'Note. 



15 



in the openings b, b, closed by tubes sliding within the larger 

 Microscope tubes. In order to regulate the intensity of the light, 

 an iris diaphragm is interposed in the left-hand tube at c. The 

 prism-box contains a pair of reflecting rhombs, and the course of 

 the rays is shown by the dotted lines. Thus the effect is to 



Fig. 3. 



appose two images which readily lend themselves to comparison. 

 Fig. 3 shows the apparatus fitted to a Spencer Lens Co.'s stand. 

 With the exception of the stand, the residue of the outfit was 

 made by Dr. Ewell in his amateur shop. 



The general idea recalls Inostranzeff s comparison chamber or 



