CHAP, in.] THE MICROSCOPE. 241 



Hint and a double-convex crown glass ; but with the very highest 

 powers various combinations of lenses of those two kinds of glass 

 are now used, some to magnify and others to correct the errors,, that, 



FIG. 178. Cainpani's achromatic eye-piece. 



after passing through the compound eye-piece, the image may be 

 approximately free from chromatic and spherical aberration. The 

 accuracy of workmanship necessary to accomplish this is so great, 

 that satisfactory results can only be obtained by repeated trials, and 

 by what may often be called accidental good fortune. 



The magnifying power given by the compound microscope is a 

 combination of the magnifying power of the object-glass multiplied 

 by that of the eye-piece. Let us suppose the real image furnished by 

 the first system magnified twenty times ; if the eye-piece magnified it 

 again five times, it is evident that the total magnification will be 

 100 times. 



In this it must be well understood that we refer only to linear 

 dimensions or to diameters. Superficial magnification is evidently 

 equal to the square of this. Thus if your object has been magnified 

 50, 100, or 500 diameters, the surface of the object has been magnified 

 2500, 10,000, 250,000 times, but no practical microscopist thinks of 

 expressing his results in any other terms than linear magnifying power. 



According to M. Arthur Chevallier. compound micro- 

 scopes are now constructed with optical systems, divided 

 into nine series according to the magnitude, from number 

 1, which gives a power from 25 to 50 diameters, to number 

 9, which magnifies from 600 to 1,300 times. With this last 

 magnification, the surfaces are multiplied by the enormous 

 number, 1,690,000. It is, therefore, possible to examine por- FIG. 179 

 tions of matter of the size of the thousandth part of a millimetre. 

 ?)iit it must not be forgotten that the art of using a microscope 



