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 lo 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 
