20 THE GENERAL PRINCIPLES OF THE METHOD 



This work consists largely of cytology, requiring 

 accurate observation as to details, and the highest 

 powers of magnification. Any good microscope stand 

 will do, but we think that the English tripod one 

 is the best, especially if the special photomicrographic 

 apparatus is adopted, in which case it is almost es- 

 sential. The larger and heavier the stand the better. 

 It must have a mechanical stage, which should be built 

 with the instrument; not an "attachable" one. The 

 lenses must give good definition. Two objectives 

 only are necessary a sixth-inch, and an immersion 

 twelfth. We use equivalents of these in a Zeiss D, 

 and a Zeiss 2-mm. apochromatic lens, which is com- 

 pensated for the long-draw-tube of 250 mm., and which 

 has a numerical aperture of 1 . 30. There is no doubt 

 that an apochromatic objective for this work is vastly 

 superior to an ordinary twelfth-inch lens, especially if 

 photography is to be used. 



The eye-pieces we employ are the No. 4 and No. 8 

 Zeiss compensated ones, and these, or their equiva- 

 lents, will be found most useful. 



The light should always be artificial; daylight is 

 not suitable for this method. We have found that the 

 inverted incandescent gas-burner gives the best light 

 for ordinary work, or if electricity is preferred, the 

 1 -ampere Nernst lamp is most suitable. If neither 

 gas nor electricity are available, the spirit-lamps which 

 give a light by heating an inverted mantle have proved 

 most suitable in our hands. No matter which light is 

 used, it is better always to use the same, in order that 

 contrasts may be detected readily. 



