596 ANIMAL PHYSIOLOGY. 
In order that such fusion shall take place, the respective 
images must fall on identical (corresponding) parts of the retina, 
b , 
d d. 
1 
P. 
e ¢; 
Z bea r q 
Fie. 437.—Ilustrates binocular vision. If the truncated pyramid, P, be looked at with the 
head held perpendicularly over the figure, the image formed in the right eye when the 
left is closed is figured on the aS and that seen when the right eye is closed is rep- 
resented by the figure in the middle. No superposition of these figures will give P, yet 
ass a a process they are combined into P, the figure as it appears to both eyes (after 
ernstein). 
As is well known, the pictures used for stereoscopes give 
different views of the one object, as represented on a flat sur- 
face. These are thrown upon corresponding points of the retina 
by the use either of prisms or mirrors, when the idea of solidity 
is produced. As to whether movements of the eyes (converg- 
ence) are necessary for stereoscopic vision is disputed. It has 
been inferred, from the fact that objects appear solid during 
an electric flash, the duration of which.is far too short to per- 
mit of movements of the ocular muscles, that such movements 
are not essential. The truth seems to lie midway; for while 
simple figures may not require them, the more complex do, or, 
at all events, the judgment is very greatly assisted thereby. It 
is of the utmost importance to bear in mind that all visual 
judgments are the result of many processes, in which, not the 
sense of vision alone, but others, are concerned; and the mutual 
dependence of one sense on another is great, probably beyond 
our powers to estimate. Reference has been made to this sub- 
ject previously. 
PROTECTIVE MECHANISMS OF THE EYE. 
The eyelids have been appropriately compared to the shut- 
ters of a window. They are, however, not impervious to light, 
as any one may convince himself by noticing that he can locate 
the position of a bright light with the eyes shut; also that a 
sensitive person (child) will turn away (reflexly) from a light 
when sleeping if it be suddenly brought near the head. The 
Meibomian glands, a modification of the sebaceous, secrete an 
oily substance that seems to protect the lids against the lachry- 
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