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lens and a screen, and the human eye exhibits in many respects a 

 close analogy with the camera. 



The outer coat of the spherical eyeball, called the " Sclerotic," 

 or in more popular language "the wliite of the eye," serves to 

 protect the more delicate interior parts from injury. The front 

 portion, which is transparent, allows rays of light from an object to 

 fall upon the crystalline lens, contained mthin the eyeball. This 

 lens, which is "double convex" and very transparent, forms a 

 small inverted image on the sensitive screen called the " Ketina : " 

 the " Optic Iferve " entering the eyeball at the back spreads over 

 two-thirds of its interior surface to form tliis inner film of the 

 retina. Wherever the light falls on the retina it effects chemical 

 changes in certain microscopic structures kno^^^l as the "Rods" and 

 " Cones," and the stimulus caused by these changes is conveyed 

 by minute nerve fibres to the brain. 



One of the principal difficulties met with in the construction 

 of optical instruments, arises from the fact, that the rays, which 

 pass through a lens near its circumference, are not brought to a 

 focus at exactly the same point as those which pass through the 

 central portion — the lens is in fact weaker at the centre than at the 

 edge. This difficulty is met in glass lenses, by cutting off, by 

 means of a screen, all those rays which do not pass near the 

 centre, and thus exactness of outline in the image is obtained at 

 the expense of brilliancy. In the eye there is a similar opaque 

 screen called the "Iris," in the centre of which is an aperture 

 known as the " Pupil." The pupil dilates when the light is feeble, 

 and contracts under a strong light, and these changes are performed 

 instantaneously and quite unconsciously. The photographer 

 imitates this action, though in a far less efficient manner, by 

 altering liis " stop " or circular screen according to the variation in 

 the intensity of the light. 



But the imperfection alluded to is again very much lessened 

 in the eye, by the manner in which the crystalline lens is 

 constructed of layers of decreasing density outward, something like 

 those of an onion. As white light, on passing through a lens, is 

 broken up into prismatic colours, the optician, to avoid the coloured 

 edt^es which would otherwise be seen in the images of all objects, 



