Objectives 57 



ing to obtain apparently impossible results or to detect 

 structure not hitherto discovered. The man who definitely 

 equips himself for original research cannot afford to have 

 less than the very best means which modern optical 

 skill can afford him, and from the point of view of actual 

 supremacy the apochromatics must then be chosen ; but he 

 would be limiting his possibilities in practically no degree 

 whatever by having lenses carefully selected from those 

 previously referred to under the title of semi-apochromatics. 



It should be remembered that the reduction of spherical 

 zones enables a higher power of eyepiece to be employed 

 with an objective than would otherwise be possible, and 

 it is due to this quality that the aprochromatic objectives 

 have been especially valuable. 



A series of compensating eyepieces is specially designed 

 to work with them, having powers varying from 2 to 27 

 diameters. Supposing, therefore, we were working with 

 a ^-inch apochromat having an initial power of 40 dia- 

 meters, with a ten-inch tube-length, we could by means of 

 the searcher ej'epiece (x 2) obtain a magnification of 

 80 diameters, and by using intermediate powers of eye- 

 pieces up to the X 27, produce any magnification that 

 might be desired from 80 to 1,080 diameters (:|-inch initial 

 power of 40 x 27 eyepiece power = 1,080). 



Further, these special eyepieces are all designed to work 

 in the same focal plane at the tube-length for which the 

 eyepieces and objectives are designed, with the result 

 that practically very little re-focusing is necessary on the 

 exchange of an eyepiece during an observation. By this 

 means the magnification with a low power objective having 

 a long working distance and a fairly high N.A. for its 

 power, as possessed by all of the apochromats, of Messrs. 

 Zeiss' manufacture, can be gradually increased, and the 

 advantage gained is one for which many microscopists 

 sighed before the days of apochromats — namely, a wide 

 range of magnifying power and great working distance. 

 For many classes of work this convenience is very great. 



