of Minerals in the Thin Section. 



!29 



scale registered •005 mm for each division on the head, while the 

 vertical (V, fig. 11) scale screw, which was constructed at a 

 later period and on a different lathe, was a trifle coarser and 

 registered a slightly greater movement for one division on its 

 head. For this reason the values K„ and K 7i are slightly 







































































































































































































































































































































































































































































































































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I 



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Fig. 6. Curve I of this figure contains the observed values of D, ; for the 

 vertical (V) micrometer screw of the double micrometer ocular (fig. 11, p. 336) 

 for the different values of E from, 0-47° ; the values of D in Curve II were 

 calculated from Mallard's formula for the same micrometer screw scale ; 

 Curve III in like manner was plotted from observed values of D Hj the 

 horizontal micrometer screw for values of E from 0-47° ; while Curve 

 IV was plotted from values of D H calculated from Mallard's formula, 

 T>k = ~Kt. sin E. 



from measurements on an optic axial angle apparatus while the angles under 

 Hi were calculated by Mallard's formula on the assumption that the K 

 obtained for topaz (1 "075) is valid for all angles. The differences between 

 observation and calculation are large and indicate that the determination of 

 the positions of optic axes near the periphery of the field is less accurate 

 than that for more centrally located points ; on comparison of this series of 

 results with those obtained by Fuess, No. 9 objective, it is evident that 

 objectives vary considerably in this particular and that for accurate work 

 the constants K of each microscopic objective and lens system should be 

 determined for a number of directions, either by using minerals of known 

 optic axial angles or a uniaxial mineral as calcite in conjunction with the 

 universal stage. 



