Theories of Visual Sensations.— These theories aim to set forth the conditions under which the sensations arise. The most import ant in the field are the rival theories of Her mann v. Helmholtz and E. Hering. The Helm holtz theory provides for three primary sensa tions, 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 pro portions, 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 Heltnholtz, was de signed primarily to account for the facts of color-mixture; that is, for the production of a large number of gintermediate qualities and of grays by means of a few gelementare sensa tions. These facts of color-mixture it covers well ; but it fails—particularly in its traditional form — to explain many other facts of color yision, notably the possibility of obtaining gray in the absence of color tone (as in color-blind ness, 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 cif antagonistic processes—processes of assimilation or build ing up and of dissimilation or tearing down. To each of these processes in each substance corresponds a 4primare color. Assimilation of one substance gives nse to blue; dissimilation of the same substance, to yellow. This sub stance 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. More over, the black-white substance appears in great est quantities, is most widely distributed through out the visual apparatus and is most easily set into function. Since assimilation and dissimila tion are opposed processes, the principle of an tagonism assumes great significance in Hering's theory: the principle is, indeed, its most char acteristic feature. Its application to comple mentary colors— where the opposed processes in the several substances cancel each other — to contrast, to adaptation, to after-images, etc., fol lows naturally from the fundamental concep tion. Although the Hering theory is open to attacic on various technical grounds, it undoubt edly covers the whole range of visual sensation more adequately than its rival. There is con siderable that is hypothetical in both theories. This is, however, a fault that recent work on the histology of the eye and on the function of the retinal elements is doing much to remedy. Within the last few years several new theories of visual sensation have come into the field. Most of them may, however, be regarded as modifications of one or other of the more clas sical theories just discussed. Their value and their relation to the older theories must be sought in the current literature of the subject Visual Perception.— It is characteristic of visual perception that all objects seen are spatial objects; that is, they occupy some place, some position in the spatial world; and they possess, lilcewise, spatial properties, form, olistance, direction, etc. This characteristic, it should be noted, is shared by tactual perceptions; but it is not a mark of perception in general—not, for example, of the perceptions of melody, harmony and rhythm. Since all visual percep tions are spatial, the chief problem they offer to' psychology is the analysis of the spatial factors and the search for the conditions under which these factors operate.
The simplest factor in visual space is exten sion. Every visual sensation comes to con sciousness as an extended sensation. A color is always espread out); its parts are adjacent.
In this respect, colors and brightnesses are essentially different from tones and noises, which lack the attribute of extension. Space, as it is perceived, is an orderly arrangement of ex tended objects. It never exists by itself alone. There is no such thing, in perception, as ((mere* space or eemptye space. Only by abstraction are the spatial properties removed from objects; only by abstraction, as in mathematics, does empty space come into existence. Even such quasi-abstract constructions as the line and the point do not properly figure in the psychology of space. It is, however, customary, within psychology, to distinguish two-dimensional and three-dimensional spaces; not because they dif fer fundamentally, but because they rest in part upon different conditions. The retina, be ing an extended organ upon which stimuli fall in patterns —thanlcs to the refractive functions of the transparent media— forms a natural sub strate for the perception of surface magnitudes. But the retina is not so well adapted for tridi mensional perception—perception of solid ob jects. A special provision for depth-perception is made, however, in binocular vision; that is to say, in the difference of the two retinal im ages which are thrown upon the two eyes by a single object. This difference in binocular images depends upon the different positions from which the two eyes observe the object. Now it is conceivable that the two unlike images should have come to consciousness as two dis tinct objects. But, as a matter of fact, they do not, except under unusual circumstances— as when one squints or presses with the fingers upon one eyeball. They do not even come to consci ousness as two discrepant views of the same object The difference in retinal images func tions solely in the perception of depth or solid ity: This is the principle involved in stereo scopic vision. Binocular perception of depth may be produced artificially by means of the stereoscope, an instrument 'which presents to the two eyes, under favorable conditions, two slightly different plane pictures of an object. Stereoscopic vision derives secondary aid, in ordinary perception, from linear and aerial per spective, from the known size of objects of reference, from change of position of the ob server as well as of objects from distribution of light and shade, etc.
The spatial functions of the eye depend as well upon- ocular movements and the sensations which these arouse as upon the immediate retinal factors just considered. It has been demonstrated by many lines of experimentation that were the eyeballs set firmly within the head without the possibility of movement (a condition which is approximately in the case of certain fishes) visual perception of objects would be enormously handicapped. In the first place, movement allows the eye to travel over the object, exploring it front point to point; in the second place, it sets up sensations both in the external muscles that rotate the eye and in the internal muscles of accommodation; and, fatally, it is probaWe that to these factors must be added articular sensations from the rubbing of the eyes in their sockets. All these sensa tions, muscular, tendinous and articular, play important roles in the determination of the spatial properties and relations of objects.
The eyes, regarded as perceiving organs, may be said, then, to fall into three parts; the retinas, which mediate visual sensations proper, and which function as a true double organ, the (lioptrie media, which focus the rays of light upon the retinas, and the movement-apparatus, which both alters the position of the retinal image and itself contributes Icincesthetic sensa tions from muscle, tendon and orbit.
Consult Calkins, 'Introduction to Psychol oW (1901), ch. ii.; James, (Principles of Ps3r chology) (1896), Vol. II, ch. xx ; Kuelpe, (Out-. lines of Psychology) (1895), 351-373; Stout, 'Manual of Psychology; (1:"), pp. 141 170 ; Wundt, (Human and Animal Psychology' ; Titchener, (Experimental Psychology,) Vol. I (1901), Part II, ch. i. For the structure and functions of the eye, consult also general text books of physiology. See EYE.