The Use of the Microscope 



31 



Fig. 35. A research microscope and suitable illuminant. The microscope is an Ameri- 

 can Optical Company model No. 5 and the illuminant is an American Optical 

 (formerly Silge and Kuhne) Ortho-Illuminator. 



turret, which should carry four lenses, is attached to the body, not to 

 the tube. 



Stage. The stage of a research microscope is both rotating and center- 

 able. The first of these features is highly desirable since it permits the 

 orientation of material both for observation and photography. The second 

 feature not only permits the stage to be centered but also serves as a 

 fine adjustment to the rectangular rack-and-pinion movement of the 

 built-in mechanical stage. It is quite common to have vernier gradua- 

 tions for both the rectangular and rotating movements, but these are 

 of more interest to geologists and metallographists, than to biologists. 



Substage. The substages of research microscopes are extremely com- 

 plicated and consist of the following parts: (1) a device to permit sub- 

 stage condensers to be interchanged, centered, and focused; (2) an iris 

 diaphragm, either mounted to permit lateral motion or supplemented 

 by a separate lateral diaphragm; and (3) some fitting, which will permit 

 a supplementary lens to be placed under the condenser when low-power 

 objectives are to be used. 



The two main methods of meeting these requirements are shown in 

 Figs. 36 through 39. Figures 36 and 37 show the substage of a Bausch 

 and Lomb DDE stand. The condenser, about to be inserted in Fig. 36, 

 is mounted on a rectangular plate that slides in a dovetail slot. The iris 

 diaphragm is mounted on a swing-out arm and has been swung out in 



