VISION. 595 
vary with the position of the eyes, head, etc., and often consti- 
tutes a very complex geometrical figure when the various 
points are united. The simpler case is when standing upright 
we look toward the distant horizon, in which instance the 
horopter forms a plane drawn beneath us—i. e., is the ground on 
which we stand. This will appear from Fig. 436. 
Estimation of the Size and Distance of Objects—The processes 
by which we form a judgment of the size and distance of objects 
are closely related. 
As we have already shown (page 583), the visual angle varies 
both with the size and the distance of an object. Knowing 
that two objects are at the same distance from the eye, we esti- 
mate that the one is larger than the other when the image one 
forms on the retina is larger, or when the visual angle it sub- 
tends is greater than in the other case, and conversely. Thus, 
knowing that two persons are at the distance of half a mile 
away, if one is judged by us to be smaller than the other, it 
will be because the retinal image corresponding to the object 
is smaller, other things being equal. But the subject is more 
complex than might be inferred from these statements, 
We have already pointed out that objects of a certain color 
seem nearer than others; also those that are brighter, as in the 
case of mountains on a clear day. And not only do all the 
qualities of the image itself enter as data into the construction 
of the judgment, but numerous muscular sensations, The eyes 
accommodating and converging for near objects, from the law 
of association, give rise to the idea of nearness, for habitually 
such takes place when near objects are viewed, so that the 
subject becomes very complex. That we judge imperfectly of 
the position of an object with but one eye is realized on attempt- 
ing to stick a pin into a certain small spot, thread a needle, cork 
a small bottle, etc., when one eye is closed. 
Solidity.— By the use of one eye alone we can form an idea of 
the shape of a solid body ; though, in the case of such as are very 
complex, this process is felt to be both laborious and imperfect. 
From the limited nature of the visual field for distinct 
vision, it follows that we can not with one eye see equally dis- 
tinctly all the parts of a solid that is turned toward us. After 
a little practice one may learn to define for himself what he 
actually does see. 
Such a figure as that following results from the combina- 
tion, mentally, of two others, which answer to the images fall- 
ing on the right and on the left eyes respectively. 
