196 PHOTOMICROGRAPHY 



made by Zeiss at Fig. 63. A beam of light is projected 

 through the opening p, and reflected by the prism through 

 half of the objective, while the image-forming rays pass up 

 through the other half. It will be noticed that the ob- 

 jective mount in the figure is very short, reflections at 

 the back lens are thereby minimised. The makers also 

 supply stops suited to different powers, which slide into 

 the mount between the prism and objective for the same 



Fig. 63 



PRISM VERTICAL ILLUMINATOR 



purpose. The prism gives a more brilliant illumination 

 than the cover-glass reflector, and can consequently be 

 used with a less intense illuminant or a higher power 

 objective. There is also less haze over^the image of a 

 highly polished metal surface, but the resulting photo- 

 micrograph is liable to show an embossed structure in 

 which the crystals stand out as a slightly raised pattern, 

 in centra-distinction to the smooth surface represented 

 when the cover -glass illuminator is used. 



Critical illumination is obtained in a manner analogous 

 to that employed for transparent objects. The lamp 

 is placed about two feet from the microscope, and a 

 condenser, preferably with iris, placed in such a position 



