powers of wide aperture, even with these objectives 

 there is an outstanding error which by itself cannot be 

 corrected. This is balanced in the eyepiece, which is 

 correspondingly over-corrected, and is called the com- 

 pensating eyepiece. It has therefore been necessary to 

 make the low powers under-corrected as well, so that 

 they may be used with the same eyepieces, for it is 

 evident that neither these objectives with the Huy- 

 ghenian eyepiece, nor the compensating eyepiece with 

 the achromatic objectives will give satisfactory results. 



The proper combination of apochromatic objective 

 and compensating' eyepiece gives a beautiful image 

 with the maximum of resolving power, unapproached 

 by any of the achromatic combinations. They further- 

 more have the advantage that they are exactly suited 

 to photomicrography, there being exact coincidence 

 between the photographic and visual image, which in 

 the achromatic objectives is not the case, as these must 

 be either corrected for photography, when they are not 

 satisfactory for ordinary purposes, or an allowance 

 must be made for the difference between the visual and 

 chemical images in the camera, which is very difficult. 



One objection to the high powers has been the 

 liability to deterioration of some of the materials used, 

 which has happily been overcome by Prof. Chas. S. 

 Hastings in the apochromatic objectives computed 

 by him. 



103 



