MICROSCOPES FOR USE IN CHEMICAL LABORATORIES 67 



which reflect the images, formed by the objectives of the micro- 

 scope, into the rectangular prisms R 1 , R 2 , situated just below 

 the ocular 0. The prisms R 1 , R 2 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 1 , P 2 , 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 T 1 is of such diameter as to fit snugly 

 into the tube of one of the microscopes, while the tube T 2 is of 

 less diameter and hence fits loosely. The microscope carrying 

 T 1 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 types, 

 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 1 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 

 1 Manufactured by E. Leitz, Wetzlar, Germany. 



