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-glass 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. 
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