CHAPTER XV 

 THE EYEPIECE 



Kinds of Eyepiece. — The eyepiece receives narrow 

 beams of only slightly converging rays, and hence does not 

 need, and cannot receive, such important corrections 

 as the objective requires. In the ordinary Huyghenian 

 eyepiece (with field lens below the diaphragm), the curva- 

 tures and positions of the two simple plano-convex lenses 

 correct sufficiently the chromatic and spherical aberrations 

 for low- and medium-power eyepieces, for one lens acts 

 on the pencils before and the other after the rays cross, 

 and the errors cancel. In the higher Huyghenian eyepieces 

 (12.5 or higher, by the new designation), however, the 

 eyepoint is too close to the upper lens (eyelens). This is 

 remedied in the special higher eyepieces (orthoscopic, 

 etc.) without a field lens, made of a correcting triplet 

 and an eyelens; which are useful with achromatic objectives 

 having apertures less than 0.65. Also, by making the 

 eyelens of the Huyghenian eyepiece a doublet, certain 

 corrections for flatness of field and color can be introduced 

 (periplane, hyperplane or planoscopic eyepieces). 



In the construction of apochromatic objectives, an error 

 in the difference of magnification for different colors is 

 usually best left to the compensating eyepieces for correc- 

 tion. This correction is made in a doublet for the eyelens 

 in the lower eyepieces (with field lens), or by a triplet 

 combination in the higher eyepieces (without field lens); 

 so that the exit pupil is always well above the eyelens. 

 (In the excellent new 10-times compensating eyepiece of 

 Zeiss, the field lens is also a doublet.) Because of the high 

 eyepoint, the 15- and 20-times compensating eyepieces 

 are comfortable to look through, though the new 15 eyepiece 

 may have too high a fitting for spectacle wearers. Com- 



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