50 PHOTOMICROGRAPHY 



a definite ray, the Sodium line "D" for example ; for 

 the above glasses it is 



1-524-1-515 



Crown =-017 



1-518-1 



1-735-1-710 

 1-717-1 



In order to make a corrected lens from the glasses 

 whose constants have been given, a double convex lens 

 of crown glass is cemented to a negative lens of flint, in 

 the way shown in the sections of lenses (Figs. 34 to 36). 

 The crown glass lens is of higher power than the flint, so 

 that the combination still functions as a positive lens, 

 and the two components are so selected as to give the 

 desired power to the combination. The dispersive power 

 of flint glass is greater than that of crown, so it is possible 

 to find a positive crown and a lower-power negative flint 

 whose dispersions, for two selected colours, will balance 

 each other. As the figures given above indicate, the 

 differences of Refractive Index of different pairs of 

 colours are not proportional in the two glasses, and it is 

 not possible to bring more than two colours to a common 

 focus by their combination. 



The Spherical error of a simple lens is comparatively 

 small when only a narrow pencil of light through a 

 central zone is employed, but as microscope objectives 

 have to utilise wide-angled pencils over their entire 

 surface, spherical aberration becomes of the greatest 

 importance. Lenses of the same power can be made of 

 different shapes, and the spherical aberrations depend on 

 their shapes, so it is possible to select a powerful positive 

 lens with small aberrations, and a less powerful negative 

 lens of greater relative aberration, which will form a 

 corrected lens when combined. As the different 

 coloured rays are differently refracted, it is necessary to 



