# The wonders of optics/Illusions caused by light itself

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ILLUSIONS CAUSED BY LIGHT ITSELF.
When playing about the Christmas fire, children frequently amuse themselves by whirling round and round a piece of wood, one end of which they have previously lighted and blown out. In proportion as the movement becomes more rapid, the path of the red-hot end becomes more and more connected, until at last a burning ring is formed, in every part of which the shining charcoal appears to be at the same time. The only way of accounting for this illusion is by supposing that the image formed by the burning stick upon the retina remains there for an appreciable period, the impression made by it at one part of its journey remaining until it returns to its former position. The power possessed by the retina of retaining impressions explains a large number of illusions of the same kind. The chord of a musical instrument, for instance, when struck, appears to occupy a longer space during the time it vibrates, than when it is at rest. A rapidly revolving wheel appears almost solid on account of the combined images of the spokes seeming to unite into one homogeneous mass.
The persistence of luminous impressions upon the retina has given rise to the invention of a number of well-known optical toys, amongst which may be mentioned the phenakistiscope, the thaumatrope, the phantascope, and many others.
​The phenakistiscope may be described (figs. 3 and 4) as consisting of an iron pin a b turning easily on its axis, and passing through two holes in a brass rod t g,
Fig. 3.—The Phenakistiscope. bent twice at right angles. Attached to one end of the pin is a disc of cardboard, divided into several equal sectors, and pierced near its circumference with as many similar sized rectangular holes (fig. 4) In each sector the same scene is represented, with this difference, that the movements of the objects are so arranged as to be progressive from one extreme to the other. The disc being fastened to the pin a b (fig. 3) by the screw v, with the figures facing outwards towards a, the whole apparatus is held before a looking-glass by the handle m. If the disc be now rotated by the button b, and the eye placed opposite one of the square holes in the card, the figures on the disc will appear to move more or less quickly according to the rate at which it is rotated. The three bricklayers in fig. 4 will be seen to pass their bricks from one to the other with perfect regularity if the drawing has been made carefully. Numberless other designs may be made for this little instrument, such as a windmill in full sail, a man working a pump, a conjurer swallowing knives—in fact, any scene with objects in motion may be drawn, and will cause infinite amusement for the long winter evenings.
The time during which the impression of any object remains upon the retina appears to be in direct proportion to its brilliancy. For a burning coal it is stated to be about the tenth of a second; consequently, if the stick mentioned at the beginning of the chapter is rotated ten times in a second, a continuous luminous ​ring will appear to be formed. That the time necessary for producing a distinct impression varies with the brilliancy of the object, may be readily guessed from the fact that an electric spark is perfectly visible, although its duration can hardly be measured, while a cannon-ball in flight is only perceptible to the practised eye of the artilleryman, owing to its reflecting only a small quantity of very diffused light.
Fig. 4.—Disc of the Phenakistiscope.
The second instrument, the thaumatrope, is constructed on the same principle. It consists of a certain number of circular discs of card three or four inches across, which are capable of being turned on their axes ​with great rapidity by means of the finger and thumb and a couple of silk threads fixed at opposite sides of their circumference. On each of these discs a design is painted, one half appearing on one side, and the other half on the other, in such a manner that the two parts form a single picture. You may have, for instance, Harlequin on one side and Columbine on the other, but on turning the card you will see them together. The body of a Turk may be drawn on one side and his head on the other, and, by rotating the card, the head suddenly finds a pair of shoulders to fit it. A sentence may be divided in the same way, or the words, or even the letters, may be divided between the opposite sides of the card: in fact, like the phenakistiscope, the designs applicable to this little instrument are endless.
The third of these instruments, the phantascope, is constructed in accordance with the peculiar power possessed by the eyes of adapting themselves to the distance of the objects they are looking at. Everybody must have noticed that in order to see objects plainly that are placed at different distances we insensibly alter the position and focus of the eyes, and that, consequently, objects even in the same plane as those we are looking at are not perceived by us until something calls our attention to them, and causes us to alter the position and focus of our eyes and fix our gaze on them. For instance, in looking at a canary in a cage, we have but a confused idea of the wires, which we will suppose to be midway between the bird and the observer. But if anything attracts our attention to the wires we lose sight of the bird, or at any rate see it only as a confused mass. If this experiment is made with care, it will be perceived that the object seen confusedly is always double,—a fact that may be verified by interposing the finger between the eyes and any object. When we look at the finger, the distant object will seem ​to be doubled; if we look at the object, it is the finger that undergoes duplication.
We know by experience that when we look at an object and press one of the eyeballs slightly with the finger, the image of it becomes doubled. The explanation of this phenomenon is not very easy, but it is generally supposed that in the case of ordinary vision the two eyes produce the sensation of a single image in consequence of the two impressions being formed at corresponding parts of each retina, and that habit causes us to see only a single object in such a case. But when the eyes are so disposed as to be capable of seeing distant objects distinctly, the two images formed by a near object are no longer found in the corresponding portions of each retina, and so produce the sensation of double vision. The same thing happens when either of the eyes is momentarily displaced.
These phenomena have given rise to the construction of a very simple instrument, the phantascope, with which many interesting experiments may be performed, and which was invented some years since by Dr. Lake, an eminent physician of New York.
In the middle of one of the edges of a thin piece of wood, say six inches or a foot in length, which serves as a base for the instrument, is fixed a rod fourteen or sixteen inches long, upon which slide a couple of ferules capable of being fixed at any height by means of thumb-screws. Each of these ferules holds a piece of cardboard five or six inches long, and of any convenient breadth, in a horizontal position. The upper card is pierced in a longitudinal direction with a slit rather less than a quarter of an inch broad, and about three inches long; that is to say, a little wider than the distance between the centres of two eyes. The second card has a similar slit of the same length, and corresponding vertically with the one above it; the width, however, in ​this instance being only about the eighth of an inch. In addition, the lower card should be marked with a fine line drawn across the centre, which we shall call the index.
Things being thus arranged, if we place two similar objects—two A's, for instance—upon the wooden stage of the instrument, about three inches apart, and look at them through the two slits, we shall see them as under ordinary circumstances; but on fixing our eyes intently on the index of the lower card, and gradually raising it, we shall see the two A's become double, the two images of each letter separating themselves more and more the nearer the lower card approaches the upper one, until the last two of the images will coalesce, and appear to be placed on the lower cardboard, the other two remaining in their proper place. The eyes must be kept firmly fixed upon the index, otherwise the illusion disappears immediately, and two A's only are seen in their true position on the base of the instrument. This is an instance of the production of an image in a place where it certainly does not exist. This illusion is seen best when the upper screen is about ten inches from the object, the lower screen being just half-way between; but, as in most of these cases, the distances will differ according to the focus of the observer's eyes. The proper distances once being found, the experiment may be varied in a hundred different ways. For example, instead of two letters and a line we may have two flowers on the stage, and the figure of a flower-pot on the intermediate screen. If the two flowers are painted different colours, they will unite and form a mixed tint. Thus a red and yellow flower will give an orange image, a blue and yellow a green image, and so on. A perpendicular stroke and a horizontal one will give a cross. A few experiments with this little instrument will throw a light upon many of the obscurer ​points that exist amongst the phenomena of vision, and will show conclusively that the two eyes rarely see in the same manner, and that it is sometimes one, and sometimes the other, that sees most distinctly. A couple of pieces of cardboard, pierced with suitable slits and held in the hand may be substituted for the apparatus above described, but of course they will be much more difficult to use, and will give less satisfactory results.