VISION. 591 
from being seen better. The full moon looks larger when near 
the horizon than when overhead, from the absence of objects in 
the latter case with which to compare it; and in like manner 
distances on the water or on a vast plain seem less than they 
really are; and so in innumerable instances the influence of a 
standard of comparison or its absence is evident. 
Subjective Phenomena.— When the eyelids are shut in a dark 
room, the eye does not seem absolutely devoid of light. Such 
sensation of luminosity as may be feebly present is sometimes 
spoken of as the “proper light of. the retina.” When the ball 
of the eye is pressed upon, colored circles of light appear when 
the eyes are closed, such being plainly due to mechanical stimu- 
lation of the retina. These are “ phosphenes,” and are akin to the 
stars seen when the eye receives a sudden blow, or to the sen- 
sations excited by electrical stimulation. But, apart from any 
stimulation of the retina, objects may apparently be seen in ex- 
cited conditions of the brain, as in insanity, delirium tremens, 
etc. Sometimes one object, instead of being recognized, seems 
to arouse the perception of another. The cause is traceable in 
many cases solely to the brain itself, especially the part of the 
cerebral cortex concerned in vision, and illustrates the impor- 
tance of this part of the central visual mechanism, and much 
more into which we can not enter now. 
Co-ORDINATION.OF THE Two EYEsS-IN VISION. 
Asa matter of fact, we 
are aware that an object 
may be seen as one either 
with a single eye or with 
both. For binocular vis- 
ion it may be shown that 
the images formed on the 
two retinas must fall in- 
variably on corresponding 
points. 
The position of the lat- 
ter may be gathered from 
Fig. 432. It will be noticed 
that the malar side of one fre. 432. a7 LO points 
(after Foster). L, R, left and rig) bt eyes ; a b,c, 
eye corresponds to the na- are points in one eye correspond ing to ay, 6), ¢), 
: in the other. The lower ape are projections of 
sal side of the other, the retina of the right (2) and the left (L) eye. 
It may be observed that the malar side of one 
though upper always an- retina corresponds to the nasal side of the other. 
