# Popular Science Monthly/Volume 40/April 1892/Science and Fine Art I

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* SCIENCE AND FINE ART.
By EMIL DU BOIS-REYMOND.
I.
WHEN we represent to ourselves the mental stature of the extraordinary man in whose honor we meet every year on this day, we are ever anew astonished at the boundless breadth of his view and the almost endless diversity of the subjects in which he was interested. It appears hardly comprehensible that the state paper which adjudged the principality of Neufchâtel to the King of Prussia came from the same pen as the Protogea, the Analysis of the Infinite, and the true measure of force from the same head as the pre-established harmony and the Theodicy. Yet on closer examination a gap is revealed in this picture which at first sight appeared all-inclusive. Aside from the Latin poem in which Leibnitz extravagantly glorifies Brand's discovery of phosphorus, we seek in vain for any relation of our hero to art. That his Ars combinatoria had nothing to do with fine art does not need to be said. Only occasionally and rarely do we meet in his writings and letters remarks on art and the beautiful. Once he permits himself to be heard at length on the pleasure we receive from music, the causes of which he seeks in a uniform though invisible order in the movements of the trembling strings "which . . . produces in us . . . a harmonious resonance, by which our vital spirits are also moved." But the world of feeling was only dimly visible to Leibnitz. He saw the Alps and the Italian art treasures with his eyes, but was, as we now say, soul-blind. The same lack of appreciation of fine art is seen in Voltaire, who was comparable for his various learning with Leibnitz; and we have to come down to a third generation, to Diderot in France, and Winckelmann and Lessing in Germany, to find decisive interest in fine art and appreciation of its position in the culture-life of the people.
The period thus defined was, aside from a few phenomenal examples, one of decline in art, while it was one of the most famous in science. When we regard the historical development of these two lines of human activity, we find no conformity in their courses. During the highest bloom of Grecian plastic art there was hardly any science. At the beginning of the art period which we are accustomed to call the cinque-cento stands out the giant figure of ​Leonardo da Vinci, who, in addition to his immortal art-creations, was a physicist of high rank, yet he was as such so far ahead of his time that the example can not be cited as evidence that the rise of science conditions also the rise of art. Michael Angelo died on the same day that Galileo was born. In the common eminence of art and science at the beginning of this century we see only a coincidence. Art has since then continued at best at the same height, while science is still careering on its course of irrepressible victory.
The two lines are in fact so different that it is easily to be seen that science can help art and art science only externally. "Nature," said Goethe, addressing Eckermann, without perceiving how sharply his words might be applied to a side of his own scientific efforts—"Nature knows no pastime; she is always true, always earnest, always severe; she is always right, and faults and mistakes are always man's." In order adequately to perceive the correctness of this expression, one must be accustomed, when he applies his own hand to work as an experimenter or observer, to look into the inexorable face of Nature, and, we might almost say, to take upon himself the immense responsibility that is involved in the determination of even the most insignificant fact. What happens at this moment, under these circumstances, will also happen, under the same circumstances, for a negatively endless time, and will likewise happen after a positively endless time; this is the pregnant significance of every rightly interpreted experiment. Only the mathematician, whose work is more nearly allied to that of the experimental investigator than we are used to conceive, can oppose eternally inviolable laws to the same feeling of responsibility. Sworn witnesses before the tribunal of reality, they both strive after knowledge of the world as it is, within the limits imposed upon us by the nature of our intellect. For this painful pressure under which he labors, the investigator is compensated by the knowledge that even the least of his achievements is a step forward above the highest stage reached by his greatest predecessors; that it may contain the germ of immensely important theoretical knowledge and practical achievements, as Wollaston's lines in the spectrum contained the germ of spectrum analysis; that such a prize invites not only the genius raised up by Nature, but also the conscientious industry of the moderately gifted; and that science, bestowing upon the human mind the mastery of Nature, is the ruling organ of civilization: that without it there never has been a true civilization; and that without it civilization, together with art and its works, might any day sink again hopelessly, as they did on the extinction of the ancient world.
​The investigator can also be comforted with the knowledge that a thoughtless multitude enjoying the benefits conferred upon it by him, hardly knows to whom it owes them; that while the name of every musical virtuoso is in all mouths, and is certain of immortality in the Conversations-Lexicons of the fashionable classes, the name is substantially unknown among us of him who achieved that supreme triumph of inventive genius of making perceptible, through a copper wire stretched over wide regions and over mountain and valley, the sound of a voice as though it was speaking into our ears. "Knowledge is earnest, art is happy," we might paraphrase the poet's expression, without lessening its applicability. Art is the empire of the beautiful; of the creation of that which inspires in us a semi-sensational, semi-spiritual pleasure; and saying this we also say that it is in its widest scope an empire of freedom. In it rule no stiff laws; no strict causality binds the events of the present to those of the past and of the future; no standard unconditionally warrants success. The changing taste of times, peoples, and men assumes to praise and blame, as when the magnificence of Gothic church architecture became the sport of the eighteenth century. Here the definition of genius as the talent for patience goes to the ground; its happy inspiration produces a picture that seizes us and lifts us up with an elementary power which seems to mock the profound interpretation subsequently imposed upon it by art criticism; and the favored hand which perfects it is also a benefactor of care-laden manhood. It unfortunately lies in the nature of things that such force is not developed in every age. Here at one time the highest development is attained in some one direction, in trying to reach which again generation after generation despondently exhausts itself. The finest art theories can neither lift the individual over the limits of his natural ability; nor in the great whole prepare a better destiny for a declining art period. Of what profit is the discussion concerning idealism and realism which has divided the art world for a considerable time? Has it protected us against the hardly tolerable excesses of the latter? Seek for something new; the bold raising of a standard which the untaught multitude blindly follows, will bear the victory, till the antiquated is in some way supplemented by the fresh, or till a personality of commanding altitude unquestionably achieves the mastery.
Still less can pure science help art; and thus, intrinsically alien to one another, without either materially influencing the other, they go each its own way—the one steadily rising, sometimes more rapidly, sometimes more slowly, the other rising and falling in majestic waves. To desire to stamp one of the two, art alone, as the mark of the highest development of the power of the human mind, as not rarely occurs to persons unfamiliar with ​science, is undoubtedly a mistake; but the human mind really shines the clearest when the luster of art is joined with the luster of science.
Furthermore, the same takes place here as in practical ethics. The lower the morals of an age or a people have sunk, the more talk there is about virtue. The more the native creative strength subsides and is dried up, the higher rises the flood of aesthetic theories. Hermann Lotze's History of Æsthetics in Germany affords a wearisome and discouraging picture of this long and fruitless movement. The philosophers of all schools have outbid one another in framing abstract formulas for determining exactly what beauty is. It is unity in diversity, or fitness without purpose, or unconscious rationality, or the absolute in sensual existence, or the enjoyed harmony of the absolute spirit, and more of the same kind. But between these qualities ascribed to all beauty nominally constituting its essentials, and the perception of the beautiful, there is no more connection than there is between the ether and sonorous vibrations and the qualities made known to us by them. It would indeed be a vain undertaking to conceive an expression which shall equally cover the various kinds of beauty; the beauty of the Cosmos in contrast to chaos, of a mountain view, of a symphony, of a poetical work, of Ristori as Medea, of a rose; or in fine art alone, the beauty of the Cologne Cathedral, of the Hermes, of the Sistine Madonna, of a genre picture, of a landscape, of a picture of still life, or of a Japanese vine-weaving. We prefer to say that we in this as in many other points meet something in our organism that is inexplicable, something inexpressible, but something none the less certainly felt for all that, without which life would pass away grimly bare.
There is in Schiller's works a discussion concerning the beauty of the human body. He distinguishes between an architectonic beauty and one that depends upon grace. Twenty years ago on Leibnitz's day, in an address on Leibnitz's ideas in later science, I attacked the rationalism in aesthetics in which the past century had been much entangled, and I ventured among other things the remark that "as little as for the effect of melody is an explanation conceivable of the charm which handsome forms of one sex have for the other." We can not in fact discern in close consideration, why this form which, according to Fechner, can be represented by a plain equation between three variables, should please us more than a thousand other possibilities. It can not be ​derived from any abstract principle or architectonics or Hogarthian wave-line. A year after my remark, appeared Charles Darwin's Descent of Man, in which the doctrine of sexual selection, which was only indicated in the Origin of Species, was treated in detail and followed out to its consequences. But I have, too, a lively recollection of how Dove, when I was once contending with him against the validity of vitalism, embarrassed me with the objection that profusion prevails in organic nature, as, for example, in the feathers of a peacock, or of a bird-of-paradise, while Maupertuis's law of the least action excludes such waste in inorganic nature. The problem seems to be solved now, under the presumption that a kind of sense of beauty in their species exists among animals. The brightly colored wedding garment of the male bird may have originated in the females giving the preference to the most highly decorated suitor, under which an ever more richly adorned posterity is developed. The male birds-of-paradise may be seen at pairing-time emulously displaying their beauty before the female. The nightingale's gift of song may likewise be accounted for if, instead of pleasure in colored feathers, we ascribe musical perceptions to the females. Darwin carries his idea further, to the extent of assuming that certain sexual marks in the human race, the grave beard of the man and the luxuriant hair of the woman, may have been derived through sexual selection. It is well known that the introduction of handsome Circassian slaves into the harems of prominent Turks has repeatedly changed the original Mongolian type into a figure of nobler pattern. Rising to a higher level, we can now find in the same idea the answer to the question. Where are the roots of the charm which female beauty exercises on man? According to our views, the woman was not made out of a rib of the first man, an assumption which encounters morphological difficulties, but it was the man himself who in the course of numerous generations made his woman by natural selection of such fashion as would please him, and, inversely, the woman her man. "We now call this type ​beautiful; but it is only necessary to cast a glance upon a Venus by Rubens or Titian, and to think of the many races of men, to recognize how little even this beauty is absolute.
An instance in which beauty seems to have allowed itself to be dismembered to the best advantage is afforded by the beauty that might be called mechanical. It is the least considered, because a peculiar training of the eye is required for its estimation. It is the beauty which a machine or a physical instrument can possess, of which every part has the right measure, the right form and position for its perfection. The definition, unconscious rationality, fits it well, for in this case the pleasure can with full right be traced back to the fact that we, by sufficient training, can unconsciously perceive how exactly that which was necessary has been done to connect firmness with lightness and as much mobility as is required, in order to obtain the most advantageous transference of force without useless expenditure of material. A driving-belt, it is true, looks neither beautiful nor unbeautiful; but since the strength of a connecting-rod needs to be greatest in the middle of its length, it pleases the educated vision to see it gradually swelling out from the ends to the middle. This kind of beauty is of course of most recent origin; and it should be remarked that it was, so far as I know, first perceived and raised to a principle in the making of our physical instruments in Germany by Georg von Reichenbach in Munich. At a time when instruments of perfect mechanical beauty were turned out of the shops of Munich and Berlin, there came to us from France and England only those on which stiff columns and fantastically ornamented cornices gave disagreeable reminders of the impure forms in the architecture and furniture of the Rococo.
I do not recollect what French mathematician in the last century endeavored to account for the impression of perfect satisfaction to the eye which the view of the cupola of St. Peter's in Rome produced. He measured the curves of the cupola, and found that their form was precisely that which under the given conditions afforded, by the rules of the higher statics, the maximum of stability. Thus, unconsciously, guided by a sure instinct, Michael Angelo solved in his model (the cupola was not built till after his death) a problem which was hardly comprehensible to his consciousness, and which had never, in his time, been mathematically discussed. The equation of beauty, if we may call it that, appears, moreover, in this case, to have had several roots; for there is at least one other form of cupola, of which that of the Val-de-Grâce in Paris occurs to me as a type, which makes quite as restful an impression, though perhaps not so elevating, as that of Michael Angelo's.
Mechanical beauty comes in here in the building art, and the ​more frequently at this time because the iron constructions of the later period give more occasion than stone structures for its exhibition. The change of material has, according to Anton Kallmann's expression, produced a changed statics of feeling. In the Eiffel Tower mechanical beauty is in conflict with plastic want of beauty, and in this reveals itself for the first time to many who would not otherwise have had occasion to perceive its effect. The new Forth Bridge is certainly not without it. Yet there is no question that even in stone buildings, besides many traditions and conventional tastes, the pleasure in definite forms, in the gentle swelling and tapering of Doric pillars upward, their expansion into the echinus and abacus, and in the profile of the architectural members, depends on mechanical beauty, as well as on the absence from the agreeable impression they make of the repulsive, which the senseless ornamentation of vulgar styles inflicts upon the refined taste.
Mechanical beauty plays a part even in the forms of organic nature, to the degree that much that is repulsive to the untutored eye delights the educated eye and fills it with admiration. That it is which the anatomist is pleased to discern in the structure of the bones, particularly of the joints; which on other grounds than its contradiction of the way the ancients painted death, makes a death-dance appear repulsive to him; which Benvenuto Cellini, to his credit, comprehended in a skeleton; and which, if only our understanding was adequate, every organized form would illustrate to us even in the aquarium and under the microscope. Even in the building up of the plant structure. Dr. Schwendener has demonstrated an economical adaptation of parts, characteristic of the organization, of which we can discern something in the sight of a broadly rooted oak pushing its massive head up toward air and light.
Mechanical beauty comes into consideration in the contemplation of animal forms, particularly of beasts of prey. A greyhound and a bull-dog, a thoroughbred race-horse and a brewer's draft-horse, a South Down and a merino sheep, an Algau Mountain steer and a Dutch milch-cow, are all handsome, though some among them, like the bull-dog and the Percheron horse, may appear ugly to a stranger; for in all of them the type of the species is modified for some adaptation.
Although science can not, as we have seen, inspire art in its departing life, nor communicate a new impulse to it, it can still afford it an inestimable service of another kind, by increasing its insight and improving its technical means, teaching it useful rules, and guarding it against errors. We are not thinking here ​of a thing so primitive as the preparation of pigments or of certain knacks in casting; the less so because, as is well known, our colors are in a remarkable way poorer than those of an unscientific primitive time, and an unexcelled thinness of the metal is the mark of a genuine Greek bronze statue. It can hardly be necessary to recollect the long story of the benefits of this kind which scientific knowledge has conferred upon art. Linear perspective was discovered by the artists themselves, by Leonardo and Dürer. The laws of reflection and shadow-construction, which were still unknown to the ancient painters, if we may judge from the Pompeiian Narcissus pictures, followed. In the representation of the rainbow, which had better be left unpainted, many and serious mistakes have been made, notwithstanding the teachings of optics. Statics furnished the sculptor important instruction concerning what is called ponderation. Aërial perspective owes its development, again, to the painters, particularly to those of northern lands.
The advance of science has added to those ancient helps much of importance, although it is not so fundamental, and many naturalists, among them some of the first rank, have interested themselves in making the new knowledge accessible to artists. The great masters of past centuries were guided by their feelings to the proper selection of colors, as, according to Johannes Müller, women of taste of all times are correct in the choice of their clothing; and the Oriental carpet-weavers are not behind them. But the significance of such unconscious success could be perceived only after the subjective physiology of the sense of sight had been created by the older Darwin, Goethe, Purkinje, Johannes Müller, and others. These matters have been discussed by our fellow member, Herr Ernst von Brücke, in his Physiology of Colors for industrial art, and his Fragments from the Theory of the Fine Arts, with such special skill as only the rare combination of the artistic culture acquired in his father's studio with his own physiological knowledge could make possible. Chevreul pursued similar aims in France. Not less did Prof, von Helmholtz embody his profound knowledge of physiological optics in public lectures in the service of art, which owes to him likewise his fruitful conclusions concerning the nature of musical harmony. He explained among other things the relation in which differences of luster of real objects stand to those which the painter controls, and showed what means he could employ to overcome the difficulties growing out of them. By imitation of the irradiation recognized by him in ​its true meaning—a fault of our vision—the painter is even placed in a position to reproduce the dazzling impression of the solar disk. Of this the Castle Gandolfo of Roqueplan in the Raczynski Gallery, through its boldness, affords an interesting example.
The representation of the stars as stars, in the shape in which the stars of decorations are drawn and from the resemblance to which star-fishes are named, rests likewise upon defects of our vision; for the stars of the sky are only shining points without rays, as indeed a few favored eyes see them. The sacred halo, the phosphorescence of holy heads, which in Correggio's Night extends over the whole Christ-child, and objectively illuminates the scene, has nothing to do with this. The origin of that kind of representation, so far as it is not a free sport of fancy, is possibly traced by Herr Exner to the crown of light which one sees in a dewy field in sunshine around the shadow of his own head. By another defect of the human eye, astigmatism, the more advanced grades of which, such as short-sightedness, belong to pathology, Herr Richard Liebreich was able to explain certain peculiarities long incomprehensible, which disfigured the later works of the distinguished English landscape-painter, Turner. It would have been easy for a modern oculist to protect him from this fault by properly fitted glasses. Color-blindness, known of old, but thoroughly studied only in our own age, is another very frequent defect of our vision, to which corresponds, in the ear, an inability to distinguish between the tones. A color-blind painter is perhaps not so inconceivable as a musician without hearing.
It might not be practicable to define the limits beyond which optical science can do no more good to the artist. In order to know the laws of the movements of the eyes, to understand wherein close vision is different from far vision, no painter will have reason to regret applying to himself Johannes Müller's remarks in his early paper on the Comparative Physiology of the Sense of Sight. Yet it must be granted that an artist could paint an eye very well without ever having heard of the Sansonian images, on which depends the soft glance of a mild eye as well as the wild fire of an angry, penetrating eye; just as the landscape-painter would paint the blue sky on his canvas no better if he had learned to take note of the yellow brush in every great circle of the heavenly sphere that passes through the sun, which continued unremarked through thousands of years, but has been familiar to physiologists since Haidinger's discovery.
​In the much-debated question of the polychromy of the ancient statues and buildings, on the contrary, and of the propriety of adopting it, one observation of the physicists, as appears to me, has not hitherto been sufficiently considered. It is that all colors become whitish under a very strong illumination, so that, on the immediate view of the solar spectrum in the telescope, nearly every impression of color disappears, except for a light-yellow shimmer at the red end. As the colors become whitish, their glaring contrast disappears, and they blend more harmoniously into one another. Therefore, under a clear sky, the fiery red petticoat of the Contadina, which is repeated so often in Oswald Achenbach's Campagna pictures, as well as the white horse of Wouverman's war-scenes, make no disagreeable impression on the eye. Under the bright Grecian sky, on the Acropolis, in the Poikile, the more or less glaringly painted façades and pillars still had a pleasant effect; in the gray northern light, and in closed rooms, they are not happily introduced.
Wheatstone has materially enriched the capacity of drawing and painting art from another side, by showing with his stereoscope the different manner in which binocular vision distinguishes nearer objects from monocular vision, and also from the binocular vision of objects so remote that the interval between the eyes vanishes before their distance. The impression of a solid body arises only when each of the eyes receives a different view of the object, and is produced by both views blending into one, the corporeal view. Therefore the painter, expressing dimensions of depth only through shading and air-perspective, has never been able to produce a real corporeal appearance on his canvas. While, then, Wheatstone's pseudoscope shows the human face concave in an unusual way, Helmholtz's telestereoscope exaggerates the distance between the eyes, and, without aerial perspective, resolves the far-off forest or mountain into its various elements. The stereoscope with movable pictures, however, confirms old Dr. Robert Smith's explanation of the fact that the moon and sun appear larger by nearly two tenths of their diameters in the horizon than in the zenith, and reduces the problem to the question why we see the vault of the sky rather flattened like a watch-glass than as a hemisphere.
Of vastly greater importance for art is photography, which originated at nearly the same time with the spectroscope. To fasten Delia Porta's charming pictures was indeed a dream of artists as well as of physicists, and after the discovery of chloride of silver the no longer unattainable object came in sight. One ​would need to have witnessed Daguerre's discovery, and Arago's report upon it in the Chamber of Deputies, to realize the enthusiasm with which it filled the world. Daguerre's particular process, of only limited application, was soon cast in the shade by one which in its essentials is still in use. But it deserves, perhaps, to be remembered that when the first still imperfect Talbotype process reached us from England nobody foresaw its immense future, and the substitution for the silver plate of paper impregnated with a salt of silver was received with shaking of heads, and was looked upon as a step backward.
Thus photography started upon its wonderful career of victory. It soon assumed the relation to art that Arago had promised for it. Not only has it lightened the work of the architectural, interior, and landscape painter, and made the camera lucida superfluous even for panoramas; it has also furnished many useful hints relative to light and shadow, reflection and half-tone, and especially as to the way to give the most natural appearance of bodily projection to figures on a flat. It might be profitable, for the sake of forming a judgment in both directions, to inquire what part photography has had in the origin of the newer schools of painting, of the mannerism of the impressionists, and of the clear-light and free-light painters. It has taught the landscape painter how to reproduce rocks with geological and vegetation with botanical correctness, and to represent glaciers, which was rarely attempted before, and never successfully. It fixed the image of the clouds, although its. pictures of the sky were somewhat defective. Finally, it helped the portrait painter without exciting his envy, for, while it caught up only a single often long-while tense expression, it was not adequate to give an average picture of the man, and the unpleasant, stiff photograph was almost proverbially a bad portrait. It furnished painters, however, in many instances with an invaluable groundwork, although it had to be enlivened by the artistic touch. But the newer form of portrait photography is calculated to attract the attention of the artist in many points. Instantaneous photography catches the expression of the countenance and the attitudes during so short an interval that it makes good what escapes in the average expression, and thus leads to most valuable observations. Duchenne and Darwin recreated the doctrine of expression in emotion; the former by counterfeiting the various expressions by means of electrical stimulation of the muscles of the face, and the latter by following their phylogenetic development through the series of animals. Both presented the artist with photographic images of such expressions by the side of which ​the drawings used in the art schools for the same purpose appear antiquated. Since then the English anthropologist, Mr. Francis Galton, has solved by photography a problem which was as much beyond the reach of the artist as the representation of the average expression of a person was of the photographer—namely, of collecting into a typical picture the average physiognomy and shape of the head of a considerable number of persons of the same age, race, like degree of mental development, or similar pathological condition or criminal propensity. This is done by causing faint pictures of faces of the same category to cover one another on the same negative. Prof. Bowditch, of the Harvard Medical School, has in this way taken average (composite) pictures or the types of American students and girl students, drivers and conductors of horse-cars. In the last cases the superiority of the intellectual expression of the conductor type over that of the driver type is very plain. It would have been something for Lavater and Gall.
Again, pathology comes into the service of fine art. Dr. Charcot has recognized, in the photographically fixed convulsive attitudes and distortions of hysterics, the classical representations of possessed persons. It is indeed most wonderful to see how Raphael, otherwise dwelling only in the ideal, portrayed in his Transfiguration the figure of the possessed boy so realistically that one can with certainty, from the Magendian position of his eyes, diagnosticate a central disease. It is in harmony with this, as was recently remarked in New York, that his left hand is afflicted with an athetoid cramp.
[To be concluded.]
* ↑ Address on Leibnitz Commemoration-day in the Academy of Sciences at Berlin, July 3, 1890.
* ↑ Die philosophischen Schriften von G. W. Leibnitz. Published by C. J. Gerhardt. Vol. ii, p. 87. Berlin, 1890.
* ↑ Gespräche mit Goethe, etc. Leipsic, 1836, vol. ii, p. 68 (1829).
* ↑ The seventh volume of Die Geschichte der Wissenschaften in Deutschland. Neuere Zeit. Munich, 1888.
* ↑ In the essay on Anmuth und Würde.
* ↑ Reden, etc., vol. i, Leipsic, 1886, pp. 49, 50.
* ↑ The Descent of Man and Selection in Relation to Sex. London, 1871, vol. n, pp. 52, 89, 379, 400, 401. In his book on Darwinism, etc (second London edition, 1889), Mr. Wallace rejected the explanation of the decorative plumage and the song of the male bird through selection by the female, and proposed other interpretations. But a writer recognized by Mr. Wallace himself as equally a student in this line, Mr. C. B. Poulton, in his work, The Colors of Animals, their Meaning and their Use (International Scientific Series), has sturdily taken up the defense of the Darwinian view against this attack, and exposed the untenability of Wallace's later explanation. Mr. Wallace has not failed to reply to this (Nature, No. 1082, vol. xlii, July 24, 1890); while Mr. R. J. Pocock, resting on Mr. G. W. Peckham's investigations, joins Mr. Poulton (ibid. . No. 1086, August 1, 1890, p. 40.'5). This is not the place to enter into the question, especially as my conclusion concerning the doctrine of sexual selection still holds, even if Mr. Wallace should be right on the single points of feather ornament and song.
* ↑ Kunstbestrebungen der Gegenwart. Berlin, 1842, p. 71.
* ↑ Handbuch der Physiologie des Menschen, etc. Vol. ii, Part II, Coblentz, 1838, p. 375.
* ↑ Physiologie der Farben, etc. First edition, Leipsic, 1866; second edition, 1887, Bruchstücke, etc., Leipsic, 1877.
* ↑ Optisches ueber Malerei. Vorträge und Reden, vol. i, Brunswick, 1884. Concerning irradiation compare the Handbuch der physiologischen Optic, second edition. 5. Lieferung. Hamburg and Leipsic, 1889, pp. 394 et seq.
* ↑ Physiologisches und pathologisches in den bildenden Künsten. Vienna, 1889, p. 17.
* ↑ Proceedings of the Royal Institution, etc., weekly evening meeting, Friday, March 6, 1872.
* ↑ Helmholtz, Handbuch der physiologischen Optic, A. a O, Fourth Part, 1887, pp. 284, 285.
* ↑ The Expression of the Emotions in Man and Animals. London, 1872.
* ↑ Inquiries into Human Faculty and its Development, with Illustrations. London, 1883.
* ↑ Compare Exner, a. a. O. S. 21 et seq.
* ↑ Sachs and Peterson, A Study of Cerebral Palsies, etc. Journal of Nervous and Mental Disease, May, 1890.