148 THE USE OF THE MICROSCOPE 



pensating eyepieces must l)e used witli most series of 

 apochromatic objectives for the best results. Since the 

 high-power achromatic objectives also have some of the 

 same error uncorrected, the compensating eyepieces can 

 be used with them with advantage, especially with a yellow- 

 green screen. If yellow-green screens are used constantly, 

 compensating eyepieces can be employed with low and 

 medium achromatic objectives also, so as to avoid changing 

 eyepieces. Or the 10 apochromatic objective may be used 

 with medium and high achromatic objectives. The com- 

 pensating eyepieces and apochromatic objectives of dif- 

 ferent makers should not perhaps be used in combination, 

 for it seems that the corrections are sometimes perceptibly 

 different. Eyepieces giving a variable degree of compensa- 

 tion were calculated by Hartridge (72). One maker 

 (Watson) has eyepieces with a flint field lens and crown 

 eyelens, and the compensation can be varied by varying 

 the distance between the two. For photography, the 

 aperture of the cones of light at the light sensitive plate 

 is so small that the differences between compensating 

 eyepieces, periplane or hyperplane eyepieces, and the 

 special projection eyepieces are small. Every small advan- 

 tage counts, however, in scientific photography, and for 

 optimum results the special projection eyepieces, or the 

 homals, are to be used. Since flatness of field is sometimes 

 of importance for a photograph, the homal diverging 

 photographic eyepieces, which are calculated especially 

 for this, are the best for cases which demand such flatness. 



Optimum Use of Different Eyepieces. — 1. For low and 

 medium achromatic objectives, use Huyghenian eyepieces 

 up to 10 times, and orthoscopic eyepieces (Zeiss) from 

 12.5 to 28 times. (With yellow-green Hght, plane or 

 compensating eyepieces may be sometimes used.) 



2. For high-power achromatic or fluorite objectives, 

 use plane eyepieces (periplane of Leitz, hyperplane of 

 Bausch and Lomb, and planoscopic of Spencer) ; or co7?i- 

 pensating eyepieces, especially with yellow-green light. 



