie as the principle on which he constructed his photometer. It is the author’s differential thermometer, with one of its balls made of black enamel, while the other is of clear glass. An instrument so prepared, when exposed to a heating cause, has its balls unequally heated. To prevent the influence of currents of air, the whole is covered with an air-tight case of transparent glass. The 1 , black ball absorbs the calorific rays which impinge on it, the air within it expands, and raises [7:13:336]the coloured liquor in the opposite stem of the instrument, to which a scale of equal parts being attached, each equivalent to ⅛th of a degree of the centigrade thermometer, affords a numerical result; and if we were sure that the intensity of the light is always in proportion to the calorific effect, the instrument would be a perfect photometer. But, unfortunately, we now know that this is not the fact, especially when we compare different kinds of light by means of this instrument. Thus the influence of a fire, so dull that it is impossible to distinguish a letter of a printed page, will affect this photometer at the distance of several feet, more than the diffused light of day sufficient to enable one to read the same book with facility; and it is more affected by radiation from a piece of iron scarcely incandescent in the dark, than by the intense light of phosphorus burning in oxygen gas. Even with the light of the sun refracted by the prism, the photometer does not indicate the point of the maximum of light. The greatest illumination is in the yellow rays; but this photometer rises highest when in or just beyond the confines of the red.

But if we employ the instrument for the purpose chiefly in the view of its ingenious inventor, the measure of the intensity of solar light, this beautiful instrument appears to us the most elegant and useful photometer hitherto proposed. Its delicacy is such, that when freely exposed, in our climate, to the light of the sky, without being acted on by the direct solar rays, it generally ranges in summer from 30° to 40°, and in winter from 10° to 15°. Exposed freely to the sun-beams at noon in summer, it usually mounts to between 80° and 90°; and in the depth of winter is generally about 25°. In the glowing language of its inventor, “the photometer exhibits distinctly the progress of illumination from the morning’s dawn to the full vigour of noon, till evening spreads her sober mantle. It marks the growth of light, from the winter solstice to the height of summer, and its subsequent decay through the dusky shades of autumn; and it enables us to compare, with numerical accuracy, the brightness of distant countries—the brilliant sky of Italy, for instance, with the murky air of Holland.”

SECT. VII.—EVOLUTION OF LIGHT WITHOUT APPRECIABLE ' HEAT.

The most familiar instance of this phenomenon is in the rays of the moon, planets, and fixed stars, in the beams of which the most delicate instruments, even the thermo-magnetic combinations of Melloni, have been unable to detect any calorific effect. In the beams of the moon and planets, the greatest portion of the incident light would probably be absorbed by the dark nucleus of those celestial bodies; and if any heating rays were emanated from them towards us, they probably are far too attenuated to produce sensible effects at our planet. The light of the fixed stars, though probably like that of the sun, radiates through too enormous a distance to become sensible to any instrument for measuring heat hitherto contrived. The luminous meteors, too, that belong to our atmosphere, have in general no sensible heat; if we except meteoric stones and condensed electricity or lightning, which has occasionally fired combustibles. We must not confound the effect of the aurora borealis on the magnetic needle with heat; it appears to be altogether magnetic, not calorific. We find, also, that certain terrestrial bodies have the power of emitting light, in some instances largely, without a corresponding degree of heat; and such are usually termed phosphorescent. Some of these have the property of absorbing light when exposed to it, and again visibly emitting it. Some become phosphorescent when slightly heated; others give out light during their spontaneous decomposition. Phosphorescence has been examined by Bartholini, Fabricius ab Aquapendente, Bayle, Algarotti, Reaumur, Father Beccari, Father Bourges, Abbé Haller, Seroi, and Canton.

1. Many bodies, when exposed to light, particularly that of the sun, absorb it, and emit it immediately on being removed into a dark place. When a diamond of some size is thus exposed, it has been observed to give out flashes of light in the dark for a short period, and it recovers this property on a fresh exposure. Several other precious stones, some calcareous minerals, almost all animal and vegetable substances, when very dry, or after solution in ni∙ trous acid, and∙even snow, are stated by Beccari to possess the same property in a greater or less degree. Several artificial compounds, when carefully calcined, have the same effect. This is particularly the case with the Bolognian stone, and with Canton’s phosphorus. The former is a calcined sulphate of baryta, found at the foot of Monte Paterno, near Bologna. Its properties were first discovered by Vincenzo Cascariolo, a shoemaker of that city, who, from its weight, mistaking it for a metal, attempted its reduction. The inventor kept the process secret, but it seems to have at length transpired: according to Kircher, the stone was reduced to a fine powder, beaten up with whites of eggs or linseed oil, and formed into a paste, which was repeatedly baked in a furnace. The Bolognian stone, as the preparation was called, has a very powerful phosphorescence, of a reddish colour; and the Italian preparation generally has this quality in a higher degree than the imitations prepared elsewhere; but it has been quite eclipsed by the phosphorus of Canton. Canton recommends oyster shells which have been long worn on a sea-beach, as the materials to be employed. They are to be calcined in a good coal-fire for half an hour. The purest parts are then to be collected, and reduced to a fine powder. Three parts of this, with one of sulphur, are to be rammed into a crucible about Iģ inch deep, till nearly full. Place it in the midst of the fire, where it must be kept red hot for at least one hour, and then allowed to cool, when it is to be removed from the crucible. The fine portions of this, which will be quite white, are to be scraped oft’, and immediately enclosed in a bottle with a well-ground stopple.

When this bottle is exposed for a short time to the light of day, to any artificial light, or, better, to the direct rays of the sun, it will be luminous for some minutes in the dark; and its light will be renewed by a fresh exposure to the sun. At one time it was a subject of controversy, whether or not these substances emitted only the light they had imbibed by exposure, as it was conceived to be a ready mode of deciding the dispute between the followers of Newton and Descartes respecting the nature of light. Galeazzo, Zanotti, and Algarotti of Bologna, tried whether, when exposed to the different rays of the prism, the Bolognian phosphorus would only show the colour of that ray to which it had been exposed; and they thought that its light was reddish, to whichever ray it had been previously exposed. But in these experiments its light was very feeble, and therefore not satisfactory. Afterwards, however, Father Beccari of Turin, by exposing pieces of more powerful phosphori in tubes of different coloured glass, found that, in the dark, they only emitted the colour of the light to which they had been exposed.

Van Helmont appears to have discovered another powerful phosphorus; and Baldwin of Misnia, in 1677, found that the residuum of a solution of chalk in aquafortis, after distillation, formed a phosphorus of considerable power, but inferior to the Bolognian. Du Fay, in 1734, found that similar properties resided in gypsum, marble, and topaz. The emerald, diamond, and many other precious [7:13:337]stones, he found to have the same property, without calcination, and by mere exposure to light. From the experiments of Margraaf, all the earthy sulphates have this property when calcined; but he thought that neither metals, metallic ores, nor agates, possess it. The analysis of topaz shows that it contains fluoric acid; and we may now generalize the observation, and state, that all substances capable of becoming phosphorescent by calcination contain some fixed acid, and probably all minerals containing such acids are capable of becoming, in like manner, phosphorescent.

The experiments of Canton are the most complete on this subject [Phil. Trans, lviii.). When his phosphorus was, for a short time, exposed to the light of a candle, the moon, or the diffused light of day, it shone for a considerable time in the dark. When exposed to the direct rays of the sun, it gave out light for two hours, at the common temperature of the air. When it had ceased to shine in the dark, the application of heat renewed its luminousness for a short time. If the glass containing the phosphorus be placed in boiling water, its luminousness will be stronger than in the cold, but will last a shorter time. When it has ceased to give light in hot water, it will again give out light on being placed on a hot iron between 400° and 700° Fahrenheit.

2. Some natural bodies become phosphorescent by a gentle heat. Thus, some kinds of fluor spar, particularly the coloured varieties, give out a pure greenish or a bluish light by being heated; and this is finely exhibited by the green varieties. The mineral called phosphorite, which is a fibrous phosphate of lime, found principally in Spanish Estremadura, gives out much light when heated. Some marbles, some ores of metals, coal, wax, butter, oil, and several other mineral and vegetable substances, so treated, become more or less luminous. In some the light is momentary, in others it lasts several minutes. It soon attains its maximum brightness, and then fades away. A stream of cooler air extinguishes the light for a moment, but it re-appears on the ceasing of the cool current. Analogous to this class of bodies in some degree are those substances which give out light on percussion; such as siliceous minerals, either with one another or with steel, hard porcelain, or the like; but they also, in such collisions, emit heat as well as light.

3. Mineral and vegetable bodies, during their decomposition, often give out light. Fish, mutton, and rotten wood, are the best known instances of phosphorescent bodies of this class.

The luminousness of fish is well known; and Dr Hulme has shown that the light of herrings and mackerels begins to appear while the fish is still eatable, and soon arrives at its maximum, but begins to decrease when they pass to putridity. To produce this change, the fish should be kept in a dark and cool place. It is not confined to the skin of the animal, for if cut into pieces, the surface of each piece becomes luminous; and it is often seen within the mouth of the fish. The luminous matter easily rubs off, and may be transferred to the hands of him who touches the fish. This light is not attended by any perceptible heat. When scraped off, it forms a gelatinous liquid, that will shine for several days, if preserved in a phial. The addition of fresh water, lime water, water impregnated with carbonic acid, of vegetable acids and alkalies, extinguishes it, as do neutral salts, infusions of pepper, and camphor, when strong; yet the same substances, in a weak solution, seem to promote it, and even render it more durable; but sea-water increases its splendour. This luminous property is also found in lobsters and in shell-fish, especially in the Pholas Dactylus, and its congeners. This shell-fish is luminous when quite fresh, and is mentioned by Pliny as rendering the mouths of those who eat it luminous. The light is readily imparted to milk and sea-water, but it is extinguished by spirit, wine, or vinegar. Sea-water, thus rendered luminous, increases in brightness by a gentle heat; but when heated to 133° Fahrenheit, it is suddenly extinguished, and cannot again become luminous. This luminous matter, when narrowly examined, sometimes appears to give out a sort of lambent flame, which closely resembles that of a solution of common phosphorus in oil; and it smells of phosphorus or of phosphuretted hydrogen. The flesh of Mammalia undergoes similar changes during its decomposition. This has often been seen in mutton, beef, and veal. This light has sometimes been observed on corpses, much to the terror of the vulgar; and in vaults where dead bodies have been deposited, it has sometimes been observed in a glairy matter adhering to the vault. This last matter, however, probably is the produce of some cryptogamian plant; and it is well known that rotten wood is sometimes highly luminous. A light of this kind is stated to have been observed round the body of a woman at Milan, but it flitted from the bed on the approach of the reporter. This appearance has been more frequently seen around graves, and has obtained in Scotland the name of elf-candles.

The light from corrupting animal and vegetable matter requires oxygen in some form for its continuance. It is soon extinguished in the exhausted receiver of the air-pump, as Mr Boyle long ago observed; but it would seem that the small quantity of air which is contained in water is sufficient to sustain its luminousness. No perceptible heat is extricated in any of these kinds of phosphorescence.

Analogous to the light from decaying organized bodies, is the curious meteor termed ignis fatuus, or will-o'-the-wisp. Its ordinary appearance is like the faint flame of a taper; sometimes it resembles the light of a torch, or a faggot; but it usually recedes as it is approached, and can rarely be observed near at hand. The colour of the light is usually pale bluish, and seems brightest when most distant. It is most frequent in marshy grounds, in churchyards, or where a considerable mass of animal and vegetable putrefaction is going on. Dr Derham once observed an ignis fatuus playing round the head of a dead thistle; and, by cautiously approaching, he got within two or three yards of it, when a slight movement of the air made it flit; and when he pursued it, he was unable to overtake it.

A remarkable appearance of ignis fatuus was, about a century and a half ago, common in the vicinity of Bologna, which has been well described by Beccari [Phil. Trans. vii.). He estimated that two, which at that time appeared almost every dark night, one to the east, the other to the north, of the city, gave light equal to an ordinary faggot. One of them accompanied a friend of Beccari for a mile along the road to Bologna, giving as much light as that of the torch carried before him. Sometimes these meteors divided into several parts, or floated like waves of flame, dropping small scintillations.

Dr Shaw, the author of Travels in the Holy Land, describes a remarkable one which accompanied him, for upwards of an hour, in one of the valleys of Mount Ephraim. Its shape was at first globular, but it afterwards spread so as to involve the party of the traveller in a pale inoffensive blaze, then disappeared; again it re-assumed the globular form, and again expanded itself, at certain intervals, over more than two or three acres of the adjacent mountains. The atmosphere that evening had been very hazy, and the dew, as it fell on their bridles, felt unusually unctuous and clammy; a [kind of weather, says Shaw, in which sailors observe the balls of fire that flit about the masts and yards of ships.

The cause of ignis fatuus has been disputed. It can scarcely be accounted for by the phosphorescence of the glow-worm, or any species of fire-fly. It differs also from electric flame, but has the greatest resemblance to the flame [7:13:338] of phosphuretted hydrogen; a gas which spontaneously inflames, on coming into contact with air, and which is given out during the corruption of organic matter. This gas is absorbable by water and by fatty oils, to which last it imparts its phosphorescent qualities; and perhaps the luminousness of fish may depend on the union of this substance with oily or mucous matter. There is some difficulty in accounting for the appearance so constant and considerable as that described near Bologna. The ground on which the largest meteor appeared is a hard clay, very retentive of water; while in the hilly district, where the Bolognian ignis fatuus was smaller, the soil was a loose sand. Beccari however states, that they chiefly frequented the banks of streams. All accounts confirm the absence of sensible heat from these meteors.

SECT. VIII LIGHT EMANATING FROM LIVING ANIMALS.

A luminous appearance somewhat similar to that given out by decaying organic matter is occasionally observed to play round some classes of living animals; and regularly emanates from the bodies of others, at particular seasons; or as a constant concomitant of motion, by another class.

Of the first kind, probably, is that light sometimes observed playing around the ears and manes of horses, which, though by some attributed to electricity, is probably an emanation from the animal itself; and may perhaps consist of a phosphuretted gas, disengaged by some unknown process of the animal economy. A lambent flame, of a similar nature, has in a few instances been remarked around the heads of children; a circumstance which is happily seized by Virgil in his fine description of the glory that appeared on the temples of the young Ascanius.

Ecce levis summo de vertice visus Iuli Fundere lumen apex, tractuque innoxia molli Lambere flamma comas, et circa tempora pasci.

Aen. ii. G83.

Living vegetables, in like manner, also occasionally give out light. This has been particularly noticed in the marigold, the orange, the Indian pink or lilium bulbosum, aconitum napellus, tropoeolum majus, &c. But there are animals in whom luminousness forms a necessary part of their economy.

The most familiar instance of this is the common glowworm, Lympyris noctiluca, and its congeners. The male of this species is a coleopterous insect, and sports in the air; while the female is apterous, doomed for ever to crawl among herbaceous plants, or to nestle on the leaves of shrubs: but when the shades of evening are drawn around, during the summer months, a spot of lucid yellow light, generally tinted with a shade of green, emanates from the extreme rings of her abdomen, and sprinkles the hedges, in some parts of our island, and the warmer parts of Europe, with brilliant stars. The final object of this light is probably to attract the notice of the male insect, who otherwise could with difficulty distinguish his wingless mate. The male of the English glow-worm is generally believed to be destitute of the apparatus for light; though Mr Waller {Phil. Trans, for 1684<) asserts, that the male of one species of English glow-worm has the luminous appendage. The winged species of glow-worm are common in Italy, Spain, the south of France, and still more so in equinoctial America, in which the flickering light of the numerous fireflies affords a pleasing and interesting spectacle. This light is found to belong to both sexes, though it is most str iki ng in the females.

But the luminous property of the Eulgora Lanternaria of South America surpasses that of all animals, in the splendour of its light. It is a large insect, of the order Hemip- tera, three and a half inches long. It has a sort of thick proboscis, about one inch in length, which is the luminous organ. The light emitted by this species is so splendid that two or three of them will illuminate a chamber.

In all these animals the light has so much the appearance of phosphorus dissolved in oil, that probably it may be a secretion of an analogous nature.

Many animals inhabiting the sea are highly luminous; and it is almost established that the luminousness so often exhibited by the ocean depends entirely upon myriads of minute phosphorescent animals.

This appearance is not constant, but is very frequent in most latitudes, and generally, whenever the night is dark, may be seen exceedingly brilliant around our own coasts. When the water is still, it is seen as numerous bright points of a bluish white phosphorescent light of considerable intensity; but when the water is agitated, as by the waves, the motion of a ship, or the dashing of oars, the light appears often in flashes so intense as to show the hours on a watch, or to render legible the pages of a large printed book. The number of the luminous points varies greatly at different times and in different places; and often in the course of a short sail this fluctuation is very conspicuous. We have observed the coruscations, during a gentle breeze, like a line of fire several hundred yards in length from the bows, or in the wake of the vessel. This appearance is found in every sea, but with some difference in intensity and colour, as it seems to proceed from various genera of animals.

The lurainousness of the sea was long ascribed to electricity; but about the beginning of this century it was proved, in many instances, to depend on the presence of animalcules in the ocean, particularly of a minute species of medusa, which abounds in our seas, and seems to be the same as Μ. Hemisphoerica of Müller. Several years before, Sir Joseph Bankes discovered other two animals which rendered the sea luminous, viz. a sort of shrimp, Cancer Eulgens, and a large medusa, Μ. Pellucens, both of which abound on the coasts of Brazil.

About twenty-two years ago, the writer of this article made many observations on this subject, both on the coasts of Britain and in the Bay of Biscay. When the water was very luminous in that sea, on several evenings he drew water and carried it for examination into the cabin. Of course it ceased to appear luminous when viewed by candle light, but he could distinguish, floating in a glass of seawater, a number of pellucid animalcules, which, when magnified by the globular form of the containing vessel, evidently belonged to ' several species of Medusaria, among which he could observe the genera Noctiluca of Lamarck, a Cayanea, and a JBeroe. The general size of these animals was from one twentieth to one thirtieth of an inch in diameter. That the luminousness of the sea was derived from these minute animals, appears to be proved by the following simple experiments. Portions of this sea-water were put into separate beer glasses, and the number of animalcules in each was carefully ascertained: they were carried successively to the deck, and when the water was dashed on it, the number of lucid points was ascertained. In most instances the number of these points coincided with the number of animals previously observed in the glass; in no instance were the lucid points more ; though occasionally they were not so numerous, probably owing to some of them adhering to the glass, or having escaped the shock that stimulated their light-making organs. The same animals he has often observed, even more numerous, in the luminous sea-water of the t British coasts; and has obtained similar results by a repetition of the experiments. The luminous points often adhere to the fingers; and on introducing such a luminous speck into a glass of non-luminous sea-water, careful inspection will show a medusary animal [7:13:339]floating in the water. The sea-water, on such occasions, if left at rest in glass vessels on deck, or even if suspended in gimbals, would occasionally exhibit luminous points; and the author conceives that the light either attends the voluntary movements of those minute creatures, or is emitted at their will. When sulphuric or other strong acid is poured into luminous water, it will produce a considerable flash of light, either by the effort of the animals to escape, or the unexpected stimulus it produces. In some climates the sea exhibits a fiery-red hue. This has often been observed in the Chinese seas, and in some parts of the Indian Ocean. It is produced, according to the best authorities, by myriads of minute animals that emit a reddish light. Captain Horsburgh and Mr Langstaff have described other kinds of light occasionally observed in the Indian Ocean. The latter mentions, that during a passage from China to New Holland, the sea’at night had a faint milky appearance, as if snow had just fallen on the water. The sailors thought it was produced by a coral shoal, but seventy fathoms of line did not find bottom; and when the water drawn up was examined,ξ it was observed to swarm with minute globular bodies, about the size of a pin’s head, linked together, and of a milky hue, probably some minute species of medusa. It is well known that the larger fish are sometimes luminous. This has been remarked in the bonito and the shark; but whether they are naturally luminous, or only in consequence of the adhesion of luminous animalcules to them, is not determined.

Some have ascribed the luminousness of the sea to electricity, others to putrescent particles in the water. The former opinion is not probable, from the appearances observed; the latter is supported by the luminous matter of dead fish being diffusible through water, and imparting to it luminous qualities; but the known phosphorescence of many small sea animals, and the coincidence of the number of scintillations, in the experiments above detailed, with that of the animals observed, incline us to believe that the luminous appearance of the ocean depends on the presence of minute animals.

SECT. IX. CHEMICAL EFFECTS OF LIGHT.

Light appears to be possessed of chemical properties and energies distinct from those of heat, which may be considered as further proofs of its materiality.

It is capable of decomposing various metallic salts. Thus, if a colourless solution of nitrate of silver be exposed to light, it gradually blackens; a powder is deposited which has the same colour, and the salt is found to have lost a portion of its oxygen. This change is more rapidly effected by the direct rays of the sun than by the diffused light of day. The neutral solutions of gold, also, when exposed to light, in contact with charcoal, with vegetable or animal matter, as cotton and silk, are decomposed. This is the principle of the beautiful process invented by Mrs Fulhame, for ornamenting muslin and silk stuffs with flowers and sprigs of gold. The salt of gold parts with its acid and its oxygen to the vegetable or animal matter through the agency of light. The dry salt formed by dissolving gold in nitro-muriatic acid is also slowly decomposed by light under similar circumstances.

Scheele examined the effects of light on metallic solutions, and discovered that the chemical effects of the different coloured rays of the prismatic spectrum were different. By enclosing solutions of silver in glasses of different colours, he found, that in red glass there was very little effect produced, whilst in violet-coloured glass the blackening was speedily produced. These interesting facts were confirmed by Senebier and by Thomas Wedgwood.

The latter showed, that in the full sunshine, the blackening of muriate of silver was produced in two minutes, and in the shade that several hours were required to produce this effect. The sunbeams transmitted through red glass have very little blackening effect; yellow and green glass are somewhat more effectual; but blue and violet glass produce the most decisive effects. The discovery of Herschel, respecting the different refrangibility of light and heat, induced Ritter and Wollaston to try the effect upon the salts of silver beyond the violet ray; and they found that the blackening was most decided beyond the visible boundary of the spectrum at the violet end. In some experiments of Senebier, muriate of silver was darkened by the violet ray in 15", by the blue in 29", by the green in 37", by the yellow in 5' 30", by the orange in 12', and by the red in 20'. These effects are wholly due to light; for no effect is produced in the hottest point just beyond the red rays.

Berthollet proved, that during the action of light on many metallic oxides, as those of gold, silver, lead, and mercury, a portion of their oxygen was extricated; and this is supposed to be the change produced in the blackening of the salts of silver,—a partial reduction of the metal. Sir H. Davy found that tritoxide of lead, when moistened, and exposed to the red rays, became red, that is, it lost oxygen, and became a deutoxide. Oxide of mercury, obtained by mixing potash and calomel, was not changed by the most refrangible rays, but became red in the least refrangible rays, which must have resulted from the absorption of oxygen. The violet rays produced on the moistened red oxide of mercury the same effect as a stream of hydrogen gas, that is, a deoxidation. Wollaston found that these rays produced an oxidizing effect on one vegetable substance, resin of guaiacum. This resin becomes green by absorbing oxygen; and he found that it underwent this change in the violet rays, but again obtained its yellow hue on being exposed to the red rays.

Some of the acids suffer decomposition by light; thus, if we expose colourless nitric acid to the sun’s rays, in a flask provided with a bent tube, and terminating in a pneumatic apparatus, the acid becomes coloured, from the formation of nitrous gas, and oxygen may be collected in the trough in the usual way, as Berthollet ascertained. Light also decomposes some vegetable acids, such as the hydrocyanic; and to preserve-such compounds pure, it is necessary carefully to exclude light.

Light also in some cases favours chemical combination. If chlorine which has been collected over water, and therefore contains water, be exposed to light, the water is slowly decomposed; its hydrogen enters into combination with the chlorine to form hydro-chloric or muriatic acid, and its oxygen is liberated. The influence of light is still more striking on chlorine and on hydrogen. If we mix equal proportions of these gases, and the access of light be carefully excluded, no action takes place, or their union is very slowly produced; but if we expose them to the diffused light of day, combination will take place in a quarter of an hour. If exposed to the direct beams of the sun, the union is instantaneous, and with a violent explosion. Davy found that when such a mixture was exposed to the red rays only, the gases united without explosion, yet more rapidly than when exposed to the violet rays; but that the conversion of a solution of chlorine into muriatic acid took place most readily at the most refrangible end of the solar spectrum.

SECT. X EFFECTS OF LIGHT ON PLANTS.

The change produced on vegetable colouring matter by light is familiar in the process of bleaching by exposure to [7:13:340]the sun. But the influence of light on living vegetables is much more remarkable.

If a plant grow in total darkness, its natural green hue is not acquired; but it will be white, though in other re-spe'ets vigorous. This is familiar in the etiolation or blanching of certain garden-stuff's, as celery, sea-kale, endive, &c. The late Professor Robison, many years ago, remarked that plants growing in darkness were not only white, but that they did not attain the natural form of their leaves, nor their natural odour. In descending into a coal mine, he accidentally met with a plant growing luxuriantly, the form and qualities of which were entirely new to him. The sod on which it grew was removed, potted, and carefully attended to in his garden. The etiolated plant languished and died; but the roots speedily threw out vigorous shoots, which, from the form of the leaves, and peculiar odour, he readily recognised as common tansy. He repeated similar experiments upon plants of lovage, carvi, and mint, with analogous results.

The green colour of vegetables, and even the form of their leaves, are materially influenced by light.

Some experiments of Senebier would lead to the conclusion that it is the violet end of the spectrum which has the greatest influence in counteracting the effects of etiolation. This production of the dark colour of plants would appear to depend upon the decomposition of carbonic acid by the living vegetable, when acted on by light, and the fixation of its carbon. That light is necessary to this process, is proved by the experiments of Priestley, Senebier, Ingenhouz, De Saussure, Davy, and many vegetable physiologists. Growing plants in sunshine give out oxygen by the decomposition of earbonic acid; but in darkness carbonic acid is evolved; and Ellis has demonstrated, in his interesting essays, that oxygen, under such circumstances, always disappears from the air in which vegetation is going on. Light, then, acts an important part in the vegetable economy, by fixing the most dense and abundant of the three general elements of vegetable matter. The influence of light in maturing fruits, is well known. The fruits which when ripe are saccharine, are previously acid ; that is, their hydrogen and earbon are combined with an excess of oxygen. Light, by favouring the evolution of oxygen, and the fixation of carbon, converts the vegetable acid into sugar, and thus provides a suitable food for the embryon in the seed. On the other hand, the influence of light is not less conspicuous in another class of vegetables. In an early stage of the germination of seeds yielding farina when ripe, the tender rudiment of the seed is enveloped in a slightly saceharine juice, as may be seen in young wheat and maize. Now, what is wanted to convert them into farina is the evolution of oxygen, and the fixation of a larger quantity of earbon. These two indications are fulfilled by the influence of light on the growing vegetables; and thus we can explain why the sun’s influence is so essential in the one ease to the formation of sugar from an acid; and in the other, of farina from a saceharine juice. The same principles will explain why a farinaceous seed, when planted in the ground, becomes again saccharine. Light is now wanting; oxygen is absorbed, and partly aids in the evolution of the redundant carbon, partly is added to the two other general elements. This view affords an instance of the important agency of light in vegetation, and a beautiful example of the simplicity of the means employed to produce diversified effects in the works of creation.

SECT. XI. EFFECTS OF LIGHT ON ANIMALS.

The facts noticed in the last section show the powerful influence of light on vegetable forms. But its effects on the exterior of animals, though less striking, are not unimportant.

1. The influence of the sun’s rays in deepening the colour, or in giving a brown tint to the skin, seems to be more due to the light than to the heat of the sunbeams; for the parts of the skin covered by the clothes, though kept thus hotter than the parts exposed, do not undergo this change. The pale visage and enfeebled vitality of those who live much in obscure apartments, in prisons, and in mines, are well known; and though probably the most violent symptoms that characterize the anhoemia of miners, in which the skin assumes a yellowish, waxy hue, and the lips become bloodless, be chiefly due to breathing a vitiated atmosphere, yet some influence is certainly attributable to want of light. The anhaemia of persons long confined in dungeons has often been remarked, and was lately described as strikingly exemplified in the person of Caspar Hausser, the young man whose mysterious birth, confinement, and assassination, have hitherto baffled conjecture.

In climates where the heat renders a state almost approaching to nudity desirable, all travellers agree that the development of the human frame is early, and the form has fewer deviations from the symmetry natural to the race than among northern nations. Deformity is said to be comparatively rare in hot climates, where the surface is much exposed; and this has been attributed by Dr Edwards in some measure to the influence of light. Perhaps, after making due allowance for the less chance of rearing sickly or deformed children among barbarous nations, and for the prevalence of infanticide among them in such cases, there is considerable justice in his remarks on this subject; without which it would be difficult to account for the fine muscular and rounded forms so often observed among nations agreeing in nothing except in the prevalence of a very free exposure of their persons to the full influence of light; as among the Mexicans and Peruvians, the Chaymas and Muyseas of South America, the Caribbs of the Antilles, the inhabitants of the numerous groups of the South Seas, or the free inhabitants of Africa. Probably, too, a part of the effects attributed to country air, in restoring to health the sickly child of the city artizan, is due to insolation, or the exposure to light, which 'appears to have the .property of invigorating the vital functions, and of elevating the spirits of those who have suffered a long deprivation of its cheering influence.

2. The effect of light on the lower animals is more marked, and is strikingly illustrated in the curious experiments of Dr W. F. Edwards upon the spawn of frogs. He enclosed portions of the spawn of the frog in different vessels permeable to water, some of which were perfectly opake, while others freely admitted light; the temperature was the same, yet the eggs exposed to light came to maturity; but those deprived of light were not hatched.

He even found that the development of the perfect form of such animals depended on light. By enclosing tadpoles of both frogs and toads in different boxes, some of which freely admitted light, while others totally excluded it, and placing them in running water, he found that the tadpoles exposed to light underwent the change to the perfect form of the animal, as usual; while the tadpoles excluded from the light, though they seemed perfectly vigorous, did not undergo the transformation, even though they had increased to double or triple their primitive weight. He also conjectures that the Anguine Syren, Proteus anguinus, may only be the larva of some reptde retained in its imperfect form by the profound obscurity of the subterranean lakes of Carniola, in which it is found. In this conjecture, however, he is mistaken; for that singular animal has been kept alive without change a considerable time, under circumstances favourable to its transformation, if really an imperfect[7:13:341] species, by various naturalists; particularly, as we have been informed, by the Austrian archduke John; and also by Mr Melly, a zealous naturalist now resident in Liverpool.

The facts, however, already detailed, show that the influence of light on the animal economy is by no means inconsiderable. But the most interesting property of light, in relation to animals, is its effect in producing vision.

3. The rays of light emanating from bodies, or reflected from their surfaces, are destined to impress the sensorium through the eye ; an admirable and complex optical apparatus, in which the rays, after passing through the transparent cornea, a, sustain various refractions, until they reach the sentient expansion of the optic nerve, f called retina. The cornea constitutes the front of the eyeball, the rest of its surface is formed of a dense white tunic, called the sclerotic coat, bb, which joins the cornea by its edges, forming with the latter an almost globular body, which contains the more delicate structures of the eye. Just within the cornea is observed a coloured ring, apparently formed of diverging fibres, termed the iris, hh. This ring is perforated by the pupil, a circular aperture in its centre, which is capable of contraction and expansion, by the action of the fibres of the iris, according to the increase or diminution of the light. This movement of the pupil is not under the will of the animal, but is regulated by the stimulus of light, so as to exclude a quantity that would be hurtful to the organ. The iris is attached to the outer coat of the eye, the sclerotic, by the ciliary processes, ηη, Within the sclerotic lies a delicately thin vascular membrane named the choroid coat, having its interior surface lined with a black secretion termed the pigmentum nigrum, the probable' use of which is to aid clear vision, by preventing a multiplicity of reflections in the bottom of the eye.

Still nearer to the centre of the eye lies the retina, which is continuous with the optic nerve,/, and is the part of the eye sentient to the impressions of light. Entering the bottom of the eye, but not just in its axis, is seen a thick cord, the optic nerve, by which the impressions upon the retina are conveyed to the brain.

The bulk of the eyeball is made up of three substances, which have been improperly termed humours. The first lies just within the cornea, and fills the space between it and the iris, as well as the smaller space immediately behind the latter. From its fluidity, it is termed the aqueous humour, g. The iris divides the chamber of the aqueous humour into an anterior and a posterior portion. The posterior wall of this chamber is formed by a delicate, transparent, capsular membrane, enclosing a lenticular body, convex on both sides, though more so posteriorly, which is named the crystalline lens, l. This body consists of concentric layers, formed of fibres externally less consistent, but increasing in density towards its centre; a structure intended for the purpose of correcting its spherical aberration. The capsule of the crystalline lens is retained in its situation by the ciliary circle ox ciliary processes ; and it lies imbedded in the most considerable of all the humours of the eye, the vitreous, m, which consists of a clear gelatinous substance, that recent anatomical investigations prove to be lodged in very delicate membranous cells. In the healthy state, especially in young persons, these humours are perfectly colourless and transparent, but they become coloured by age; and the crystalline lens has been seen in very old persons of a bright amber colour, though still transparent. Opacity of the lens forms the disease termed cataract. The obvious use of the humours of the eye is to refract the rays of light entering the eye by the transparent cornea, and to concentrate them on the retina. The difference in their consistence seems intended to make that organ a true and nicely adjusted achromatic optical instrument.

The ray from any object falling directly in the centre of the eye, perpendicular to its surface, will not undergo any refraction, but pass straight through to the retina; and its direction may be termed the axis of the eye. All the rays which fall obliquely on the eye will, by the refraction of the humours, be bent from their course, and cut the axis somewhere between the lens and the retina; and the refractive power of the humours is such as to converge on the retina all rays proceeding from any one point of a visible object, whether these rays fall on the cornea in a parallel or a diverging condition. But as these rays have all crossed the axis of the eye by the refractive power of the humours, it is obvious that the rays from the upper part of the object will impinge on the lower part of the retina, and those from the lowest point, on the upper part of that membrane. Hence the image of the object will be inverted on the retina. That this actually takes place we can easily satisfy ourselves, by removing from the posterior part of the eye of a sheep a portion of the sclerotic coat with a sharp knife, until it becomes translucent, when the image of a candle placed before the cornea will be seen inverted on the back part of the eye thus prepared.

In the usual state of the eye, its refractive apparatus is so contrived that all the rays from the same points of objects at some distance from the eye will converge in the retina, or the principal focus of the eye will lie in that membrane ; but divergent rays from very near objects would converge in a point behind the retina, were there not some power of adjustment in the eye for correcting the confused image which would thus be formed. This may be effected in two ways; either by increasing the convexity of the cornea, or by bringing the lens to a greater distance from the retina. Some have attributed this adjustment to an alteration in the form of the eyeball, and consequently of the cornea, by the action of the motor muscles of the organ; but there is much more probability in the opinion that this adjustment is produced by the contractions and expansions of the iris. This part [7:13:342] of the eye is attached to the ciliary ligament, which is also connected with the capsule of the crystalline lens. Hence contraction of the pupil would tend to draw forward the lens, or remove it farther from the retina; and thus the rays which have their point of convergence behind the retina, would meet in that membrane, and a distinct image would be produced. That some such adjustment is necessary is obvious; for if, after contemplating a distant object, our attention be suddenly turned to a very near one, it is some time ere the second object is distinctly perceived; and the movement of the iris will be seen to take place before the second image is well defined. The retina is capable of affording the perception of external objects in its whole extent, except at the insertion of the optic nerve, a spot about ⅛th of an inch in diameter in man. Any image falling on this spot is invisible, as multiplied experiments have shown. This insensible point in either eye may be indicated by the familiar optical experiment of placing three coloured wafers horizontally on a wall, on a white ground, about two feet from each other. Let the operator place himself about two feet from the wall, opposite the middle wafer; shut one eye, and then gradually retire backwards, while he fixes his open eye on the wafer nearest the closed eye. When an ordinary eye is distant from the wall about five times the distance of the wafers from each other, the image of the middle wafer will have fallen on the insertion of the optic nerve, and will no longer be visible.

Two very interesting speculations connected with vision have long exercised the ingenuity of physiologists and metaphysicians, viz. how it happens that animals with two eyes see objects single, by means of double images on the retinae; and how they appear erect by means of inverted images?

The first has been explained on the supposition of there being corresponding points in each eye that convey similar perceptions to the mind, and that such afford only the idea of unity. In the natural movements of the eyes, the images of external objects are supposed always to fall on such points; but should any cause throw the images on dissimilar points, then we have double vision. Hence, if with the finger we push one eyeball slightly out of its parallel position with regard to the other, two objects are perceived. In persons who squint, the parallel movement of the eyes is lost; and it has been supposed that in cases where the squinting has come on in adults from imperfection in one eye, that habit enables the individual to correct the illusion of double vision, and that the person ceases to notice the impressions made on the least perfect of the two eyes.

The second fact alluded to is that of erect vision by inverted images. That images of external objects are depicted inverted on the retina is perfectly well ascertained; and philosophers have been much puzzled to explain how the indications of vision and touch are reconciled. Some have imagined that at first we really see objects inverted, but learn by experience and by the sense of touch to correct the illusion. Other attempts have been made to explain it by what has been termed the law of visible direction ; by which it is supposed that when any point of an object is viewed, the rays proceeding from that point must fall on the eye with different degrees of obliquity, yet that point will be only seen in the direction of the central ray of the cone of light proceeding from that point: And as the lines of visible direction must necessarily cross each other at the centre of visible direction, those of the lower part of the image on the retina must go to the upper part of the object, and those of the upper part of the image to the lower part of the object; and hence an erect object is considered as the necessary result of an inverted image.

This is very ingenious, but it is obviously hypothetical and obscure. The physiological explication of single vision by double images suggested by Newton, and since supported by Wollaston, is based on what has been termed the semi-decussation of the optic nerves at their commissure, whereby the right half of the retina of each eye is placed in direct nervous communication with the right optic lobe, or right half of that part of the brain termed corpora quadrigemina, and vice versa. Hence impressions made on corresponding points in each retina may, in fact, be impressions on the same points of the common sensorium, and therefore co-operate in producing the same perception. The anatomical proof of this is still defective, but many facts give it probability. The subject has engaged the attention of Professor Alison of Edinburgh, who considers that this arrangement of the optic nerves is but a part of the provision by which nature has secured harmony between perceptions afforded by sight and touch. His explanation of one of the most perplexing questions in animal physiology is so ingenious, that we gladly avail ourselves of his permission to lay it before our readers.

According to him, the peculiar contorted or involuted course of the optic nerves (in all vertebrate animals) around the crura cerebri, until they are lost in the optic lobes, seems designed to secure that the position of the impression on the sensorium should be conformable to the true position of the object.

It is only in those animals intended by nature to contemplate objects with both eyes at once, as in mammalia and birds, that the semi-decussation of the optic nerve is found. The contrivance in fact implies that both eyes are to be at the same time directed to the same object, or that both optic lobes are to be constantly employed in vision at the same time; the right half of each retina being in connection with each optic lobe, and the left half of each retina in like manner connected with the left optic lobe. Now, the right half of each retina contains the image of the left half of the field of vision; and therefore the impressions made by the left half of the field of vision fall on the right optic lobe, and have, on their left, the impression resulting from the right side of the field of vision; on which, as both optic lobes are necessarily exercised together, the attention of the mindjs equally directed.

Dr Alison considers that the grand contrivance adopted by nature to secure harmony between the indications of sight and touch is the decussation at the pyramids of the nervous fibres concerned in common sensation and in voluntary (movements; the effect of which must be, that while the right side of the brain is that to which impressions from the left half of the field of vision are brought, it is also that on which all the other sensations of the left side of the body depend; or, in other words, we both see andfeel τvhat is on our left by the right side of the brain. Accordingly, it is in those animals only in which the semi-decussation of the optic nerves exists (namely, in Mammalia and birds) that the decussation at the pyramids exists, or that the sensations and voluntary motions of each side of the body appear to be in connection with the opposite side of the brain.

As the admirable mechanism of the eye, and its beautiful adaptation to the necessities and comfort of the animal creation, afford a striking instance of that wisdom and beneficence so conspicuous in the handy work of the Deity; so the extinction of this exquisite and important organ must be considered as one of the severest of human calamities; a misfortune that perhaps cannot be fully appreciated by those who have not experienced the loss of sight, so feelingly deplored by our mighty bard in poetry that can only perish with the language in which it is expressed.

Thus with the year Seasons return, but not to me returns [7:13:343]Day, or the sweet approach of even or morn, Or sight of vernal bloom or summer’s rose, Or flocks, or herds, or human face divine; But cloud instead, and ever during dark Surrounds me, from the cheerful ways of men Cut off, and for the book of knowledge fair Presented with an universal blank

Of nature’s works, to me expunged and raz’d, And wisdom at one entrance quite shut out.

Some of the works that treat of light are, Aristotelis Opera de Anima et de Meteor. ; Senecae Opera ; Plinii Hist. Nat.; Euclidis Optica; Heliodorus Larissaeus De Opticis ; AIhazen, Optica cum Comment. Vitellionis; Baconis Opus Majus; Maurolycus De Lumine et Umbra ; Baptista Porta, Magia Nat. et de Refractione; Bacon’s Works ; Antonio de Dominis De Radiis Visus et Lucis; Borelli De Vero Telescopii Inventore; Galileo Galilei, Opere et Vita; Kep ler, Parolipomena ad Vitellionem; Schotti Magia Universalis; Descartes, Opera; Gassendi, Opera ; Athan. Kir-cheri Opera; Huygenii Opera, also Snell’s Discoveries ; Newton’s Optics and Principia ; Boyle’s Works ; Hooker’s Micrographia, axAPosth. Works; Muschenbroeck’s Tentamina et Introd. ; Grimaldi De Lumine ; Beccaria Delľ Elettricismo, &c.; Beccarii Comment, de Phosphoris ; Bouguer, Traite d’ Optique; Lambert, Photometrie; Montucla, Histoire des Mathématiques; Smith’s Optics ; Priestley on Vision; Hutton on Light; Cavallo’s Philosophy ; Leslie on Heat; Biot, Traité de Physique; Muncke, Handbuch der Naturlehre ; Treviranus, Beitraclie zur Lehre von den Gesichtwerkzengen und dem Sehen; Mémoires de l’ Acad. Royale des Sciences; Mémoires de l’ Institut, et de l ’ Acad. des Sciences; Phil. Trans. London; Phil. Trans. Edin. (i. 1.1.)

LiGirr-Horse, an ancient term, signifying an ordinary cavalier or horseman lightly armed, in opposition to the men-at-arms, who were heavily accoutred, and armed at all points.

LiGHΤ-House, a building erected upon a cape or promontory upon the sea-coast, or upon some rock in the sea, and having on its top in the night-time a light, such as to be seen at a great distance from the land. As the British light-house system is at present under the consideration of parliament, we feel that it would be premature and unsatisfactory to give any article under this head; and we are therefore under the necessity of referring to another, viz. that of Sea-Lights, where the whole subject will be fully discussed.

LiGHτ-Room, a small apartment, enclosed with glass windows, near the magazine of a ship of war. It is used to contain the lights by which the gunner and his assistants are enabled to fill cartridges with powder to be ready for action.