MICROSCOPE LAMPS 161 



obtaining .oblique illumination in the study of opaque objects 

 and as radiant for vertical illuminators. For use in this way the 

 cross- wire just outside the projection lens is removed as well as 

 the cross-wire diaphragm sliding into the tube. It sometimes 

 happens that owing to a drop in the voltage and a high resist- 

 ance of the " ballast " in the lamp, the heater will not raise the 

 glower to the necessary temperature to permit the passage of 

 the electric current. In such an event carefully unscrew the 

 lamp from the tube and hold a lighted match under the glower. 

 The glower will usually become incandescent and the lamp can 

 be screwed back in place. 



The chief difficulty encountered with single glower Nernst 

 lamps is the fact that the radiant is long and very narrow and 

 its image projected into the field fails to give uniform illumi- 

 nation 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 uniform 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 10 to 30 per cent, 

 according 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 lamps have round bulbs and are made for no 

 volt circuits in from 60 watt to 1000 watt sizes. A variety of 

 different housings are obtainable. Employed with screen and 

 suitable condensing lenses these lamps leave little to be desired 

 where a 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 excel- 

 lent results in illumination by transmitted light in the usual 

 manner by means of the microscope mirror or as a source of 



