32 CONSTRUCTION OF THE MICROSCOPE. 



the object, p a portion of the cornea of the eye, and d 

 the stop, or limiting aperture. 



Fig. 20. 



Now it will be observed that each of the pencils of light 

 from the extremities 1 1 1 of the object is rendered exccntri- 

 cal by the stop ; consequently, each passes through the two 

 lenses on opposite sides of their common axis op; thus 

 <3ach becomes affected by opposite errors, which to some ex- 

 tent balance and correct each other. To take the pencil I, 

 for instance, which enters the eye at r 6, r b : it is bent to 

 the right at the first lens, and to the left at the second ; 

 and as each bending alters the direction of the blue rays 

 more than the red, and moreover as the blue rays fail 



