DESCRIPTION OF THB ACHROMATIC MICROSCOPE. 125 



merits, unless an erecting eye-piece is applied, as men- 

 tioned in the following chapters. 



SECT. 7 Single Lenses mounted in imitation of Achro- 

 matic Object-glasses. A series of single lenses mounted 

 as object-glasses, and of the same foci and angular 

 aperture as achromatics, will prove an excellent 

 method of instituting a comparison between their relative 

 values. The object-glasses are the most costly portion 

 of an achromatic microscope, notwithstanding to ordi- 

 nary observers they appear not to differ exteriorly from 

 common lenses mounted in the manner described. Some 

 persons are very apt to imagine that the additional price 

 of the one above the other is owing to the achromatics 

 superior magnifying power ; but as this does not 

 necessarily follow, these single lenses will manifest at once 

 wherein the superiority consists. 



By way of trial, screw to the body of the microscope 

 one of the single lenses ; place an object under it, and 

 examine it with attention : then remove the single 

 lens, and substitute an achromatic object-glass in its 

 stead, allowing the object, eye-piece, and other portions 

 of the instrument, to remain as before. The difference 

 will be apparent enough in a few minutes to leave a full 

 and lasting impression upon the mind, of the extensive 

 improvement which has of late years taken place in this 

 respect. I know of no better mode than this by which 

 persons unacquainted with optical science can judge 

 fairly of what has been effected for the microscope. 



