164 SENSORLAL FUNCTIONS. 



passing through it, from the air, to a point, or focus, it is 

 plain that two sections of a solid transparent sphere, with 

 their plane faces applied to each other, is one of the forms 

 which is indicated. This form, made of solid glass, is 

 called a double-convex lens, and which corresponds very 

 nearly to the shape of the crystalline lens in the human 

 eye. 



495. The amount of refraction being in proportion tc 

 the obliquity of the refracting surface, if the rays of light 

 proceed in parallel lines, then the refractive power will be 

 greatest at the greatest distance from the central ray, and 

 thus all will be concentrated at the same point. 



496. It will be observed on inspecting the lens, a. Fig 

 102, and comparing its form with the law of refraction 



Fig. 102. 



above stated, that both sides of a sphere conspire to bring 

 parallel rays of light to a focus ; the first side by turning 

 them toward, and the other from a perpendicular raised 

 from their respective surfaces of refraction. After the 

 rays have been made to converge to a focus, e, they cross 

 each other, and again diverge from that point, as shown 

 by the figure. 



497. It will be seen in the sequel, that the lens of the 

 eye operates precisely on the rays of light, like the convex 

 lens- just described, only that the former is a more per- 

 fect instrument than any which can be constructed by 

 the hand of man. But before we proceed to describe 

 the manner in which vision is performed, we will show 

 by a very simple experiment, in what manner the ima- 



What is the form of a double-convex lens ? In parallel rays what part 

 of a double-convex lens has the greatest refractive power ? Do both sides 

 of the lens conspire to form the focus, or not ? 



