108 BELL SYSTEM TECHNICAL JOURNAL 



of the carbons are due to lack of proper adjustment of apparatus or of 

 current and voltage or to the use of an unsatisfactory grade of carbons. 

 It may be of interest to know that the type of automatic arc kimps 

 used in the Bell System Laboratory are so steady and uniform in 

 their ()]X'ration that they occasion no concern except for the usual 

 maintenance. Since very small diameter carbons must be used to 

 appro.ximate the point source of light condition, these lamps will 

 operate continuously with one set of carbons for about thirty minutes 



^ % t • A 



6 





Fig. 7 — Condensers and illuniiiialors used in microscopy 



only. Frequently exposures are made at high powers (6,000 to 9,000 

 diameters) lasting from 45 seconds to 3 minutes, during which time 

 the carbons may feed several times and no ill effects result, so perfect 

 is the operation. 



Critical illumination is nothing more than bringing the rays of 

 light from the source of illumination to a state of proper focus and 

 optical alignment so that the surface of the specimen under exami- 

 nation will be uniformly and brilliantly illuminated. This matter 

 of securing uniform illumination is by no means the simple operation 

 that the designation implies since usually an optical train consisting 

 of the light source, condenser, diaphragms and an object illuminating 

 de\ice of some sort must all be brought into exact optical alignment 

 with the optical sN'stem of the microscope. 



For very low-power work or for gross jjliotography of specimens, 

 a gas-filled, metal filament lamp with a suitable condenser and mounted 

 on a portable pedestal which may be adjusted to all angles is very 

 useful. In this case the optical train is dispensed with and the light 

 thrown at the proper ant;le on the specimen to be photographed. 



