8. Association in the case of spacial ideas is of the most comprehensive character. It is not very noticeable in the sphere of touch when vision is present, on account of the importance of tactual ideas in general and especially for memory. For the blind, on the other hand, it is the means for the rapid orientation in space which is [p. 230] necessary, for example, in the rapid reading of the blind-alphabet. The effects of assimilation are most strikingly evident when several tactual surfaces are concerned, because in such cases its presence is easily betrayed by the illusions which arise in consequence of some disturbance in the usual relation of the sensations. Thus, for example, when we touch a small, ball with the index and middle fingers crossed, we have the idea of two balls. The explanation is obvious.
In the ordinary position of the fingers the external impression here given actually corresponds to two balls, and the many perceptions of this kind that have been received before exercise an assimilative action on the new impression.
9. In visual sense-perceptions assimilative processes play a very large part. Here they aid in the formation of ideas of magnitude, distance, and three-dimensional character of visual objects.
In this last respect they are essential supplements of immediate binocular motives for projection into depth. Thus, the correlation that exists between the ideas of the distance and magnitude of objects, as, for example, the apparent differing the size of the sun or moon on the horizon and at the zenith, is to be explained as an effect of assimilation. The perspective of drawing and painting also depends on these influences. A picture drawn or painted on a plane surface can appear three-dimensional only on condition that the impression arouses earlier three-dimensional ideas which are always with the new impression. The influence of these assimilation most evident in the case of unshaded drawings that can be either in relief or in intaglio.
Observation shows that these differences in appearance are by no means accidental or depend upon the so-called "power of imagination", but that there are always elements in the immediate impression which determine completely the assimilative process. The elements that thus operative are, above all, the sensations arising from the [p. 231] position and movements of the eye. Thus, for example, a design which can be interpreted as either a solid or a hollow prism, is seen alternately in relief and in intaglio according as we fixate in the two cases the parts of the which correspond ordinarily to a solid or to a hollow object. A solid angle represented by three lines in the same appears in relief when the fixation-point is moved along of the lines, starting from the apex, it appears in intaglio when the movement is in the opposite direction, from the of the line towards the apex. In these and all like cases assimilation is determined by the rule that in its movement the fixation-lines of objects the eye always passes from nearer to more distant points.
In other cases the geometric optical illusions (§ 10, 19 and 20) which are due to the laws of ocular movements, produce certain ideas of distance, and these not infrequently eliminate the contradictions brought about in the by the illusions. Thus, to illustrate, an interruptedstraight line appears longer than an equal uninterrupted line (p. 125); as a result we tend to project the first to a greater depth than the latter. Here both lines cover just the same distances on the retina in spite of the fact that their length is perceived as different, because of the different motor energy connected with their estimation. An elimination of this contradiction is effected by means of the different ideas of distance, for when one of two lines whose retinal images are appears longer than the other, it must, under the ordinary conditions of vision, belong to a more distant object.
Again, one straight line is intersected at an acute angle by another, the result is an overestimation of the acute angle, sometimes gives rise, when the line is long, to an apparent bending near the point of intersection (p. 125). Here contradiction between the course of the line and the [p.
232] increase in the size of the angle of intersection, is often eliminated by the apparent extension of the line in the third dimension.
In all these cases the perspective can be explained only as the assimilative effect of earlier ideas of correspending character.
10. In none of the assimilations discussed is it possible to show that any former idea has acted as a whole the new impression. Generally this is impossible because we must attribute the assimilative influence to a large number of ideas, differing in many respects from one another. Thus, for example, a straight line which intersects a vertical at an acute angle, corresponds to innumerable cases in which an inclination of the line with its accompanying increase of angle appeared as a component of a three-dimensional idea. But all these cases may have been very different in regard to the size of the angle, the length of the lines, and other attending circumstances. We must, accordingly, think of assimilative process as a process in which not a single definite idea or even a definite combination of elements from ideas, but as a rule a great number of such combinations are operative. These need agree only approximately with the new impression in order to affect consciousness.
