PHOTOMICROGRAPHY AND TECHNICAL MICROSCOPY 793 



For preliminary adjustment a 20 X achromat objective and a 5 X Huyghens glass 

 ocular are fitted to the microscope. The substage condenser, with front lens removed 

 and with the condenser adjusted centrally in its centering mount, is inserted in the 

 substage holding ring. Light from the mercury-vapor lamp is then reflected into the 

 base of the microscope. A suitable specimen is placed on the microscope stage and 

 the microscope focused in the usual way. The condenser is focused to bring the field- 

 of-view diaphragm into focus. When the diaphragm is partly closed, the bright 

 circular area should be centered by turning the microscope prism and by adjusting the 

 leveling screws of the optical bench of the spark-generating apparatus. The uranium 

 glass disk is now substituted in place of the substage condenser; the reflecting mirror in 

 front of the prism is swimg out of position; and the mercury-vapor lamp is extinguished. 



Fig. 21. — Ultraviolet microscope fitted with graduated half circle, graduated slider, 

 and the aluminum pointer attached to the slow-motion thumb-screw. In this particular 

 assembly, 1° on the protractor corresponds to a change in focal planes of }^ ^l. A sensitivity 

 of }/2° in adjustment may easily be attained. By means of a different slow-motion 

 mechanism a spacing in focal planes of about 3^1 6 /" may be secured, although a spacing of 

 J^ fx seems adequate for most work. 



The spark is generated with cadmium electrodes and the 2750 A slit image sharply 

 focused on the uranium glass disk, as indicated by a small circle marked on the disk. 

 The spark is now turned off and a low-power quartz objective and quartz ocular of 

 5 X or 10 X power substituted for the glass optical parts. The mercury-vapor lamp 

 is again used as the illuminant. The specimen, mounted on a quartz slide and covered 

 with a quartz slip, is placed on the stage and focused with the visible light. The 

 apparatus is now ready for final adjustment with the ultraviolet light. The searcher 

 eyepiece is swung around into position over the ocular. This searcher eyepiece con- 

 sists of a uranium glass wedge on the surface of which are ruled two cross lines. This 

 is the surface on which the image fluoresces, and the image is observed by a small 

 a(j[justable magnifier which forms part of the searcher eyepiece. The image, as it 

 appears on the uranium glass wedge, is very small and so must be enlarged by the 

 magnifier to make even the gross details of structure visible. The magnifier is 



