THE MONOCULAR MICROSCOPE 57 



Changing Eyepieces. — It is probably an advantage to 

 have the eyepieces "sprung" into the sleeve of the draw- 

 tube. One may have too much changing of eyepieces in 

 the use of the microscope, as already stated. There is a 

 maximum useful magnification, somewhere near 1,000 

 times the working aperture of the microscope. Anything 

 over this is mostly useless for normal eyesight. With 

 regard to the highest powers, it is obvious that anything 

 under this maximum may be a loss. The low and medium 

 powers, however, are usually employed with a less magni- 

 fication than the maximum, since this procedure hides 

 errors due to wrong centering, variation in cover-glass 

 thickness, etc. It is not good to use so low an eyepiece that 

 the illuminated part of the eyepiece circle nearly equals 

 the pupil of the eye, because of fluctuations of the hght 

 with slight motions of the head, as already noted. If 

 more brightness in the image is needed, the preferable 

 method would appear to be to increase the used aperture 

 of the condenser by opening its iris, instead of taking a 

 lower eyepiece. With an uncorrected condenser, of course, 

 low eyepieces are obHgatory. With a corrected immersion 

 condenser, however, and neutral or colored screens for 

 lessening the intensity of the light without altering its 

 aperture, high condenser cones and high eyepieces can 

 be used, especially with yellow-green light. The writer, for 

 instance, in several years' work, has not needed to use an 

 eyepiece lower than 12.5 times (new measurement), with 

 achromatic, fiuorite, or apochromatic objectives. With 

 low eyepieces and high objectives, the curvature of the 

 field is usually pronounced, but this curvature is scarcely 

 noticeable with the maximum useful magnification. It 

 is possible to select the three or four objectives used, so 

 that one eyepiece gives just the maximum for the highest 

 power objective, and well below the maximum for the low 

 or medium-power objectives. Thus the writer has used, 

 on an achromatic microscope, objectives 13, 40 fiuorite 

 dry, 90 water immersion, and 100 fiuorite oil-immersion 

 (all with yellow-green glass), with the paired compensating 



