Appendix A : The Microscope and Its Optical Principles 151 



etc. The illumination may be increased by means of a bull's eye con- 

 denser or a mirror. In some microscopes the mirror can be swung above 

 the stage for the purpose of illumining an object which is to be studied 

 by reflected light. 



The best light for microscopical work is light reflected from white 

 clouds. Direct sunlight is never used. The light should come from in 

 front of the observer or from one side. 

 Various kinds of artificial light are used for 

 microscopical work, such as an ordinary 

 lamp with flat wick, the Welsbach, or the 



FIG. 66. High-Power Dissecting 

 Lens, Bruecke Type. 



It may be used on the stand of 

 a dissecting microscope or in a 

 lens holder. 



FIG. 65. Stand for Dissecting Lens. 



ordinary electric light. The Welsbach is 

 perhaps the best. Whatever the source, the 

 rays must be steady and brilliant. If a 

 lamp with flat wick is used greater bril- 

 liancy is secured when the edge of the 

 flame is turned toward the microscope; the object should be lighted 

 directly by the image of the flame. To do this with low powers, the 

 lamp may have to be turned so that the flame is oblique to the microscope. 

 In artificial light the rays are divergent, not parallel as in the case 

 of sunlight, hence they will not come to focus at the same point when 

 reflected from the mirror as the latter do. This should be corrected by 

 using a large bull's eye condenser between the source of light and the 

 mirror, or by sliding the mirror along the mirror-bar farther away from 

 the stage so that the concave mirror will have a longer distance in which 

 to bring the rays to focus. If a substage condenser is used the same 

 results may be obtained by depressing the condenser somewhat below 

 the level of the stage. Lamps made for the microscope often have a 

 metal chimney with a bull's eye in one side. 



