136 ELEMENTARY CHEMICAL MICROSCOPY 



unless great care is taken in adjusting the distance of radiant, 

 condensing lenses, diaphragms, etc. Multiple glower lamps are 

 far superior in this respect. Unfortunately they are so fragile 

 and require such care in handling as to render them expensive 

 and therefore impracticable for the average chemical laboratory. 



To obtain a uniformly illuminated field with single glower 

 Nernst lamps recourse must be had to a screen of ground-glass. 

 This causes a diffusion and softening of the light, but greatly 

 reduces its intensity, the loss being from 30 to 60 per cent, accord- 

 ing to the thickness and nature of the glass and the character 

 of the ground surface. 



The most satisfactory electric lamps for general purposes now 

 available are Mazda projection lamps with concentrated fila- 

 ments. These General Electric Company lamps have round 

 bulbs and are made for no volt circuits in 100 watt and 60 watt 

 sizes. 1 They may be obtained with plain glass bulbs or with 

 frosted glass having a circular unfrosted window. On the Cor- 

 nell University lighting circuit the 100 watt lamps have ap- 

 proximately 65 and the 60 watt lamps 49 candle power. Em- 

 ployed with screen and suitable condensing lenses these lamps 

 leave little to be desired where a moderately powerful radiant is 

 required. The tungsten filament will stand rougher treatment 

 than Nernst glowers and is not subject to burning out through 

 short circuit. They yield excellent results in illumination by 

 transmitted light in the usual manner by means of the micro- 

 scope mirror or as a source of light in dark-ground illumination 

 or with vertical illuminators, but for oblique reflected light in 

 the study of opaque objects, the size of the lamp bulb and the 

 position of the tungsten filament renders the lamp and condens- 

 ers somewhat clumsy and apt to be in the way. Fig. 82 shows 

 the construction of these lamps. They are now supplied by the 

 Bausch & Lomb Optical Company with the same style of stand 

 as shown in Fig. 81. Because of the very little difference in the 

 candle powers of the two lamps, it will be found that in general 

 the 60 watt lamp is more convenient on account of its smaller 



1 Since the above was written, 200, 40x5 and 500 watt nitrogen filled concen- 

 trated filament tungsten lamps have been placed upon the market. 



