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secondarily applied to the relations of external objects in regard to one another. Thus, we
come to assign to two points which lie at different distances along the line of orientation a
direction and distance in relation to each other. All such ideas of spacial distance
referring to various positions along the line of orientation, when taken together, are called
ideas of depth, or when they are also ideas of particular single objects ideas of three
dimensions.
29. An idea of depth arising in the way described varies according to objective and
subjective conditions. The determination of the absolute distance of an isolated point in
the field of vision, is always very uncertain. Even, the determination of the relative
distance between two points a and b lying at different depths is generally certain only
under the condition assumed above, that they are connected by a line along which the
points of fixation for the two eyes can move in changing the convergence from a to b. We
may call such lines which connect different points in space with one another lines of
fixation. The principle may then be formulated: points in space are apprehended in their
true relations only when they are connected by lines of fixation, along which the point of
fixation may move. This principle is explicable on the ground that the condition of a
regularly connected change in the local signs of the retina and in the accompanying
sensations of convergence, that is, the condition for the rise of ideas of depth as we found
before (p. 133), is obviously fulfilled only when impressions are presented which can
arouse the appropriate local signs.
30. When the condition mentioned is not fulfilled, there arises either an imperfect and
indefinite idea of the different relative distances of the two points from the subject, or
else [p. 136] the two points seem to the equally distant -- a phenomenon which can
appear only when one of the points is rigidly fixated. Under the latter condition still
another charge, always arises in the idea; only the fixated point is seen as single, the other
as double. The same thing happens while looking at objects when they are not connected
with the binocular fixation-point by means of lines of fixation. Double images that arise
in this way are uncrossed -- i.e., the right belongs to the right eye, the left to the left eye --
when the crossed fixated point is nearer than the observed object and crossed when the
point is beyond the object.
Binocular localization in depth and binocular double images are, accordingly, phenomena
directly interrelated; where the former is indefinite and imperfect, we have double
images, and where, on the other hand, the latter are absent, the, localization in depth is
definite and exact. The two phenomena stand in such a relation to the line of fixation that,
when it is present, localization is aided and double images removed. Still, this rule is not
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OUTLINES OF PSYCHOLOGY
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without exception, for when a point is ridgidly fixated with both eyes, double images
arise easily in spite of any lines of fixation that may be present. This is explained by the
necessary conditions, for both of depth as mentioned above (p. 133). Just as the absence
of lines of fixation results in the lack of the required succession of the local signs, so in a
sigular manner the sensations of conference connected with movement, are absent in
rigid fixation.
c. Relations between the Location of the Elements in regard to one another and the
Location in Regard to the Subject.
31. When the field of vision is thought of as merely a location of impressions in relation
to one another, we represent it to ourselves is a surface, and call the single ob- [p. 137]
jects lying in this surface ideas of two dimensions, in contrast to the ideas of depth. But
even an idea of two dimensions must always be related to the seeing subject in two ways.
First, every point in the field of vision is seen ill a particular direction on the subjective
line of orientation mentioned above (p. 131). Secondly, the whole field of vision is
localized at some distance or other from the subject, though this distance may be very
indefinite.
The location in a particular direction results in an erect ideational object corresponding to
an inverted retinal image. This relation between the objective localization in direction and
the retinal image is as necessary a result of ocular movements as the inversion of the
image itself is a result of the optical properties of the eye. Our line of orientation in space
is the external line of regard or, for binocular vision, the middle line resulting from the
combined effects of movements of fixation. A direction upward on this line of orientation
in external space corresponds to a direction downward in the space where the retinal
image lies, behind the centre of ocular rotation, and vice versa. It follows that the retinal
image must be inverted if we are to see the object erect.
32. The location at some distance or other, which is also never absent, brings about the [ Pobierz całość w formacie PDF ]