baldus' in the church of that dedication at Nuremberg; and the Tomb of Eitel-Friedrich II. von Zollern and of his wife in the church at Hechingen'; etc.

Vischer, vish'er, William Lightfoot, American author and actor: b. Owingsville, Ky., 25 Nov. 1842. He was graduated from the law denever practised, subsequently engaged in jourpartment of the University of Louisville, but nalism and is now (1904) on the stage. He has published several novels, among which are Way Out Yonder'; and Peter Vansant'; 'Blue Grass Ballads'; etc.

Visconti, vis-kon'tě or vēs-kōn'tē, an old Milanese family, celebrated for its political consequence and its patronage of science. History makes mention of the Visconti in the 11th century; but they disappear from the time of the destruction of Milan by Frederick Barbarossa, when, with some other noble families, they were obliged to yield to the superior power of the opposite party, the Torriani or family Della Torre. The first of the Visconti who laid the foundation of their greatness was OTTONE: b. 1208; d. 1295, created archbishop of Milan in 1263, and perpetual lord of Milan in 1277, who gained the ascendency over his enemies, and bequeathed his power to his nephew, MATTEO: b. 1250; d. 1322. The latter was, however, driven into banishment by the Torriani, but after living in exile seven years, had the address to obtain the title of imperial vicar 1294, which he soon exchanged for that of lord-general of Milan (1311). Matteo transmitted the supreme power to his eldest son, GALEAZZO: b. 1277; d. 6 Aug. 1328, who was overpowered by his enemies and thrown into prison by Louis of Bavaria in 1327, but was soon after released. His son Azzo: b. 1302; d. 14 Aug. 1329, who succeeded him and increased the extent of his dominions, acquiring nearly all of Lombardy, was not less distinguished for his pacific virtues than for his military talents. His uncle LUCCHINO: b. about 1287; d. 24 Jan. 1349, succeeded him. The latter extended the dominions of the family, and was the first of the name who was distinguished as a

patron of science and art. He corresponded with Petrarch. After his death in 1349 his brother, GIOVANNI: b. 1290; d. 5 Oct. 1354, archbishop of Milan, assumed the reins of government. He was a zealous patron of letters. He appointed a commission of six learned men to compose a commentary on Dante, fostered the University of Bologna, and received Petrarch on his arrival at Milan with the highest marks of distinction. Giovanni was succeeded by his nephews, MATTEO II., d. 1355; BARNABO, d. 1385; and GALEAZZO II., d. 1378. Matteo's two brothers, who shared their estates on his death, though eminent for their warlike talents, rendered themselves obnoxious by their cruelty and other vices. Galeazzo, however, continued to treat Petrarch with the same respect that his predecessors had shown him, and employed him in several negotiations. He was succeeded in 1378 by his son, GIAN GALEAZZo: b. about 1347; d. 3 Sept. 1402, who imprisoned his uncle Barnabo in the castle of Trezzo, and took upon himself the sole government (1385). In him the Visconti family reached the summit of its grandeur and splendor. In 1395 he received from the Emperor Wenceslas the ducal dignity; and his territories were more extensive than

those of any of his predecessors. Pisa, Siena, Perugia, Padua, and Bologna were subject to his sceptre; and he had already shown a disposition to assume the title of king of Italy, when his ambitious projects were cut short by his death of the plague. He fostered science and art, collected the most distinguished scholars at founded a large library. He built the celebrated his court, restored the University of Piacenza, and connected that of Pavia with it, and bridge over the Ticino at Pavia, and began the magnificent cathedral at Milan. Gian Galeazzo left three sons, GIAMMARIA; d. 16 May 1412; FILIPPO MARIA; d. 13 Aug. 1447, and an illegitimate child, GABRIEL; d. September 1408. Giammaria succeeded to the dukedom, and was assassinated, after which Filippo Maria reigned alone till his death. His natural daughter, Bianca, had been married to Francesco Sforza, who was named duke of Milan in 1450. Consult: Sismondi, 'Histoire des républiques italiennes? (1826-33); Symonds, 'The Age of the Despots' (1875).

Visconti, Ennio Quirino, Italian archæologist: b. Rome 1751; d. Paris 7 Feb. 1818. He was the son of Giovanni Battista (q.v.), and from his earliest years was trained in the habits of an antiquary. He continued the work begun by his father, entitled 'Il Museo Pio Clementino descritto, of which he wrote the greater part and which extended to seven volumes, the last appearing in 1807. In 1785 he was appointed keeper of the Capitoline Museum. In 1798 he was one of the five consuls of the Roman Republic, and during a consulate of seven months founded the Roman Institute. In 1799 he left Italy and settled in Paris, having been appointed an administrator of the museum of the Louvre, and professor of archæology. His archæological works are very numerous, and among the most popular are 'Description des Antiques du Musée Royal'; 'Description des Vases peints du Musée'; 'Iconographie Grecque'; and 'Iconographie Romaine.'

Visconti, Louis Julius Joachim, French architect: b. Rome 11 Feb. 1791; d. Paris 1 Dec. at Paris under Percier and Fontaine, and at 17 1853. His architectural studies were prosecuted he entered the Ecole des Beaux Arts. In 1817 Wine Market; in 1822 inspector of building to he was superintendent of building on the Paris the city government; and 1825 architect of the great library of Paris. His most important works are the tomb of Napoleon I., under the dome of the Invalides; and the additions to the Louvre, which latter he did not live to complete

Visconti-Venosta, vēs - kōn'tē - vā - nos'tä, Emilio Morquis, Italian statesman: b. Milan 22 Jan. 1829. In 1846 he entered journalism as a liberal and for a time was an adherent of Mazzini. In 1859 he was appointed by Cavour royal commissioner at Garibaldi's headquarters in Lombardy, was elected to the chamber of deputies in 1860 and was minister of foreign affairs 1863-4, 1866-7, and 1869-76. He became senator in 1886, was minister of foreign affairs for a fourth time 1896-8, and in the Pelloux cabinet held the same post 1899-1900, as also in that of Saracco, 1900-1. His foreign policy was characterized by an ardent desire to retain the good will of France.

Viscosity, is that property of matter by virtue of which resistance to change of shape

VISCOUNT

depends on the rapidity of the change. All substances, ranging from gases to solids, are supposed to possess this property in a greater or less degree. A noteworthy instance of viscosity is exhibited by sealing wax; for while it is quite rigid in resisting forces quickly applied, it will change shape greatly under the action of a small force (its own weight, for example) applied continuously for a long time. Molasses, tar, asphalt, and many other substances, also illustrate this property in a striking way; while water, alcohol, air, and other liquids and gases, as shown by suitable experiments, possess the same property in a less degree. The viscosity of a medium is measured by the quotient of the tangential stress developed along any plane in the medium, by relative motion of its parts, divided by the rate at which the velocity of the medium is changing with distance perpendicular to that plane. More briefly, this measure, which is commonly called the coefficient of viscosity, may be defined as the quotient of the resultant tangential stress at any point of the medium divided by the resultant angular velocity of the medium at the same point.

Viscous Fermentation, an undesirable form of fermentation that sometimes accompanies alcoholic or acetic fermentation. It yields a gum-like ropy substance that injures the fermenting mass.

In a

Vishnu, vish'noo ("the Preserver"), the second god of the Hindu triad, and by his special worshippers considered to be the greatest. The development of the Hindu religion is naturally divided into three periods. In the first, or Vedic period, Vishnu is one of the gods who represent the various qualities of the sun. few hymns he is specially distinguished, but in general he is classed without distinction with gods who were regarded as inferior to Indra, and fewer hymns are dedicated to him than to some others. In the second or epic period Vishnu is elevated to the rank of first of the Adityas or luminous offspring of Aditi (space). Of the two great epics-the Rāmāyaṇa and the Mahabharata the former is entirely devoted to the glory of Vishnu, one of whose avatars it celebrates the latter is divided between him and Siva, the third god of the triad, whose worship during its period began to prevail over that of Vishnu. In the third period, which is represented by the Puranas, the worshippers of Vishnu and Siva are divided into different sects, and their respective creeds are propagated in separate Puranas. The distinctive characteristic of the worship of Vishnu is his avatars or incarnations, which greatly excel in number and importance those of Siva or any other god. The number of the avatars of Vishnu varies in different authorities. Sometimes they are regarded as almost innumerable; but there are ten principal avatars distinguished by specific names. Some of these avatars appear to have had at first a purely cosmical character and others to have been founded on great historical movements of

- VISIGOTHS

a religious nature. sented as becoming incarnate to oppose the power of the demons, restore the authority of the gods, or otherwise to effect some beneficial change in the government of the universe. The last avatar is yet future. See INDIA, Religions; VAISHNAVAS; VEDAS.

Vishnu is always repre

Vishnu, Institutes of. A work of standard authority in the religious faith and practice of the Hindus. In its recognized and partly modern form it consists of 100 chapters of legal precepts and aphorisms, put under the name of Vishnu, originating from the early Vedic school called the Cathas, but much added to in subsequent times. See INDIA; SANSKRIT LANGUAGE AND LITERATURE; VEDAS; VISHNU.

Visible Church, an ecclesiastical term

meaning the Church, as seen by man, not as it appears to God. It includes the whole body of professing Christians, some of them regenerate, others unregenerate, as distinguished from the Invisible Church, consisting of all those who are worthy in the sight of God to be members of

His Church.

Visible Speech, the name given by its inventor, Alex. Melville Bell, to a system of alpossible articulate utterance of the human organs phabetical characters designed to represent every of speech. It is based on an exhaustive classification of the possible actions of the different organs concerned in speech, and to every organ and every mode of action of an organ a symbol is assigned. Of these radical symbols there are in all 30, and by combinations of these 30 symbols some hundreds of characters may be formed to represent the sounds of the human voice, but little more than 100 characters are actually required for all the ascertained sounds of different languages, and only two thirds of the 30 radical symbols are required in forming the signs or letters necessary for the writing of any European language. In form the letters are as simple as those of the Roman alphabet. After completing this system, Mr. Bell submitted it to the consideration of a

large number of gentlemen well qualified to judge of its value, and allowed it to be tested in their presence in such a manner as to make quite clear its efficiency for the purposes for which it was devised. See DEAF-MUTES, DEFECTIVES, EDUCATION of.

Visigoths, viz'i-goths, or West Goths, the western branch of the Goths (q.v.), who after the death of Theodosius broke into Italy under Alaric, and captured Rome in 410. Alaric died later in the same year before he had matured his plans, and after his death his brother-in-law Athaulf, who was placed at the head of the nation, turning toward Gaul, made new conquests on both sides of the Pyrenees. He reached Barcelona, where he was murdered in 415; but his successors continued to extend their dominions in Gaul and Spain. Wallia made a treaty with the Romans, and in return for certain services received under their nominal suzerainty, though in virtual independence, western Aquitania, with Toulouse as capital. He died in 419. His successor, Theodoric I. (419–451), was treated by the Romans as an independent ruler, and took part in the victory over Attila on the Catalaunian Fields in 451. Euric (466-483), third in succession to Theodoric, conquered the Suevi and other races, and is considered the

founder of the Visigothic kingdom. Clovis, king of the Franks, on pretense that it was unjust to let the heretic Visigoths possess the fairest portion of Gaul, attacked the peaceful Alaric, Euric's successor, and defeated him at Vouglé in 507. The Franks obtained possession, without resistance, of most of the cities in southern Gaul, and the kingdom of the Visigoths would have been in great danger had not Theodoric, king of the Ostrogoths, undertaken its defense. While guardian of the Visigothic prince, his grandson, he embraced the favorable opportunity to make himself master of a part of the territories still belonging to the Visigoths in southern Gaul; and after a long separation of the two nations there existed for a time an intimate connection of the Ostrogoths and Visigoths. After his death dissensions soon arose among the Visigoths, and the pernicious influence of differences of religion became more and more evident. The kingdom of the Visigoths arose again with new energy under Leovigild (568-586), who totally subdued the Suevi, improved the laws, limited the power of the nobles, made Toledo the royal residence, and tried to render the regal power hereditary. His son Recared adopted, on his accession (586-601), the Catholic faith, upon which the divisions of the people ceased, and Goths and Spaniards became one nation. Under his reign was held the Council of Toledo. His conversion had the most important influence on the character of the government. As soon as the Catholic faith became the established religion, the clergy, who had been completely subservient to the king under the Arian form of Christianity, acquired a predominant influence, and constituted a hierarchy under the direction of the Papal authority. The Arian bish/ops had lived quietly in their dioceses, and had no influence on the public administration; but the Catholic bishops obtained an active participation in public affairs. The grandees of the kingdom, usurping the rights of popular representatives, remained no longer the first class in the state; the mode of choosing the king was altered in favor of the bishops; and under weak kings these found it easy to place themselves at the head of the state and to procure exemption from all public burdens. As early as 633 the regulation was made that those secular grandees alone should be admitted to the councils who should be pronounced worthy by the bishops. Internal disturbances facilitated the conquest of the country by the Saracens, who were settled on the north coast of Africa. In 675 these Mohammedans had begun their attempts to settle in Spain, and during the reign of the weak Roderick were enabled to execute their project. The Goths were defeated in 711 at Xeres de la Frontera; the king was slain, and the Saracens spread themselves over the greatest part of the country. (See SPAIN.) The remainder of the Goths, who, after the downfall of the kingdom, fled to the mountains of Asturia and Galicia, founded there new kingdoms, in which the constitutions of the Visigoths were in part retained, and which, when their descendants broke forth from their fastnesses and wrested from the Moorish settlers one tract after another, finally gave rise to the kingdoms of Spain and Portugal. The traces of the public institutions of the Visigoths were preserved in the laws. The most ancient collection of Spanish laws, the Fuero Juzgo, or Forum Judicum (see FUERO),

is drawn from the ancient laws of the Visigoths; and many of their institutions have been retained to the present day in the provincial laws of Castile and Catalonia. The Spanish language contains a strong admixture of the Gothic element. Consult Hodgkin, Italy and her Invaders' (1880–5).

Vision, the sense of sight. Visual sensations include brightness qualities (blacks, whites, and grays), and color qualities (reds, greens, yellows, purples, etc.). They are produced by the vibratory action upon the eye of the luminiferous ether and, in conjunction with certain other sensations, set up in and about the eye by the stimulation of muscles and tendons, they give rise to the visual perception of objects and processes in the outside world. A descriptive account of vision has to deal (1) with the visual apparatus, both as a dioptric or refracting mechanism, which conveys light-rays to the retina, and as a nervous organ, which transforms the vibratory stimulus into neural excitation and transmits it to the brain; (2) with the sensations of brightness and color as regards their quality, number, classification, etc.; (3) with the specific processes which underlie and condition visual sensation; and (4) with those perceptions of things and events which come to mind through the avenue of sight.

The Visual Apparatus.- This apparatus consists of the eye, the optic nerve, subcortical centres (of the corpora quadrigemina and thalami), and terminal or central areas in the cortex of the occipital lobes. It is generally believed that visual sensations are directly correlated with the functions of the last or cerebral part, only, of the visual mechanism; the office of the eye, the optic nerve, and the lower centres being the transmutation of ether-vibrations, the transmission of nervous impulses to the brain, and the setting up of various movements-such as rotation of the eyes, accommodation, and winking - which are important factors in visual perception.

In the human adult, the eye is a spheroidal mass having an antero-posterior diameter of about one inch. The central part of its front surface (the cornea) possesses a higher convexity than the remaining opaque surface. It is seen from a lateral position as a bulging transparent covering. The eye is invested with three coats and its interior is divided into two chambers; a smaller anterior cavity containing the aqueous humor and a larger posterior cavity filled with the vitreous body. Between the two chambers lie the iris (the colored part of the eye), its circular opening (the pupil), and the crystalline lens.

The function of the refracting mechanism, which includes the cornea, the aqueous and vitreous humors, and the lens, is to focus the light entering the eye, and to project upon the retina a small inverted image of the object seen. The eye is essentially a small camera. It differs, however, from the ordinary photographic camera in adjusting its focus for different distances by changing the convexity of its lensnot by altering the position of its sensitive surface, the retina. The process of increasing the convexity of the crystalline lens is known as accommodation. It is compassed by the reflex action of the ciliary muscles which permits the lens to bulge forward in viewing

near objects and to flatten out in viewing distant objects. The retinal focus is thus maintained. The eye rests in its socket on a cushion of fat, and is turned in its orbit by the joint action of three pairs of antagonistic muscles, the internal and external recti, the superior and inferior recti, and the superior and inferior obliqui. Since the two eyes function as a single organ (binocular vision), it is important that they move together and thus bring the images of an object upon corresponding retinal areas. The turning inward of the eyes in their common fixation of an object is called convergence.

The true nervous end-organs for vision lie in the retina. The retina is a complex structure, of no less than eight strata or layers, which forms the innermost coat for the posterior part of the eye. Within it are the rods and cones, which stand closely connected with the neural elements leading to the brain, and which are probably the seat of those changes that transform the vibrations of the luminiferous ether into the immediate stimulus for nervous excitation. The neural elements (nerve-fibres) which transmit the excitation unite near the centre of the retina, pierce the outer investments of the eye at the porus opticus-called also the "blind spot," because the retina at this point is insensitive to light and continue as the optic nerve. The two optic nerves come into communication below the brain, in the median plane of the body, and form the optic chiasma. Reyond the chiasma there is a second division, a part of the fibres of both nerves passing to each hemisphere of the cerebrum.

Sensations of Color and Brightness.— Visual sensations include the colors seen in the solar spectrum, a series of purples ranging between red and violet, and all the grays, whites, and blacks. Colors proper (that is, excluding mere brightness qualities) form a closed series in which one may pass by small gradations from any quality, as red or green, through every other quality, and arrive finally at the starting point. For this, it is only necessary to join the red and violet ends of the spectrum by inserting the purples. The closed color-series naturally falls into halves. The one half contains the reds, oranges, and yellows, which obviously belong to a single group of qualities, the "warm" colors; the other half, the greens, blues, and violets, the "cold" colors. The line of division falls in the purples on the one side, and in the yellow-greens on the other, both these colors including transitional points between the two types. Within each group, again, may be distinguished two sub-types, the red and yellow types in the one, the blue and green in the other. To each group belong several neighboring qualities which merge gradually into one another. To complete the classification of visual sensations it is necessary to bring the color sensations into relation with brightness (the gray series). That an intimate relation obtains between the two series is shown by the fact that each color has itself a certain brightness; the yellow of the spectrum, for example, has a brightness corresponding to a light gray, while spectral blue corresponds in brightness to a medium gray. Moreover, it is possible to pass gradually from any color to a pure brightness. This transition occurs, for example, in the fading of painted surfaces into gray. In such a case, the richness or saturation of the color is said to be reduced. The relation of color tones

(colors in the narrower sense) to each other, to brightness, and to saturation is represented in the accompanying figure (Fig. 1). The vertical axis (W Blk) of the double pyramid represents the gray series from white to black. The letters around the base (G, Ol, Y, Or, etc.) stand for color tones at maximal saturation. The saturation of any tone is represented by the length of a perpendicular dropped from the point in the base where the color is found upon the gray axis; the brightness of any tone, by the point where this perpendicular strikes the axis. The inclination of the base brings the perpendicular from yellow (Y) higher up the axis than the perpendicular from blue (B). This is in accord with the greater brightness of the former color. The number of pure brightnesses that can be discriminated is estimated to lie between 650 and 800; the number of color tones- reds, greens, violets, etc.- is about 150.

To produce the large number of possible color tones it is not necessary to have recourse to a corresponding number of wave-lengths; W

for, given a small number of tones, properly chosen, it is possible to produce all the others by the process of color-mixture. One of the most common methods of mixing colors is by the use of pigments; another is by rapid rotation of the color-wheel, which carries two or more sectors of colored paper or cardboard. At a certain rate of revolution of the color-wheel the observer ceases to see the separate sectors and sees instead a single homogeneous color which is different from either of the sectors used. The tone of the "mixed" color depends upon the relation of the colors mixed. If two colors stand near each other in the spectrum, the "mixed" color is intermediate; for example, red and yellow mixed give an orange, violet and red, a purple. There is, however, for each color, a "complementary" color which when mixed with it in a certain proportion gives-not a color tone at all, but pure gray or white. It should be noted that, in

-a

every case, it is stimuli that are mixed and not sensations; that the mixture is of physical or physiological processes, not mental processes. A "mixed" color is as simple, psychologically, as any other.

Visual sensation depends not only upon the light stimulus to which the eye is exposed and upon the mixture of stimuli, it depends also upon (a) the part of the retina stimulated, (b) the stimulation of adjacent areas, and (c) certain more or less permanent effects left by the stimulus upon the visual organ.

(a) It is only in the central part of the retina- the part lying about the fovea or spot of clearest vision that all the colors are seen; further out toward the periphery of the retina lies a zone in which reds and greens are not sensed (only blues and yellows), and, still further out, an outermost zone whose stimulation gives rise to brightness sensations, only. That is to say, the normal eye is totally color-blind over a part of its sensitive area and partially color-blind over a second part. Abnormal colorblindness, then, which is relatively common in the race, may be regarded - at least in its commoner forms-as an extension to the fovea of the normal color-blindness of the normal eye.

The eye is a chemical sense; that is, chemical processes are interpolated between the reception by the eye of light-waves and the neural processes that are ultimately set into function by them. Two special results follow from this fact. First, stimulation is not strictly confined to the part of the retina directly affected by light; and, secondly, nervous excitation continues after the external stimulus has ceased to operate. (b) The first of these results, retinal irradiation of chemical or photo-chemical processes, is strikingly demonstrated by the phenomena of contrast. A patch of white looks whiter if it is placed on a black surface; black looks blacker in the neighborhood of white. In a similar manner, the saturation of a color (for example, red) is enhanced if it is brought near a complementary color (blue-green); and, finally, a gray in the immediate neighborhood of a color (for example, green) is tinged, under certain conditions, with the complementary of that color (purplish red). The last case, the induction of a contrast color upon a gray, may be observed in the shadows of snow, which often look blue under yellow sunlight. The general effects, both of brightness and of color contrast, are, moreover, frequently to be seen in clothing and in interior decorations. The fundamental law of contrast is that the contrast effect is always in the direction of the opposite brightness or of the complementary color. (c) The result of continued stimulation of the eye is to change the condition of excitability of that organ. If large colored glasses be worn before the eyes for several minutes the unnatural hue of objects, which is at first very noticeable, gradually disappears. With blue glasses, objects at first look bluish, but finally return to their proper tones. The eyes are said to have become blue-adapted. A similar effect is produced if only a small part-not the whole of the retina is exposed for some moments to colored light. A piece of red paper, for example, hung on a gray wall and fixated steadily gradually loses its saturation and approaches a gray. But not only is the excitability of the eye altered under continued stimulation; it remains altered,

as noted above, even after the exciting cause has ceased to operate. When the blue glasses are removed the landscape looks yellowish; the observer is yellow-sighted. Similarly, when the red paper is removed, a bluish-green patch appears upon the gray wall. The first result is due to general adaptation, the second to local adaptation. In both instances, adaptation forms a predisposition for seeing colors which are complementary to the original stimulating colors. The result of local adaptation (as the bluishgreen patch induced by the reddish paper) is a negative after-image. Adaptation and afterimages follow the same course with brightnesses as with colors. It is, for example, a matter of common observation that confinement in a darkened room enhances the brightness of objects seen subsequently in full light; that exposure to a strong light (sunlight on snow) tends to reduce the apparent brightness of objects seen afterward in moderate illumination, and, finally, that fixation of a dark or a light area induces a negative after-image of the opposite brightness. All these phenomena illustrate the effects of adaptation.

Theories of Visual Sensations. These theories aim to set forth the conditions under which the sensations arise. The most important in the field are the rival theories of Hermann v. Helmholtz and E. Hering. The Helmholtz theory provides for three primary sensations, red, green, and blue (or violet), which rest upon three distinct processes of excitation, in the visual apparatus. To explain the large number of spectral qualities, it is assumed that these three processes, combined in varying proportions, give rise to all possible color qualities, and that in equal amounts they produce gray. The theory, which was outlined by Thomas Young and elaborated by Helmholtz, was designed primarily to account for the facts of color-mixture; that is, for the production of a large number of "intermediate» qualities and of grays by means of a few "elementary" sensations. These facts of color-mixture it covers well; but it fails-particularly in its traditional form-to explain many other facts of color vision, notably the possibility of obtaining gray in the absence of color tone (as in color-blindness, peripheral vision, and from stimuli of small extent). Recently, important modifications and additions have been introduced into the theory which have, in a measure, removed its deficiencies.

The Hering theory rests upon a basis quite different from that of the Helmholtz theory. Hering posits three retinal substances; but he makes each of these the seat of antagonistic processes-processes of assimilation or building up and of dissimilation or tearing down. To each of these processes in each substance corresponds a "primary" color. Assimilation of one substance gives rise to blue; dissimilation of the same substance, to yellow. This substance is called briefly the blue-yellow substance. Similarly the other two substances are called the red-green substance and the black-white substance. Blue, green, and black correspond to the three assimilative processes; yellow, red, and white to the three dissimilative processes. All forms of stimulation affect the black-white substance; but certain wave-lengths produce no effect upon the other two substances. Moreover, the black-white substance appears in great