200 THE USE OF THE MICROSCOPE 



time abundantly rewarded. These objectives also contain 

 fluorite, which has special properties of refraction and 

 dispersion that the experimental glass works had in vain 

 tried to imitate in useful glasses. (Fluorite had been 

 previously used by Abbe in his high-power achromatic 

 objectives.) The apochromatic objectives are more costly, 

 not only because they contain fluorite lenses, and this 

 crystal is rather rare in a state fit for lenses ; but also because 

 they have more components, usually containing one or 

 more triplets. (Some excellent modern apochromatic 

 objectives are stated to contain alum or sodium nitrate 

 lenses.) Modern achromatic objectives have been so 

 improved by the introduction of new glasses, that Spitta 

 classed them all as semi-apochromatic — a term usually 

 restricted to fluorite combinations. It must be remem- 

 bered that there are corrections in the apochromatic 

 objectives for zonal spherical aberration of different 

 colors, which cannot be had in the semi-apochromatic or 

 fluorite objectives. (But, by the use of a yellow-green screen, 

 achromatic and fluorite objectives can be much improved.) 



At the same time Abbe put some of the needed corrections 

 into the eyepiece, forming the well-known compensating 

 oculars for apochromatic objectives. These corrections 

 were also useful for the high-power achromatic objectives, 

 but not for those below 0.8 aperture. 



A list of Abbe's inventions and discoveries with regard 

 to the microscope would be large. He discountenanced 

 the patenting of microscopical inventions (though he 

 freely patented his field glasses, etc.) because they were 

 mostly used by scientific men. His example was followed 

 by others. 



In all, there appear to have been two great inventors 

 in the history of the achromatic microscope, Abbe and 

 Lister; and of these Abbe is undoubtedly the chief. Abbe 

 naturally made a few slight errors of prevision; and piety 

 to his memory does not demand that one should also 

 countenance these errors, which, in fact, only the change of 

 conditions has caused to be errors. 



