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University of California Publications. [Geology 



hundred ellipses, the time consumed in racking the microscope 

 tube back and forth over a large dead area would be very consid- 

 erable. 



The cone was trimmed back three times in the early stages 

 of the work, and was used for most of the tests with a diameter 

 fZ„=1.20 mm. The total diameters of the ellipses varied from 

 3.00 — 6.00 mm., most of them being very near 4.00 mm. The 

 traveling microscope which was employed read distances to 

 1/400 mm. and throughout the work this smallest reading of the 

 instrument was taken as unity. Thus the constant subtracted 

 for d was 480, since 480 (1/400 mm.)=1.2 mm. 



Thus the ratio of two axes might be 1334/1673. Prom this, 

 the corrected ratio is found to be (1334-480)/ (1673-480), or 

 .717. 



To facilitate finding the limits of the wax figures under the 

 microscope, it was found advisable to scratch a number into the 

 wax on the inner flanks of the figure, and, in a few cases, to 

 delimit the field by scratches, as in fig. 6. Oblique natural light 



Fig. 6. 



was used. The shadows which it east around the figure as a 

 whole were of no value, but it brought out very clearly, by high 

 lights, the extremely fine-grained, uniform texture of the twice 

 melted wax, as compared with that which surrounded the figiire 

 and which had been melted but once. It was very easy, by ob- 

 serving this distinction, to follow the line of contact entirely 

 around under the microscope, and any figure which showed irreg- 

 ularity in outline was rejected. 



The ratios of the three axes of the triaxial ellipsoidal iso- 

 therms are uniquely determined by measurements in two prop- 

 erly chosen sections, but a third section of each rock was taken 

 as a check. 



