VISION, the sense of sight. Visual sensa tions include brightness qualities (blacks, whites and grays), and color qualities (reds, greens, yellows, purples, etc.). They are peoduced by the action of light upon the eye and, taken in conjunction with certain other sensations, which are set up in and about the eye by the fume tioning,of muscle.s and tendons, they give rise to the vistialipereeption of obcts and processes in the outside world. A descriptive actount of vision has to deal (1) with the visual appa ratus, .both as a diiiptric or refracting mech anism; which conveys light-rays to the retina, atid as a nervous organ, which transforms tke Vibratory stirmilns into n.eural excitation and fransmits it to the brain; (2) with the sensationa 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 This apparatus consists of the eye, optic nerve, subcortical centres (of the corpora quadrigemina and thal ami), and terminal or central arras in the cor tex of the occipital lobes. It is generally believed that visual sensations are directly cor related with die functions of the last or cere bral part, only, of the visual mechanism; the office of the eye, the optic nerve and the lower centres .being the transmutation of ether-vibra tions, the transmission of nervous irnptdses to the brain and the setting up of various move ments— such as rotation of the eyes, accommo dation and winking—which are important fac tors in visual perception.
In the human adult, the eye is a spheroi dal mass having an antero-posterior diameter of about one inch. The central part of its front surface (the cornea) possesses a higher convex ity than the remaining oqaque surface. It is seen from a lateral position as a bulging trans parent covering. The eye is invested with three coats and its interior is divided into two cham bers; 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 die 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 dif fers, however, from the ordinary photographic camera in adjusting its focus for different dis tances by changing the convexity of its lens — not by altering the position of its sensitive sur face, 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 olistant 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 intenial and external reed, the superior and inferior recti and the superior and inferior obliqui. Since flie 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 in ward of the eyes in their common fixation of an object is called convergence.
The true nervous end-organs for vision lie in the retirra. 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 trans form the light-energy transmitted to the eye into the immediate stimulus for nezvons excita tion. 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 insensi tive 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. Beyond the chiasma there is a second division, a part of the fibres of both nerves passing to each hem isphere of the cerebreni, 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 whic-h 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 pur ples. 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 aware) 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 neces sary to bring the color sensations into relation with brightness (the gray series). That an inti mate 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 (\V Blk) of the double pyramid represents the gray series from white to black. The letters around the base (G, 01, Y, Or, etc.) stand for color tones at maximal saturation. The saturation of any tone is rep resented by the length of a perpendicular dropped from the point in the base where the color is found upon the gray axis; the bright ness of any tone, by the point where this per pendicular strikes the axis. The inclination of the base brings the perpendicular from yellow (Y) higher up the axis than the perpendicular from the blue (B). This is in accord with the greater brightness of the former color. The number of pure brightness that can be discriminated is estimated to lie between 650 and 800; the number of color tones— reds, greens, violets, etc.— is about 150.