278 WRIGHT: MEASUREMENT OF REFRACTIVE INDEX 



• from the prisms varies with the refractive index. Its 



\i/ position can be fixed by means of a scale on the side of 



the draw tube of the microscope. For two prisms of 

 angle 15° the variation in distance between the focal 

 points for liquid of refractive index 1.50 to one of index 

 1.75 is 25.0 mm. If it were possible to focus sharply to 

 ^ 0.01 mm. on the indistinct bands of light, the probable 

 :^ error would be ±0.001 but this is not the case. The 

 bands of light are not sharp and the exact point of cen- 

 tral overlap is not easy to recognize. At best one might 

 obtain the second decimal place by this method which is 

 ■p- j4 not satisfactory and neither the hollow prism nor the glass 



plate method of this paragraph are to be recommended. 



SUMMARY 



The refractive index is the most important optical constant 

 with which the petrographer has to do, since its determination 

 oii a single mineral grain enables him practically to identify the 

 mineral within narrow limits. The determination of the re- 

 fractive indices of irregular mineral grains measm*ing 0.01 nrni. 

 in diameter is best accomplished by means of the immersion 

 method, in which the refractive indices of the mineral grain are 

 compared with that of the liquid in which it is immersed. In 

 the preceding pages the measurement of the refractive index 

 of a single drop of liquid is considered in some detail. The 

 accuracy of the standard methods (with the exception of the Abbe 

 refractometer methods) for this purpose is discussed and certain 

 new modifications are suggested which render the application 

 of such methods to a single drop of liquid possible. The use of 

 a diffusing screen in front of monochromatic light sources is 

 emphasized. A new type of hollow prism is suggested which has 

 proved satisfactory and convenient, and with which accurate 

 measurements (accurate to the fourth decimal place) can be 

 made either by a method of autocollimation or by the minimum 

 deviation method or by Gilford's method. The use of a cover of 

 specially prepared tin foil on a drop of liquid to be measured 

 on the Abbe-Pulfrich crystal total-refractometer is suggested as 

 a satisfactory method for obtaining the phenomena of grazing 

 incidence even on a thin film of liquid. Five new methods are 

 also described for measuring the refractive index of a drop of 



