MICROSCOPES FOR USE IN CHEMICAL LABORATORIES 65 
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^ 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^ improved the Inostranzeff in- 
* Jahrb. f. Min., 2 (1885), 94; J. Roy. Micros. Soc., 1886, 507. 
^ Van Heurck, J. Roy. Micros. Soc., 1887, 463. 
