To give a concrete illustration, it is observed that complex stereoscopic pictures generally require several movements of convergence back and forth before a clear plastic idea arises. Furthermore, the effect of the parallax appears in looking at stereoscopic pictures whose parts are movable in respect to each other. Such movements are accompanied by changes in the relief which answer exactly to the corresponding changes in binocular parallax.

This parallax is dependent on the distance of the two eyes from each other, so that ideas of depth can be produced even in the case of objects too distant in reality to give a plastic effect, by combining in the stereoscope pictures taken from positions much further apart than the two eyes are.

This is done, for example, in making stereoscopic photographs of landscapes.

The result is that these photographs when combined do not look like real landscapes, but like plastic models regarded from a short distance.

34. In monocular vision all the conditions are absent which are connected with movements of convergence, and with binocular differences in the retinal images, and which may be [p. 140] artificially reproduced with the stereoscope.

Still, not all the influences are wanting even here to produce a localization in the third dimension, although this localization is more imperfect.

The direct influence of movements of accomodation have in comparison with other conditions a relatively small, perhaps entirely insignificant influence. Still, like movements of convergence, they too are accompanied by sensations which can be clearly perceived in the else of greater changes of accommodation from distant to neighboring points. For smaller changes in depth these sensations are very uncertain. As a result the movement of a point in the direction of the line of regard, when it is looked at with only one eye, is generally not clearly observed until a change in the size of the retinal image appears.

35. For the development of monocular ideas of depth the influences which the components of the so-called perpective exercise, are of the greatest importance. These are the relative magnitude of the of vision, the trend of limiting lines, the direction of shadows, the change in colors due to atmospheric absorption, etc. All these influences, which act in exactly the same way in monocular and binocular vision, depend on associations of ideas, and will, therefore, be treated in a later chapter (§

16).

35a. We have in general the same opposing theories for the explanation of visual ideas as for tactual ideas (p. 114). The empirical theory has sometimes committed the fallacy of limiting itself to optics and turning the real problem of space perception over to touch. In such cases it has tried to explain only how a localization of visual ideas can take place with the aid of experience, on the basis of already existing spacial ideas from touch. Such an interpretation is, however, not only self-contradictory, but it also conflicts with experience, which shows that normal persons with vision, visual space-perception determines tactual, not the reverse (p. 104). The fact of general development, that touch [p. 141] is the more primitive sense, can not be applied to the development of the individual.