100 THE USE OF THE MICROSCOPE 



a flat double-ground glass near the iris of the condenser 

 (Hartridge). But he should replace this condenser, if 

 possible, by a dry long-focus achromatic condenser giving 

 large images (Leitz, Zeiss, Spencer, Watson). In this case 

 he should use an electric lamp at the proper distance, with a 

 double-ground glass disc (or two singly ground superposed) , 

 and a diaphragm in front of it. A yellow-green light filter 

 will markedly improve his achromatic objectives. The use 

 of the oil-immersion objective of 3.5-millimeter focus, and 

 0.85 aperture (or other similar objective), is of service 

 in some cases where the 2-millimeter (or the l.S-milHmeter) 

 objective is used; and the former objective is more com- 

 fortable to work with. Finally it may be recommended 

 that, however experienced the microscopist, the compensa- 

 tion of the unoccupied eye by a disc of ground glass may add 

 to his comfort when using the monocular. 



Summary. — For careful routine work, the writer recom- 

 mends the use of a dry achromatic condenser corrected 

 for 25-centimeter lamp distance and 1.0-millimeter slides. 

 It is also thought that the low oil-immersion objective of 

 0.85, or somewhat higher, aperture may often replace the 

 dry 4-millimeter objective with better results. A screw 

 cover-glass gage should be available, and only No. 1 

 covers should usually be obtained. Covers of 0.17 milli- 

 meter should be put on important objects. For routine 

 and research, the monobjective binocular is unsurpassed. 

 For research, the use of yellow-green light, and a corrected 

 water-immersion condenser giving at least a cone of 1.2 

 aperture, is dwelt on. Water-immersion objectives are 

 regarded as worth using with objects in water. It is usually 

 best to replace the plane mirror by a prism. The use of 

 the apochromatic objective 60, of 1.3 or 1.4 aperture with 

 eyepiece 20, has some advantages over the usual use of the 

 apochromatic 90 with eyepiece 15. 



