USE OF THE COM/'OrXI) M /('ROSCO/'E 



27 



condensers, the aperture as defined above is to be multiplied 

 by the index of refraction of the substance between the 

 front lens and the object; and, in the case of several such 

 substances, is kept to the limit of the lowest such refractive 

 index.) 



Classification of ITseful Magnifications in the Compound 



Microscope 



Hence low-power microscopes in this scheme are those 

 that have useful magnifications reaching to 100 times, and 

 working apertures (which at these low powers are usually 

 identical with objective apertures) reaching at least to 

 0.1. Such microscopes are: the monocular microscope, 

 used without a condenser, and with objective magnifica- 

 tions from 2 to 8; the monobjective binocular, used similarly; 

 and the popular twin-objective (Greenough) binocular, 

 which has paired objectives of a single doublet or triplet 

 each. 



Medium-power microscopes have useful magnifications 

 up to 700 times, and working apertures up to 0.7. They 

 are either monocular or monobjective binocular. They 

 have, usually, dry objectives with initial magnifications 

 from 8 to 40, and apertures from 0. 15 to 0.75. An achroma- 



