MICROSCOPICAL DIAGNOSIS. 



imperfection in the objective is exaggerated by using "deep" eye- 

 pieces, so any imperfection, in the denning power can be thoroughly 

 tested in this way; for an image wtrch appears clear and distinct 

 with a low eye-piece may be found to be very deficient under higher 

 amplification. 



Spherical aberration exists when the peripheral and central rays 

 do not actually reunite in a single point. Those rays passing near 

 the periphery, being more strongly refracted, come to a focus sooner 

 than those which pass through the more central portions. Now if 



Fig. j. Microtome. (R. & J. Beck.} 



some parts of a lens bring the rays to a focus sooner than other 

 parts, then the lens will magnify unequally, thereby distorting the 

 figure. This distortion is present with all objectives not properly 

 corrected, causing what is known as "aberration of form." 



Chromatic aberration exists when a ray of light is not refracted 

 as a whole, but is decomposed into rays of various colors, which 

 being refracted to different degrees, give rise to a spectrum. 



All objects examined now. are seen fringed with colors. An 

 objective is called "achromatic" when it is free or nearly free from 

 this aberration. It is impossible to perfectly remedy these two 



