28 THE USE OF THE MICROSCOPE 



tic or aplanatic dry condenser gives the best results; for the 

 uncorrected condenser in common use allows much glare. 



High-power microscopes, in this scheme, may have 

 useful magnifications up to 1,350. They may be monocular, 

 or monobjective binocular. They employ dry objectives 

 of high aperture, up to 0.85 or 0.95; and also water-immer- 

 sion objectives up to 1.2 or 1.25, and oil-immersion objec- 

 tives up to 1.3 or 1.4 aperture. They require an immersion 

 condenser, achromatic or aplanatic. Yellow-green hght 

 adds decidedly to their defining power, especially if they 

 are not apochromatic. 



Structure of the Compound Microscope. — The construc- 

 tion of the compound microscope is well described, with 

 figures, in the catalogues and booklets of instruction 

 issued by the six or more chief optical firms of the world. 

 As is well known, a more or less corrected condenser 

 throws an image of a source of light (radiant) on the object. 

 Then a highly corrected objective throws an image of the 

 object (above or) in the plane of the diaphragm of the 

 eyepiece. A virtual image of this image is formed by 

 the eyepiece (specially corrected for this work), usually at 

 an indefinitely large distance. (For the purposes of cal- 

 culation, however, the distance of the virtual image is 

 taken as 250 millimeters.) The field of view (Fig. 8) is 

 bounded primarily by the diaphragm in the eyepiece 

 Sz and the magnified image of this diaphragm bounds the 

 image- field seen through the eyepiece. The field of view 

 on the object S2 is a small circle equal in diameter to the 

 breadth of the image-field divided by the total magnifica- 

 tion. This is the object-field. The field of view at the 

 source of hght *Si is equal to the object-field multiplied 

 by the inverse of the diminution caused by the condenser. 

 This is the source-field. All three, image-field, object- 

 field, and source-field, depend for their size on the diaphragm 

 of the eyepiece. 



One of the first rules of modern microscopij is that the 

 source of light should be diaphragmed to be equal to the source- 

 field, as above defined. 



