76 F. E. Wright — Oblique Illumination in 



this method is then not over ±*002; but as noted above, this 

 probable error is noticeably larger when strongly dispersive 

 liquids, such as methylene iodide, are used. For more accu- 

 rate work recourse should be had to monochromatic light where 

 differences of ±'001 in refractive index can be readily detected 

 either by the oblique illumination or by the Becke line method 

 on clear grains* measuring at least , 001 mm in diameter. 



In view of the ease with which this method can be applied 

 to the determination of relative refractive indices of minerals 

 both in thin sections of rocks and in grains immersed in liquids, 

 it is surprising that it is not more frequently used by penol- 

 ogists. For the detection of small amounts of orthoclase in a 

 rock in which plagioclase is the predominating feldspar, the 

 writer has found oblique illumination (by method 3 above) 

 with a relative low power objective exceedingly useful. The 

 low refracting grains of orthoclase appear to stand out in relief 

 (as hollows) in the slide and can be detected at a glance, whereas 

 if the usual Becke line test be applied with a higher power 

 objective to individual grains, the chances are that the minute 

 orthoclase grains will be overlooked and missed altogether. In 

 the first case it is like finding a needle in a haystack ; in the 

 second, like finding the same needle when it reflects the sun's 

 rays which the observer cannot fail to see. 



A simple field method for distinguishing calcite and dolomite. 



To one accustomed to the above immersion method for 

 determining the refractive indices of mineral grains the fol- 

 lowing method for distinguishing calcite and dolomite in the 



*For a description of a set of suitable refractive liquids see Journal Geol- 

 ogy, xx, 494, 1912 ; also Carnegie Institution of Washington Pub. 158, in 

 which the piactical working details of this and other methods are given. 

 Attention may here be directed to a method described by Anton Pauly 

 (Zeitschr. Wiss. Mikroskop., xxii, 344-848, 1905) for measuring the refrac- 

 tive index of a liquid by means of sections of calcite and siderite cut parallel 

 with the axis. From the angle 2<£ between the two positions at which the 

 mineral plate disappears in the liquid, Pauly calculates the refractive index 

 of the liquid from the equation 



we 

 n = / 



V £' 2 sin 2 (j> + w 2 cos 2 ^ 



where w and e are the principal refractive indices of the mineral. Pauly 

 claims an accuracy of 2 or 3 in the fourth decimal place for this method. 

 His fundamantal assumption, however, that the mineral plate actually disap- 

 pears, is incorrect. For every position of the plate two waves emerge, one 

 vibrating parallel to u and the other parallel to e. The intensity of the 

 wave o) varies with cos 2 <j>, where <p is the angle included between w and the 

 principal section of the polarizer ; the intensity of the second wave £ varies 

 with sin 2 0. Two waves emerge and, except for </> = 45° their intensity is 

 different. There is consequently no position where the plate disappears 

 entirely (i. e., has the same refractive index as the liquid), and the method 

 cannot furnish accurate results. 



