12 



THE MICKOSCOPE. 



ities of the object. The central pencil, it is obvious, would 

 pass both .lenses symmetrically; the same portions of light 

 occupying nearly the same relative places on both lenses. The 

 blue light would enter the second lens nearer to its axis than 

 the red, and being thus less refracted than the red by the 



second lens, a small amount of compensation would take place, 

 quite different in principle and inferior in degree to that which 

 is produced in the eccentrical pencils. In the intermediate 

 spaces the corrections are still more imperfect and uncertain; 

 and this explains the cause of the aberrations which must of 

 necessity exist even in the best-made doublet. It is, however, 

 infinitely superior to a single lens, and will transmit a pencil of 

 an angle of from 35 to 50 without any very sensible errors. 



