Petrographic Microscope Work. 75 



conditions the left margin of the grain would appear tinged 

 with red. For the right margin, on the other hand, the blue 

 rajs are less steeply inclined and this margin would accordingly 

 be blue under these conditions. By shifting the Nernst glower 

 the angle of the incident cone of light can be decreased grad- 

 ually, whereupon the red margin on the left becomes orange- 

 red and this color in turn passes into the yellow tint of the 

 strongly illuminated field ; at the same time the blue of the 

 right margin passes into green and thence into the yellow hue 

 of the field. 



Similar phenomena, though less brilliant, are visible at the 

 indistinct shadow edges produced in the field by placing the 

 finger below the condenser (method 3 above, daylight illumin- 

 ation) or by using the sliding stop in the lower focal plane of 

 the condenser (method 2 above). The red and blue fringes 

 described above appear and their explanation is similar to that 

 given for the fringes observed with the Nernst glower. The 

 angle between the incident rays which enter the semi-dark edge 

 is in all cases sufficiently small to allow minute angular differences 

 between the slightly dispersed colored rays to be detected. The 

 greater the difference in dispersion between liquid and mineral 

 grain, the more brilliant and purer the red and blue margins thus 

 obtained. With high-power objectives the semi-dark shadow 

 zone covers the entire field and the colors are spread out and 

 correspondingly less distinct and brilliant. For this reason 

 high-power objectives are as a rule less favorable than low 

 objectives for the determination of relative refractive indices 

 by this method of oblique illumination. 



A remarkable display of these colored margins resulting from 

 prismatic color dispersion can be seen on a clear quartz crystal 

 immersed in a mixture of clove oil with a little cinnamon oil 

 and illuminated by oblique rays entering the glass base of the 

 containing receptacle (beaker glass or trough of axial angle 

 apparatus). Attention was first directed to these phenomena 

 by O. Maschke in 1872.* 



The color fringes as ordinarily obtained are rarely pure red 

 and blue but rather orange and pale blue, the blue being often 

 more pronounced than the orange. In case the liquid has a 

 very strong dispersion and the mineral a weak dispersion, such 

 color-rims appear over a considerable range of refractive indices 

 and the refractive index values obtained by the method are 

 correspondingly less accurate. Ordinarily, when colored rims 

 are observed on a mineral grain and both sides present about 

 the same intensity of illumination, the refractive index of 

 liquid and mineral are equal for a wave length of about 550/*//, 

 and the error of the determination of the refractive index by 

 * Pogg. Ann., cxlv, 568, 1872 ; Wied. Ann., xi, 722, 1880. 



