358 OSCAR W. RICHARDS 



the surface of the specimen polarizes light, or introduces optical path 

 differences, polarization or phase equipment assists in obtaining the 

 best contrast and detail in the image. The vertical illuminator is of 

 use only with specimens that have adequate reflecting properties 

 and has attained greatest use in metallurgical microscopy {1,3,4)- 



4. Epi-illumination 



The epi-illuminator has a condenser built around the objective, 

 which concentrates the light onto the specimen so that only the light 

 reflected from the specimen goes through the objective (Fig. 5). 

 Thus much less glare is involved and specimens with lower reflecting 

 power can be examined. Special, slender objectives are required and 

 these may be obtained for all magnifications. For a range of magni- 

 fications, however, several condensers may be required. Some of the 

 units are small enough to be used on a revolving nosepiece. Trans- 

 parent specimens are examined on a black glass or other opaque re- 

 flecting slide. Insect appendages, tissue circulation, and micro- 

 organisms have been studied in this way. When the optical appara- 

 tus is made of materials transparent to ultraviolet, the equipment 

 can be used for fluorescence and ultraviolet microscopy. Some skill 

 is required and the method has not yet come into general use {58,65, 

 66,68). 



E. SMALL AND ULTRAMICROSCOPIC SPECLVIENS THAT 



SCATTER LIGHT 



1. Darkfield Microscopy 



Light is concentrated at the specimen by the darkfield condenser 

 in the form of a hollow cone at such an angle that none enters the ob- 

 jective unless a specimen is present to change its direction (Figs. 

 6D, 7A). Small particles that scatter light are readily seen and ma- 

 terials with large optical path differences show better with darkfield 

 than those with small path differences. 



Two types of darkfield condenser are in general use, the bispheric 

 and the cardioid. Both have two spherical reflecting surfaces to 

 control the reflection and concentration of light. A nonspherical 

 cardioid shape for the outer reflector of the latter type would give 

 better concentration of light at the specimen, but the difficulties of 

 grinding such a shape have so far prevented commercial use of the 



