20. Added to these two illusions, which arise from the special structure of ocular muscles in their adaptation to the purposes of vision, there are certain other variable optical [p. 125] illusions that are due to certain attributes of all voluntary movements and have their analoga in the movements of the tactual organs. These illusions may also be divided into those of direction, and those of magnitude. The former follow the rule that acute angles are overestimated, obtuse underestimated, and that the direction of the intersecting lines varies correspondingly. For the illusions of magnitude we have the rule. forced or interrupted movements require more exertion than free and continuous ones. Any straight line that necessitates fixation is, accordingly, overestimated in comparison with an open distance marked off by two points, and a straight line interrupted by several dividing lines is overestimated in comparison with an uninterrupted line.
The tactual analogon of the illusion in angles is the tendency to overestimate small articular movements and to underestimate large ones. This comes under the general principle that a relatively greater expenditure of energy is required for a short movement than for a more extensive one, because it is more difficult to begin a movement than to continue it after it is already started. The tactual phenomenon analogous to the overestimation of interrupted lines, is that a distance estimated by a movement of one of the limbs always seems shorter when it is passed through in a single continuous movement than it does when the movement is several times interrupted. Here, too, the sensation corresponds to the expenditure of energy, which is, of course, greater for an interrupted than for a continuous movement. The overestimation of interrupted lines by the eye, takes place, as we can easily understand, only so long as no motives arise from the way in which the division is made, to hinder the movement of the eye over the interrupted line. Such a hindrance is present, for example, when the line is interrupted only once. This one point of division makes fixation [p. 126] necessary. If we compare such a line with a continuous one, we tend to estimate the first without any movement, with the point of division as a fixation-centre, while the second is apprehended by a movement of the eye. As a result the continuous line seems longer than the interrupted line.
21. All these phenomena point to the immediate dependence of the apprehension of spacial directions and magnitudes on ocular movements. As further evidence pointing in the same direction, we have the negative fact that the arrangement of the retinal elements, especially their proximity to one another, normally has no influence on the ideas of direction and magnitude. This is most strikingly evident in the fact that the distance between two points appears the same whether served in direct or indirect vision. Two points that are clearly distinguished in direct vision, may become one in the eccentric parts of the field of vision, but so soon as they are distinguished at all, they will appear just as far apart in one region as in the other.
This independence of the proximity of the retinal elements, in our perception of magnitude, holds even for a part of the retina that is not sensitive to light at all -- for the blind spot, where the optic nerve comes into the eye. Objects whose images fall on the blind spot are not seen.
The size of this spot is about 6 o, and it is located 15 o inward from the point of fixation. Images of considerable size, as, for example, that of a human face at a distance of six feet, may disappear entirely on it. Still, when points appear at the right and left or below and above this region, we localize them just as far from each other as we should in any other, uninterrupted part of the field of vision. The same fact is observed when some part of the retina becomes blind through pathological conditions. The resulting break in the field of vision shows itself only ill the fact that images falling on it are not seen, never [p. 127] through any changes in the localization of objects lying on opposite sides of the blind region. [ 2 ]
22. All these phenomena teach that the keenness of vision and the apprehension of directions and distances in the field of vision, are two different functions, which depend upon different conditions: the first depends on the proximity of the retinal elements to one another, the second on ocular movements. It follows directly that spacial ideas from sight can not be regarded as original and given immediately in the action of impressions of light with their spacial arrangement, any more than can the spacial ideas of touch. The spacial order is, here too, developed from the combination of certain sensational components which, taken separately, have no spacial attributes whatever. Other conditions also indicate that the sensational elements are related here in the same way as in the case of touch, and that the development of visual space under normal conditions runs entirely parallel to the development of space in congenital blindness, the only condition under which touch attains a similar independence. Retinal impressions correspond to impressions of contact, and ocular movements to touch-movements. Tactual impressions can gain spacial qualities only through the local coloring of the sensations connected with them -- the local signs and in like manner -- we must presuppose the same for retinal impressions.
22a. To be sure, a qualitative gradation of local signs on [128] the retina can not be demonstrated with the same evidentness as for the skin.
Still, by the use of colors it can be established in general that for greater distances from the retinal centre the sensational quality gradually changes.