THE MICROSCOPE. 



glass and the eye-glass, neither of which are achromatic, the 

 image viewed by the observer through the eye-glass must be 

 affected by as much positive aberration as is due to the combination 

 of the field-glass and the eye-glass. 



This defect might, it is true, be remedied by making both 

 the field-glass and eye-g\ass achromatic ; but independently of 

 other objections to such an expedient, it would be needlessly 

 expensive ; and the same purpose is attained in a much more 

 simple manner, upon the principles of positive and negative 

 chromatic aberrations, which have just been explained. 



The method practised for this purpose may be briefly and gene- 

 rally explained thus : The field-glass and the eye-glass being 

 simple convex lenses, produce positive chromatic aberration. The 

 object-glass, on the other hand, being a compound lens, may be 

 so constructed, according to what has been just explained, as to 

 produce negative chromatic aberration. 



1G 



