THE ROUTINE MICROSCOPE 63 



about 100 watts and 110 volts. A large screen of thin 

 metal should be placed before it, with a circular centered 

 opening corresponding to the size of the source-field of 

 the objective 10 with the most-used eyepiece; and with 

 25 centimeters between the lamp and the condenser. Two 

 other diaphragms may be pivoted on, corresponding to 

 the source-fields of the two higher objectives. The bulb 

 of the lamp should be centered near the diaphragms. 

 With ground glass and a lamp of 150 watts, a magnifica- 

 tion of 1,250 can be sufficiently illuminated on the binocular, 

 with the objective 100, of 1.3 aperture, and an achromatic 

 water-immersed condenser. 



Triple (or quadruple) nosepieces, taking in all the 

 objectives, should be employed. Apochromatic objec- 

 tives 10, and 20, as well as the 50 (or 70) achromatic oil- 

 immersion objective, with a periplane or 15 compensating 

 eyepiece may be best in the writer's opinion; though 

 achromatic objectives 10, 40, and 90 (or 100) oil immersion, 

 with a 10 Huyghenian eyepiece have been, in the past, 

 usually recommended. The objectives should always be 

 made concentric and the nosepiece centered by the manu- 

 facturer. The eyepieces may be compensating, no matter 

 what the objectives, if used with yellow-green screens; 

 and periplane (hyperplane, or planoscopic) eyepieces 

 are also available. 



The microscope tube may well be non-extensible, being 

 fixed at the correct length. For the best work, cover- 

 glasses 0.17 millimeter thick should be selected, at any 

 rate for all important objects. 



In working without cover-glasses, the (20 or) 40 objective 

 needs a special correction, which could be supplied by the 

 maker fitting a readily removable appropriate achromatic 

 converging lens in the back of the objective. The 90 

 (or 100) oil-immersion objective usually needs an increased 

 tube length of about 15 (or 25) millimeters when used with- 

 out a cover-glass. 



For counting blood corpuscles through the usual 0.4- 

 millimeter cover, a readily detachable achromatic diverging 



