XI. WHEN TO USE SPECIAL MICROSCOPES 373 



can be imaged in the microscope than of anj^ other shaped filaments 

 so that its ilkiminating efficiency is higher. Lamps of the same type 

 of filament with longer lives burn at lower temperatures and give both 

 yellower and less light than those burning at higher temperatures 

 with correspondingly shorter lives. 



The carbon arc is the brightest artificial source available and 

 suitable for microscopy. Its disadvantages stem from the frequent 

 carbon replacement and a certain amount of necessary continuous 

 adjustment. With automatic line voltage control and dry carbon 

 arcs much of this difficulty may be avoided. The zirconium arc is 

 less bright than the carbon arc and brighter than tungsten filament 

 lamps. The light from it is very uniform but special lenses are neces- 

 sary to magnify its small area and special equipment is required to 

 control the electrical supply. Heat-absorbing glass should be placed 

 between an intense source and the microscope to protect living or- 

 ganisms. 



2. Accessory and S}3ecial Eqiiipnieiit 



A back surface plane mirror gives two images, a weak one from 

 the front glass surface and a strong one from the silvered surface. 

 When this double image is objectionable in critical microscopy, it can 

 be avoided by using a prism, or a front-surfaced mirror, mounted in 

 place of the usual mirror. 



The concave mirror, used without a condenser, does not give a 

 wide enough cone of light to illuminate fully the aperture of other than 

 low power objectives. A condenser and the plane mirror should be 

 used with all objectives having a numerical aperture greater than 

 0.25. Unless the aperture of the objective is filled with light it will 

 not resolve the optimum detail otherwise possible. The better con- 

 densers are corrected for aplanatism (coma and spherical aberration) 

 and for chromatic aberration. Corrected condensers should be pro- 

 vided with centering mounts, unless these are built into the micro- 

 scope substage. 



Dry objectives of large numerical aperture are sensitive to spheri- 

 cal aberration arising from cover glasses of other than the standard 

 0.18 mm. thickness. They should have a correcting collar or be used 

 with an adjustable draw tube, or with a tube length corrector. Im- 

 mersion lenses are less sensitive to cover glass thickness because of the 

 homogeneous oil contact, but the dispersion of the cover glass may 



