USEFUL MICROSCOPE ACCESSORIES 



135 



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 care- 

 fully 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 FIG. 80. Galvanometer Lamp of the 



in place. Cambridge Scientific Instrument 



A more powerful Nernst lamp Co ' Nemst Type " 

 is shown in Fig. 81. This lamp 1 is intended primarily for use 

 with dark-ground illuminators. As supplied by the dealers this 



FIG. 81. Microscope Lamp. Nernst Type. 



lamp has a ground-glass globe. A more convenient arrangement 

 consists in substituting for the globe, a tin shield as illustrated. 

 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 illumination 



1 Sold by Bausch & Lomb Optical Company, Rochester, N. Y. 



