168 



SATURDAY LECTURES. 



are two forms of deflection. In one the light is thrown 

 back by the surface of the interposing body in the direction 

 from which it came ; that is to say, it is reflected. In the 

 other it passes through the body, but its course after its 

 passage is not the same as before ; in which case it is said 

 to be refracted. It is with this latter we have especially to 

 do in treating of the physiology of vision. Without enter- 

 ing into a detailed consideration of all the laws of refrac- 

 tion, we will state that when the refracting body has a cer- 

 tain form, which we call a lens, all the rays coming from 

 any one point of an object are, after refraction, brought to- 

 gether in another point. Such a lens is shown in the ac- 

 companying figure. 



Fig. I. 



All rays proceeding from the point A are, after their pass- 

 age through the lens, united in the point a, and all rays 

 proceeding from the point B are, after refraction, united in 

 the point b, and raj's proceeding from all points between A 

 and B are united at points between a and h. The result of 

 this is that in ah we have an image of the object AB. There 

 are two peculiarities of this image to which I want to call 

 your attention. The first is that it is much smaller than 

 the object AB, and the second is that it is turned upside 

 down. Xow, when we come to regard the eye as an optical 

 instrument, we will find it to differ in principle in no essen- 

 tial particular from this simple lens. The whole function 



