WRIGHT: MEASUREMENT OF REFRACTIVE INDICES 107 



shape, and size of the grain and its edges. In many grains, 

 especially cleavage flakes, the phenomena are so masked by 

 local irregularities that it is difficult to determine whether the 

 grain has a slightly higher or slightly lower refractive index 

 than the immersion liquid. With central illumination the 

 differences, noted above, in refractive indices of the light waves 

 which emerge in the different quadrants find expression in a 

 noticeable difference in the behavior of the different sides of the 

 grain on raising the objective. 



Experience has proved that the two methods furnish, in 

 general, results which are of the same order of exactness; in 

 special instances, however, central illumination is superior to 

 oblique illumination because of the smaller angle of inclination 

 of the cone of its incident rays; in other cases oblique illumi- 

 nation is preferable because the phenomena, which it produces, 

 are then more distinct. As a rule it is good practice to use 

 both methods and to verify the results of the one by those of the 

 second. 



Summary. In the exact measurement of refractive indices 

 of minute crystal particles by the immersion method it is essen- 

 tial: (a) that correctly oriented sections be selected which are 

 normal to at least one principal optic section; (b) that in case 

 oblique illumination be used, only those pencils of light be 

 employed whose direction of propagation is included in the 

 plane normal to the principal optical section; in other words, 

 the metal sliding-stop should be so inserted that its front edge 

 is parallel to a principal section; (c) that with central illu- 

 mination special weight be given to the effects along those edges 

 of the grain, which trend approximately parallel to the prin- 

 cipal section. If these conditions be disregarded the value 

 obtained for the maximum refractive index will be too low, 

 the value for the minimum refractive index, too high, while 

 that of any intermediate refractive index, as /3, may be too 

 high or too low. Failure to recognize these factors may lead, 

 especially in the case of strongly birefracting crystal particles, 

 to refractive index determinations which are appreciably in 

 error. 



