40 THE REASON WIIY : 



Each friend by fate snatched from us, is a plame 

 Plucked from the wing of vain humanity.&quot; YOUNO. 



136. Why does a flash of light from the eye generally follow 

 from a violent blow on that organ or the surrounding parts ? 



Because light, or some degree or modification of light, is the only 

 impression of which the eye is susceptible ; and therefore every 

 effect upon the parts of the eye which are essential to vision only 

 must be a sensation of light. 



137. A violent blow on the eye appears as an instantaneous flash of light, and this 

 effect is produced, not only when the eye itself is struck, but when there is a violent 

 concussion of the head, which we may suppose to be transmissable to the surround 

 ing parts of the eye, and so to the eyeball itself. If a person walking hastily and 

 heedlessly in the dark strikes his forehead violently against an object of resist 

 ance, then instantly, and before any pain is felt in the part which has received the 

 stroke, a flash of light is seen by both eyes, brighter in proportion as the stroke is 

 more violent. Thus, any sudden agitation produced in the exterior coats of the 

 eyeball, from whatever source it may arise, always tells upon the sentient part of 

 the eye as light, and never as pain or any other sensation. 



138. Why cannot a person distinguish minute objects in the 

 water with his head likewise immersed in the fluid ? 



Because, although there is sufficient difference between the den 

 sity of the humours of the eye and that of the air, to bring the rays 

 transmitted by the latter to a focus on the retina, there is not suffi 

 cient difference between the density of these humours and that of 

 water, to do the same by rays transmitted through this fluid, so that 

 such rays are not brought to a focus sufficiently soon. Hence divers 

 in some places are in the habit, when they descend into the water, 

 of using extremely convex glasses, in shape almost like the lens of 

 fishes, and turning their eyes by this means, as it were, into those 

 of an aquatic animal. 



139. Why are the eyes furnished with lashes? 



Because they serve to ward off insects, and to protect the eye 

 from particles floating in the air. When the eyelashes are humid, 

 the little drops of moisture decompose the rays of light, and, even 

 independently of humidity, they also partly resolve the light passing 

 into the interior of the eye. 



140. When the eyelids are placed near each other, and the eyelashes admit only a 

 quantity of light to pass at a time, the eyelashes, by separating into pencils 



