190 Botanical Microtechnique 



definite cc)ui\alent focal length. This value nia\ l)e obtained from 

 the catalogues, l)ut a more useful designation, \\hich is engraxed on 

 modern oculars, is the magnification value, which ranges from 1 to 

 30 X- YoY routine work and for classwork lOx is the most satisfactory 

 magnification. The lower magnifications are likely to ha\e marked 

 curvature of the image. Higher magnifications cause increasingh 

 greater eyestrain, which is very pronounced \\ith the 30X- Further- 

 more, there is an upjjcr limit, beyond which the ocular produces only 

 empty magnification, with no gain in the revealing of detail. 



The maximum effecti\e ocular magnification, which may l)e used 

 with a given objective, can be computed easily. Assume that a 13 X 

 objective of N.A. 0.65 is l^eing used; the formula is 



1,000 (X.A. of objective) (1000) (0.65) 



Magnification of objective 43 



^ 15Xj tl'^e approximate maximum ocular magnification. 



It is exident that with a microscope on \vhich the 43 X objecti\e 

 is the highest }K)wer used, an ocular magnification of over 15X 

 is of no value with respect to resolving power but of possible 

 value for counting or drawing by projection. 7 his simple calcu- 

 lation will enable a purchaser to specify the most useful lens 

 combinations. Modern ocidars are parfocal, making it j)()ssiijle 

 to interchange ocidars of diiferent magnifications xvithout reciuii- 

 ing nuK h change of focus. 



OPTICAL CATEGORIES OF OCULARS 



Huygenian oculars are of comparati\ely simple two lens con- 

 struction. They are designed for use xvilh achromatic objedives and 

 yield inferior images with apochromatic objet ii\es. 



Comjjensatiiig (xulars aie designed to correct certain residual 

 aberrations inhereni in ajxx hromatic objectives. It is, therefore, im- 

 perati\c' to use compensating oculais with apoc In oniatit objecti\es, 

 and oculars and objectives must be of the same make. These oculars 

 mav be used with .uhroniatic and Ihiorite objedixcs ha\ing magnifi- 

 cation oxer lOX- 



Flat IrU! (xuhu's are ol the noiuompeiisaling t\pe and \ ield 

 imaues in wiiich cin\aiuie has been considerably reduced. Ihesc 

 oculars have various iiade nanus. H\|)eri)lane and Planascopic being 

 the l)est-kn()\\ n. A serious oljjection to some oculars ol this type is 

 that the eve nuisi be held rigidly at a restricted eye position. The 



