USK OF THE COMPOUND MICROSCOPE 35 



.1 fourth rule of microscopy is thai the best images are 

 attained with cover-glasses 0.17 millimeter thick, even with 

 oil-im mersion objectives . 



Eyepieces. — Too low eyepieces give a curved field, 

 and do not bring out the full capabilities of the objective. 

 To do this requires a total magnification of 500 to 1,000 

 times the working aperture. Perhaps no eyepieces giving a 

 total magnification less than 750 or 1,000 times the working 

 aperture need usually be employed, when the adjustments 

 of the microscope are fairly correct. There are several 

 practical disadvantages in changing eyepieces. (With 

 yellow-green hght, compensating eyepieces can be fairly 

 well used for the lower achromatic objectives, as well as 

 for the higher achromatic and the apochromatic objectives.) 

 It is not difficult to select the three or four objectives on the 

 nosepiece so that one eyepiece will give nearly the best 

 results for all. This eyepiece may be selected so as to give 

 about the maximum useful magnification with the highest 

 powers. Since compensating eyepieces have higheyepoints, 

 a high compensating eyepiece can well be used with a low 

 apochromatic and two or three high achromatic objectives 

 on the nosepiece. 



Summary .^ — A classification of useful magnifications 

 into low, medium, and high, is suggested. This classifica- 

 tion applies primarily to objectives; but condensers cannot 

 be omitted from consideration, since they aid in fixing 

 the working aperture. A short account is given of some 

 of the optical phenomena in the compound microscope. 

 The terms, image-field, object-field, source-field, condenser 

 circle, objective circle, and eyepiece circle are defined. The 

 illumination of the low-power microscope with the plane 

 or the concave mirror, of the medium-power microscope 

 with the dry achromatic condenser, and of the high-power 

 microscope with the corrected water-immersion condenser 

 are described. Methods of focusing the condenser and the 

 high-power objective are given. The proper use of dry, 

 water-immersion, and oil-immersion objectives is stated. 

 Four important rules of modern microscopy are given in this 

 chapter. 



