390 WRIGHT: DETERMINATION OF REPRINGENCE 



four available lines (546, 560, 578, bSSfx/j.) the refractive indices of 

 mineral and liquid coincide, the liquid having the higher refractive 

 index for the shorter wave length, and the mineral the higher index 

 for the longer wave length. Now the refractive index of solids 

 increases about 0.001 for a decrease in wave length of 10 to 20mm, 

 while for liquids the change is approximately twice as great. If, 

 therefore, the refractive index of a mineral be accurately measured 

 for any wave length between 546 and 588, its index for the wave 

 length 589/xM (D line) can be estimated with an error not exceed- 

 ing ±0.001 and a liquid then prepared of exactly this index, where- 

 upon the estimated refractive index of the mineral grain can be 

 checked by immersion in the new liquid. By use of this arrange- 

 ment a considerable amount of time has been saved in the routine 

 measurement of the refractive indices of fine crystal grains. 

 Occasionally the monochromatic illuminator (Hilger type with 

 Nernst light filament and ground glass diffusing screen) has 

 been found useful for ascertaining approximately the wave * 

 length for which the refractive index of the grain coincides with 

 that of the enveloping liquid. 



New method involving two-fold oblique illujnination. Oblique 

 illumination is obtained ordinarily by means of a shding stop 

 below the condenser of the microscope. ^ This stop is purposely 

 not sharply imaged in the object field but appears as a shadow 

 with a hazy edge which passes gradually into the brightly illumi- 

 nated part of the field. The mineral grains are placed in this 

 transition shadow edge between light and dark, and the illumina- 

 tion of their edges both in white and in monochromatic light is 

 observed. Because of the prismatic refraction of the inclined 

 edges of such grains the intensity of illumination of edges adjacent 

 to the shadow is different from that of the opposite edges, when 

 the refractive index of the grains is different from the refractive 

 index of the liquid in which they are immersed. These differences 

 become less distinct as the refractive index of the liquid ap- 

 proaches that of the mineral; and, if the refractive indices differ 



1 The principles of oblique illumination are discussed yi detail in Am. J. Sci. 

 (4) 35: 63-82. 1913. 



