102 WRIGHT: MEASUREMENT OF REFRACTIVE INDICES 



mersion method and can only be approached by a careful ob- 

 servance of certain favorable conditions of observation. It is 

 the purpose of the present paper to consider these conditions 

 briefly and to ascertain their relative importance in accurate 

 work. 



The methods for the detection of differences in refractive 

 index between crystal grain and immersion liquid are based pri- 

 marily on differences in intensity of illumination in different 

 parts of the grain. In order that these differences be readily 

 visible even for slight changes in refractive 'index it is essen- 

 tial that attention be given to the following factors: (a) the 

 source of light; (b) the optical condenser system of the micro- 

 scope for producing the different types of illumination — cen- 

 tral or oblique — of the crystal grain; (c) the optical system for 

 observing the differences in illumination thus produced. 



(a) For accurate measurements intense, uniform, monochro- 

 matic light is essential. The most satisfactory light source is 

 the mercury lamp used in conjunction with either a dispersion 

 prism or the Wratten ray filters. It furnishes a very intense 

 green light (546.1 mm) and an intense orange-yellow light (576.9 

 and 579 mm)- The ordinary sodium, thallium, lithium, and 

 calcium flames, the helium and hydrogen tubes, and the cadmium 

 and iron arcs are also useful. The light source is placed in the 

 focus of a condenser lens, and either the light itself or the il- 

 luminated condenser lens is imaged in the object plane of the 

 microscope by means of the substage condenser. In case still 

 more uniform, but less intense, illumination is desired a finely 

 ground glass disk may be placed in the lower focal plane of the 

 substage condenser. 



(b) Two types of illumination, central and oblique, are em- 

 ployed in refractive index determinations by the immersion 

 method. In central illumination a narrow cone of light whose 

 axis coincides with that of the optical system, is used and 

 is obtained either by closing the iris diaphragm below the sub- 

 stage condenser or by lowering the condenser. The angle which 

 the marginal rays of the central cone include with the axis 

 can be measured by means of a graduated apertometer scale 



