CH. IX] 



HIGH POWER MICRO-PROJECTION 



273 



that the cone of light is converging to the focal point and diverging 

 beyond that point (fig. 122, 132 and 320-323). If the converging 

 cone is used the substage condenser brings it to a focus too soon 

 and if the diverging cone, then the substage condenser brings it 

 to a focus too far beyond it. 



402. Methods of rendering converging or diverging light 

 parallel. There are two principal ways of utilizing the light cone 

 from the main condenser. 



Object 



FIG. 1506. ABBE SUBSTAGE CONDENSER SHOWING PARALLEL AND 

 CONVERGING INCIDENT LIGHT. 



In this form of condenser the iris diaphragm is below both condenser lenses 

 (compare fig. 150). 



With parallel, incident light the condenser focuses the light just above the 

 condenser, with converging light the focus is within the upper lens and the 

 light is diverging on leaving the upper lens. 



o, o Object. 



Objective The front lens of the projection objective. 



A. Rendering the converging cone of light approximately 

 parallel by means of a concave lens. As it is desirable to use all the 

 light in the cone, the concave lens is put in the cone where its 

 diameter is slightly less than the diameter of the substage con- 

 denser, that is about 25 mm. (i in.). The trial glasses used by the 

 oculist are excellent for the purpose. A fork with stem is desirable, 

 and this is placed in the socket for the mirror stem. This brings 

 the fork carrying the spectacle lens near the substage condenser. 

 Concave spectacle lenses of 10 to 20 diopters (100 to 50 mm., 4 to 2 

 in. focus) have been found excellent. The microscope for projec- 

 tion is so placed that the fork carrying the concave lens is about 



