VISION. 



549 



face. These are tkrown 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 (conver- 

 gence) are necessary for stereoscopic vision is disputed. It has 



i 



p 





Fig. 397.— ^luetratep binocular vision. If the trnncatecl 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 right, and that seen when the right 

 eye is closed is represented by the figure in the middle. No superposition of these 

 figures will give P, yet by a pyschic process they are combined into P, the figure 

 as it appears to both eyes (al^ter Bernstein). 



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 ETE. 



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- 

 mal fluid, and prevents the latter running over their edges as 

 oil would on the margins of a vessel. The lachrymal gland is 



