The colors are not so saturated in indirect vision, and the color-tone also changes; for example, yellow appears orange. There is, indeed, in these properties of the retina no strict proof for the existence of pure local differences in the sensations, at least not in the fine gradations that must be assumed in the retinal centre, for example. Still, they show that local differences in sensations do exist, and this seems to justify the assumption of such even beyond the limits of demonstration. This is all the more justifiable because here, where the gradations are much finer, the tendency to translate sensational differences directly into local differences, which has already been noticed in the case of touch, will be much more apt to destroy their specifically qualitative character. As a confirmation of this view we have the fact that the clearly demonstrable sensational differences at greater distances from the retinal centre, can be observed only under favorable conditions of limited impressions, and disappears entirely when surfaces of uniform color are looked at. This disappearance of qualitative differences which are in themselves considerable, must be attributed in part at least to their relation to local differences. When, however, such relatively great differences disappear as a result of this relation, so that special methods are required for their demonstration, it can not be expected that very small differences will be demonstrable at all.
23. We assume, accordingly, qualitative local signs, which, judging from the data derived from the keenness of vision, are graded in the finest stages at the retinal centre and more slowly in the eccentric parts. The formation of visual space may then be described as a combination of this system of local signs arranged in two dimensions, with a system of intensive sensations of movement. For any two local signs a and b there will be a corresponding sensation of movement a arising from the movement through the distance a b and [p. 129] serving as a measure of the same. A longer distance a c will have a more intense sensation of movement, g . Just as the point of finest discrimination on the finger is the centre of reference, so in the same way the retinal centre is such a point of reference for the eye.
In fact, this is from the laws of ocular movements more obvious for the eye than it is for the tactual organ. Any luminous point in the field of vision is a stimulus for the centre of occular innervation, and tends to turn the line of regard reflexively upon itself. This reflex relation of eccentric stimuli to the retinal centre is probably an essential condition for the development of the synergy of ocular movements mentioned above, and is, at the same time, an explanation of the great difficulty of observing objects in indirect vision. This difficulty is evidently due to the greater reflex impulse toward a point in indirect vision when the attention is concentrated upon it, than toward less favored points. As a result of the preeminent importance which the retinal centre has for ocular movements, the point of fixation necessarily becomes the centre of reference in the field of vision, and all distances in this field are brought under a unitary standard by being determined with reference to the fixation-point. The excitation of local sign is due to the action of external impressions, and both together cause the movement towards the retinal centre. The whole process of visual space-arrangement is thus due to the fusion of three different sensational elements: first, the sensational qualities depending upon the character of the external stimulus, second the qualitative local signs depending on the places where the stimuli act, and third, the intensive motor sensations determined by the relation of the stimulated points to the centre of the retina. The latter elements may either accompany actual movements -- this is the original case -- or, when the eye remains at rest, are [p. 130] mere motor impulses of a particular intensity. Because of the regular connection between qualitative local signs and intensive sensations of movement, they may both together be regarded as a single system of complex local signs. The spacial localization of a simple visual impression, is a product of a complete fusion of the sensation caused by the external stimulus with the two interconnected elements belonging to this system of complex local signs. The arrangement of a number of simple impressions in space consists in the combination of a great number of such fusions, which are graded in quality and intensity according to the elements of the system of local signs. The predominating elements in these fusions are the sensations due to the external stimulation. In comparison with these, the elements of the system of local signs are so obscure, even in their original quality and intensity, that for the immediate apprehension of objects they are entirely lost except as spacial qualities.
Connected with this complex process of fusion, which determines the order of the elements in the field of vision, is still another. This latter process, which takes place in the formation of every spacial idea, arises from the relation of the object seen to the subject. We pass now to the consideration of this second process. b. The Location of Visual Ideas in Relation to theIdeating Subject.