Ch. IX] CORRECTION OF COMPENSATION OCULARS 



293 



aberrations are inevitably introduced which must be eliminated by ;i 

 subsequent part of the optical train. The most striking and trouble- 

 some defect is the so-called difference of chromatic magnification; 



Fig. 175. Huygenian Ocular Showing the Ordinary and the Compensating 



Action. 



(From Spitta, p. 106). 



Ordinary action. (H). 



If the rays are traced on the left it will be seen that the field-lens ((') brings the 

 rays to a focus at the diaphragm (D), and that they cross and pass on to the eye-lens 

 slightly divergent, but in passing through the eye-lens (O, the red and blue con- 

 stituents are made parallel to each other, and are projected into the field of vision 

 in close parallel (virtual) bundles and hence appear achromatic-. 



Compensating action (C). 



For this the field-lens is of flint glass (/•'). and the eye lens <>f crown glass (C). 

 Or the eye-lens may be an over-corrected combination. The end result is the same, 

 viz., the red image is magnified more than the blue image by the ocular, and this 

 balances the excess magnification of the blue image by the objective and in the 

 projected (virtual) image the red is on the outside, producing the orange b 

 at the margin of the field when looking through the ocular, toward a window, or 

 the sky. 



that is, the differently colored constituent imams Conning the final 

 image are of different magnitudes, the blur one being larger than tin- 

 red one. This defect is more easily corrected in the ocular than in 

 the subsequent combinations of the objective. The ocular is then 



