168 



SENSORIAL FUNCTIONS. 



ing 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. 



The amount of refraction being in proportion to 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. 



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 towards, and the other from a perpendicu- 

 lar 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. 



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 images 



What is the form of a double convex lens 1 In parallel rays what 

 part of a double convex lens has the greatest refractive power 7 Do both 

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



