MICROSCOPES FOR USE IN CHEMICAL LABORATORIES 67 
which reflect the images, formed by the objectives of the micro¬ 
scope, into the rectangular prisms R^, R^, situated just below 
the ocular 0 . The prisms R^, R^ consist of rectangular pieces 
of glass cut through diagonally and cemented together, the 
inclination of the cut surfaces being parallel to the reflecting 
surfaces of P^, P^, respectively. Upon looking into the ocular O 
the field is seen to be divided into an upper and a lower part by a 
line passing from left to right. It is obvious that the image of 
half the field of one microscope will be seen in one of the halves 
of the ocular, while the other half of the ocular will exhibit half 
the field of the other microscope. In order to facilitate focusing 
the microscopes the tube is of such diameter as to fit snugly 
into the tube of one of the microscopes, while the tube is of 
less diameter and hence fits loosely. The microscope carrying 
T^ is therefore focused first. Objects to be carefully compared 
by means of this instrument must necessarily lie in the same 
plane, otherwise the magnification in one half-field will be greater 
than in the other. Where slight variations in magnification 
can be neglected, the thicker preparation is placed upon the 
stage of the microscope carrying the tight tube of the comparison 
eyepiece, or if chemical microscopes (Fig. 25, page 62) are em¬ 
ployed, one or both preparations may be supported upon the 
auxiliary stage and turned down until the upper surfaces of the 
two preparations lie in the same plane. This, however, is only 
possible when no substage condenser need be employed. 
Comparison microscopes proper are of two different t3^es, 
either they have a single eyepiece and make use of reflecting 
prisms or they consist of two microscopes with two eyepieces, 
the observer using both eyes. 
The Leitz^ comparison microscope. Fig. 28, consists of two 
microscope tubes A, B attached to a single pillar P movable by 
rack and pinion. A single stage S is provided with two open¬ 
ings, one for each microscope tube. Under each stage opening 
is placed an Abbe condenser with iris diaphragm and rings 
for stops, or for blue, green or ground glass. Each condenser is 
illuminated by means of a separate mirror on a swinging bar and 
^ Manufactured by E. Leitz, Wetzlar, Germany. 
