of Minerals in the Thin Section. 



325 



a sphere. It is necessary, therefore, to focus sharply on 

 points about midway between the center and margin of the 



i/iiii > 



Fig. 5. In. this figure, C represents the condenser lens system ; O that 

 of the objective, B, the Bertrand-Amici lens, P, the mineral plate and Oc, 

 the ocular of the Uamsden-Czapski type. The primary image EF (cl) is 

 reproduced as a secondary image at E'F" id') and later still more magnified 

 by the ocular at E"F" (D), where it is observed by the eye back of the small 

 aperture S. In the writer's microscope with Fuess No. 9 objective, of 

 equivalent focal length, 2 - 7 mm , and numerical aperture - 97, the diameter 

 of the primary interference figure measures about 5 mm and this is magnified 

 very considerably by the Bertrand lens and by the ocular in the secondary 

 interference figure as shown in the figure. By choosing a Bertrand lens of 

 suitable focal length and inserting the same at the proper point between 

 objective and ocular, it is possible to increase the magnification of the 

 secondary interfei'ence figure and thus to enhance the accuracy of the 

 measurements, providing the interference figure is sufficiently sharp and 

 distinct to permit magnification to such an extent. 



