278 HIGH POWER MICRO-PROJECTION [Ca. IX 



brilliant pictures can be produced by using also a parallelizing lens 

 as indicated in 402 A. 



If one has an optic bench apparatus (fig. 121, 158, 159) one can 

 get good results with the condensers of all foci by placing the 

 microscope so that a diverging cone of light enters the substage 

 condenser (fig. 1546). It will then be necessary to lower the 

 substage condenser slightly for the higher powers. 



403. Kohler method of using the substage condenser. The 



general principle is shown in fig. 170. The microscope is moved 

 toward the main condenser until the focus is at the iris diaphragm. 

 One can tell when the main condenser is focused on the iris dia- 

 phragm in the same way as that in focusing on the black hood of the 

 objective ( 375) viz., by noting when the image of the crater and 

 the tip of the lower carbon appear on the iris. After the image is 

 focused on the iris diaphragm the iris is opened to admit the cone 

 of light, and the substage condenser is raised or lowered slightly to 

 get the most brilliant light. As one can see by the diagrams of 

 light cones and the plates of the light rays and the light cones, the 

 light is diverging beyond the focus so that diverging and not 

 parallel light enters the substage condenser. As the condenser 

 cannot focus diverging light at the same level that it would focus 

 parallel light it may be necessary to lower the substage condenser 

 somewhat to get the most brilliant image with high powers. Fur- 

 thermore, if a concave lens of 10 to 20 diopters is put in the fork as 

 described in 402 A the image will be markedly brighter unless a 

 very long focus main condenser is used (fig. 171). (See also Ch. 

 XIV, 864). 



404. Aperture of the substage condenser. The purpose of 

 the substage condenser in projection, as in direct observation 

 with the microscope, is to increase the aperture of the illuminating 

 cone. And as it is now one of the fundamental doctrines, that the 

 resolution or making visible of minute details depends directly 

 upon the aperture of the objective used, naturally as much as 

 possible of the aperture of the objective is employed. For this, 

 the substage condenser diaphragm should be wide open, so that the 



