USEFUL MICROSCOPE ACCESSORIES 133 



inexpensive but very convenient type of more powerful arc lamp 1 

 is shown in partial section. 



Arc lamps for microscopic illumination should always have 



FIG. 77. Microscope Lamp; Bausch & Lomb. Arc Type. 



their carbons at right angles, or approximately so. Direct cur- 

 rent arcs are far better than alternating current. The horizontal 

 carbon should be the positive pole and the carbons should be soft 



FIG. 78. Microscope Lamp; Spencer Lens Co. Arc Type. 



cored. By this means the crater is maintained at a fixed point 

 and the condensing lenses of lamps or of special stands will 

 project an image of the crater upon the microscope mirror or 



1 Sold by Wm. Gaertner & Co., Chicago, 111. 



