SENSE ORGANS. IO n 



proper place. Now if the images of all objects at all 

 distances were thrown to the same place, we might find 

 that place and fix the screen permanently there. But 

 we have already seen (page 105) that as the object is 

 farther away the image comes nearer the lens, and as 

 the object approaches the image recedes. Now there 

 are two ways of adjusting this : if the lens retains its 

 form then the screen must be moved "back and forth to 

 the proper place, or else, if the screen be fixed, the lens 

 must change its form so as to throw the image on the 

 fixed screen — i. e., it must become more refractive as the 

 object comes nearer. The former is the method of the 

 camera and of nearly all optic instruments, such as the 

 opera glass, the field glass, etc. In these the tube is 

 drawn out so as to carry the screen back for near ob- 

 jects, and is pushed in so as to carry the screen nearer 

 the lens for distant objects. The microscope is an ex- 

 ception. Usually, when the object is brought near (as 



bQ 



^^ 



Fig. 77. — A, eye observed ; B, eye of observer ; C, section of candle flame ; 

 /, a distant point of sight, and n, a near point of sight. (After Helm- 

 holtz.) 



in magnifying greatly) the lens is changed and the image 

 is thrown to the same place. 



Now in the eye the adjustment for distance is per- 

 fect, for objects at all distances from five or six inches 

 to infinite distance ; for the moon or sun is seen perfectly 

 defined. How is it done ? It is done by changing the 



