COORDINATION 



399 



Fig. 16-5. A comparison of the camera and the human eye. Note the likenesses and differences. (For names 



of parts of the eye see Fig. 16-4.) 



shaped so that it is uniformly curved on 

 both sides ( a convex lens ) the rays of liejht 

 are bent toward one another so that they 

 come to a point, or focus, as it is called. Just 

 how light passes through the lens depends 

 on the angle at which it strikes the surface. 

 The amount of bending; increases with the 

 increase in angle between the hght ray and 

 the surface of the glass. Therefore, a highly 

 curved surface will bring the rays to focus 

 at a very short distance (focal length), 

 whereas a more flattened surface will bring 

 them together at a greater distance. De- 

 pending on conditions, therefore, a lens is 

 said to have a long or short focal length. 

 For example, in the objectives of the com- 

 pound microscope the low power lens has 

 a focal length of 16 mm., whereas the high 

 power lens has one of only 4 mm. The lens 

 with the shorter focal length has the greater 

 curvature and consequently magnifies the 

 greater, also. 



Light coming from an object on one side 

 of a convex lens passes through and comes 

 to focus on the other side at the focal length 

 of the lens. The light comes from an infinite 

 number of points on the object, and passes 



through the lens with the result that the 

 image is completely reversed. With this 

 knowledge of the working of the convex 

 lens we can better understand how the eye 

 functions. 



The remarkable similarity of a simple 

 camera to the eye will help us to under- 

 stand how we see (Fig. 16-5). Both have 

 convex lenses which bring the rays of light 

 to focus upon a sensitive plate, the film in 

 the camera and the retina in the eye. The 

 amount of light entering the chamber is 

 controlled by the iris diaphragm in both 

 cases. The housing is a lightproof case 

 which allows the rays of light to pass 

 through unobstructed; in the camera it is 

 made of an adjustable tube or collapsible 

 bellows and in the eye it is composed of a 

 tough outer covering, the sclerotic coat and 

 a highly pigmented inner lining, the cho- 

 roid coat (Fig. 16-4). In the eye the space 

 behind the lens is filled with a semi-liquid 

 substance, the vitreous humor, and the cav- 

 ity in front of the lens is occupied by the 

 aqueous (watery) humor. These maintain 

 pressure within the eye and keep it from 

 collapsing. 



