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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. The chief evidences in support of nativistic theories are, first, the
metamorphopsia after dislocation of retinal elements (p. 119) and, secondly, the position
of the line of orientation (p. 131), which indicates united functioning of the two eyes
from the first. It has been noted already (p. 120) that the metamorphopsia and other
related phenomena prove the exact opposite as soon as the chances to which they are due
become stationary. Furthermore, the fact that in long continued use of only one eye the
line of orientation comes to coincide with the line of regard (p. 131), proves that the
position of this line is not given from the first, but that it has arisen under the influence of
the conditions of vision. Still another fact against nativistic and in favor of the genetic
theory is the development in the child of the synergy of ocular movements under the
influence of external stimuli and the organization of space-perceptions which apparently
accompanies it. Here as in many other respects the development of most animals is
different. In the latter cases the reflex connections of retinal impressions with movements
of the eyes and head function perfectly immediately after birth. (v. inf. § 9, 2).
The genetic theory has gained the ascendency over older nativistic and empirical views
primarily through the more thorough investigation of the phenomena of binocular vision.
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OUTLINES OF PSYCHOLOGY
84
Nativism has difficulty with the question why we generally see objects single although
they produce images in each of the two eyes. The effort is made to avoid the difficulty by
assuming that two identical retinal points are connected with the same optic fibre which
divides in the chiasma, and that in this way they represent in the sensorium only a single
point. This doctrine of the "identitv of the two retinas" was, however, untenable as soon
as the actual conditions of binocular vision in three-dimensions began to be investigated.
Especially the invention of the stereoscope thus brought with it a new era for the genetic
theory of vision.
[1] A process analogous to this elimination of the metamophopsia is sometimes observed
in binocular vision when the disturbances arising from squinting are gradually overcome.
When the squinting begins, the two lines of regard no longer meet in the field of vision,
so that double images of objects arise. These may gradually disappear, however, if the
condition of the eyes remains perfectly stationary; a new set of relations is developed for
the retinal elements of the squinting eye.
[2] In this connection, we have the fact that the blind spot does not appear as a break in
the field of vision, without sensational contents, but as a continuation of the general
brightness and color of the whole field; for example, as white when we are looking at a
white surface, as black when we look at a black one. This filling out of the blind spot is
possible only through reproduced sensations, and is to be considered as one of the
phenomena of association to be discussed later (§ 16).
[3] The habit of seeing with two eyes results in exceptions to this rule. Often when one [ Pobierz całość w formacie PDF ]