32 THE USE OF THE MICROSCOPE 



the distance of the lamp is fixed, the concave mirror is 

 focused on the object by moving it up and down. vSince 

 the aperture of a concave mirror, so used, may be sometimes 

 0.3 or more, it should be cut down with diaphragms for the 

 low-power microscope, which only needs about 0.1 aper- 

 ture. If placed directly on the mirror, such diaphragms 

 should be elliptical. (Since both plane and concave mirrors 

 are oblique to the incident light, it is obvious that they, 

 too, should not be circular, but long rectangular or 

 elliptical.) 



For the medium-powered microscope with dry objectives, 

 an achromatic dry condenser (or an aplanatic condenser 

 used with yellow-green hght) is to be employed by prefer- 

 ence. If the user of the microscope has to employ a 

 cheap, uncorrected condenser, he loses by being unable 

 to get the maximum useful magnification; because he 

 cannot get a large enough condenser aperture without 

 glare, and so has to be content with low eyepieces. 



For the high powers of the microscope, a corrected 

 immersion condenser, of at least 1.25 aperture when used as 

 a water immersion, is essential for the best results. If, 

 for instance, the 100 oil-immersion objective of 1.3 aperture 

 is used with an uncorrected dry condenser, its maximum 

 useful magnification (without glare) will be cut down from 

 nearly 1,300 to something less than 800, in the writer's 

 experience; that is, instead of using a 12.5 eyepiece, a 

 7 eyepiece is used for best vision. 



A third important rule of modern microscopy is that 

 the light should approximate to yellow-green monochromatic, 

 whenever possible. 



Focusing. — With all condensers, the focusing is to be 

 made on the edge of the diaphragm on the source of light. 

 This diaphragm is to be adjusted to equal the source- 

 field, as already defined. Having put the object in the 

 field of the microscope, and turned the hght on it (which 

 can be done easily while looking at the back of the objective 

 without an eyepiece), the object is focused with the lowest 

 objective on the nosepiece. Then the mirror is turned so as 



