Eyepieces 79 



Compensating Eyepieces. 



Under the description of * Apochromatic Objectives ' on 

 page 57, reference is made to compensating eyepieces. 

 These are specially designed to correct an outstanding 

 colour defect (of the nature of under-correction) which is 

 inherent in all high-power objectives, whether they be 

 apochromatic or achromatic, on account of the peculiar 

 construction of the front lens. For the sake of uniformity 

 of eyepiece, Zeiss imparts the same colour effect to the 

 lower-power lenses of the apochromatic series. The eye- 

 pieces, then, have an equal error of the opposite kind 

 (over-correction), and when the objective and eyepiece are 

 combined, a perfect correction is obtained. 



The apochromatic objectives are ' under-corrected,' while 

 the achromatic objectives of low power are ' over-corrected.' 

 The compensating eyepieces for the former are over- 

 corrected, and the Huyghenian eyepieces for the latter 

 under-corrected. With low powers of the achromatic type, 

 the compensating eyepieces are disadvantageous ; but with 

 high powers, where the defects caused by the hemispherical 

 front lens give rise to error identical with that in the 

 apochromatic objectives, and which the compensating eye- 

 pieces are designed to overcome, these special eyepieces 

 can be employed, but the result is not sufficiently beneficial 

 to justify the purchase of them for use with high-power 

 achromatic objectives only. 



A most advantageous feature is imparted to the Zeiss 

 compensating eyepieces. They are all designed to work in 

 the same focal plane, so that when two eyepieces of this 

 series of different powers are interchanged in the body of 

 the microscope no alteration in the focusing is necessary. 



A number is engraved on each eyepiece, which, multi- 

 plied by the initial magnifying power of the objective, will, 

 when used at the tube-length for which the eyepiece is 

 designed, indicate the magnifying power that is being 

 employed. 



The compensating eyepieces designed for the 6-inch 



