132 ELEMENTARY CHEMICAL MICROSCOPY 



addition an adjustable back the added comfort thus secured 

 cannot be overestimated. 



Radiants for Microscopic Illumination. The modern micro- 

 chemical laboratory employs as sources of artificial light for 

 microscopic illumination the electric current or the acetylene 

 light. Gas light illumination, using Welsbach mantles, made 

 incandescent by coal gas, alcohol or gasoline vapors, have al- 

 ready become radiants of the past, and the oil lamp is now so 

 very rarely used as to need no comment. If Welsbach lights 

 must be employed owing to lack of electric current or calcium 

 carbide, preference should be given to lamps of the inverted 

 mantle type. 



Cylinders containing compressed acetylene gas are now so 

 widely distributed and the gas relatively so inexpensive (exclud- 

 ing the first cost of the container) that few investigators will care 

 to be bothered with carbide gas generators. A piece of thin 

 faintly blue glass placed between the acetylene flame and the 

 mirror of the microscope yields light approximately equivalent 

 to daylight, so far as color values are concerned. 1 



The development of dark-ground and of vertical illuminators 

 and their applications has been accompanied by a corresponding 

 improvement in electric lamps. These now fall in one of several 

 groups: carbon arc lamps, Nernst glower lamps, tungsten 

 filament incandescent lamps or mercury vapor lamps. 



Ordinary microscopic work rarely requires an arc lamp draw- 

 ing a current of more than 4 or 6 amperes, but for ultramicro- 

 scopic investigations an arc of 15 to 30 amperes is desirable and 

 in many instances absolutely essential. Many styles of con- 

 struction are found on the market. Several typical lamps are 

 here illustrated. Fig. 77 shows the 4 ampere hand-feed arc 

 lamp of the Bausch and Lomb Optical Company; Fig. 78 that 

 of the Spencer Lens Company; and Fig. 79, the automatic 4 to 

 5 ampere lamp as manufactured by E. Leitz. In Fig. 24 an 



1 Wright, Artificial Daylight, Amer. J. Sci. (4) 27 (1909), 98. Quite re- 

 cently the Corning Glass Works of Corning, N.Y., has perfected a combi- 

 nation of glasses such that, when employed with large tungsten lamps, true 

 artificial daylight is obtainable as shown by spectroscopic tests. 



