MICROSCOPES FOR USE IN CHEMICAL LABORATORIES 23 



be found a great saver of time, labor and material. Its appli- 

 cations are many. In laboratory work involving the study of 

 plates of bacterial cultures it will be found to be far superior 

 to microscopes of the ordinary type, since plates of large size 

 may be examined at any point within their areas. 



The compound microscope is provided with rack and pinion 

 coarse adjustment and with a quick acting screw adapter F 

 fitted to the end of the body tube for fine adjustment. 



Comparison Microscopes. It not infrequently happens that 

 it is found desirable to carefully compare two preparations or 

 two different samples. This is especially true in quantitative 

 microscopy. With ordinary microscopes it is necessary to place 

 first one sample, then the other, under the microscope, make 

 drawings, measurements and take mental note of the appearance 

 of each preparation in turn and then compare the mental pictures 

 by the aid of the data at hand. This process is not easy, and the 

 results not always trustworthy even in the hands of an expert 

 without long and exceptionally thorough studies. Photomicrog- 

 raphy offers a fair solution but here again the time required 

 and the additional manipulations necessitated prevent its general 

 application. 



This need of some device whereby quick and rapid compari- 

 sons might be possible has long been felt, but no suitable instru- 

 ments were placed upon the market until very recently. These 

 new instruments have received the name Comparison Micro- 

 scopes. They are so constructed that the images formed by 

 two different optical systems are brought into juxtaposition, 

 so that the observer is able to simultaneously see the images of 

 two different objects. 



As long ago as 1885, Inostranzeff 1 employed what he desig- 

 nated as a comparison chamber, consisting of two sets of totally 

 reflecting prisms so mounted in a rectangular chamber as to 

 reflect, into a single eyepiece, the images of half the field of each 

 of two microscopes. 



Two years later Van Heurck 2 improved the Inostranzeff in- 



1 Jahrb. f. Min., 2 (1885), 94; J. Roy. Micros. Soc., 1886, 507. 



2 Van Heurck, J. Roy. Micros. Soc., 1887, 463. 



