274 PROGRESS IN MICROSCOPY 



the condenser aperture and Becke's line is observed by shifting the 

 microscope setting. This Une shifts to the higher-index phase when 

 the microscope is moved away from the specimen. In order to elimi- 

 nate dispersion effects, a monochromatic filter is used. Measuring the 

 index of the melted substance consists in causing the glass particles 

 to vanish by suitable temperature adjustment. At the temperature 

 considered and the wave-length used, the melted substance and the 

 glass particles have the same refraction index. In most types of glass, 

 the index increases by one unit of six places of decimals per degree 

 centigrade (10""). It may therefore be considered that the glass- 

 particle index remains constant from 50 to 300° C. Conversely, the 

 index of melted organic substances varies much more. Such variations 

 may attain and even exceed 10"^ per degree. Rising temperature 

 lowers the index. Indices are balanced by increasing the temperature 

 if the melted substance has a higher index than that of the glass 

 particles. If the index of the melted substance is lower, the powder 

 is to be changed. 



As temperature rises, some compounds may be sublimated or 

 decomposed, this making it difficult to determine the refraction index. 

 Measurements are yet feasible if the compound can be supercooled. 

 Once the substance has melted, the hot stage is cooled in order to 

 achieve supercooling. The resulting liquid has a lower temperature 

 than the melting point and its index is therefore higher than that of 

 the non-supercooled liquid. Then, equalization can be made with 

 high-index glass particles. 



The foregoing procedure requires temperature variations substan- 

 tially above the mehing point. In some cases, such variations make 

 the method inapplicable. 



The refraction index can be measured, using interference methods 

 without calling for temperature adjustments. First, the index for the 

 temperature of the specimen may be determined. If the temperature 

 varies, the corresponding variations of the refraction index can be 

 followed up. Polarizing interference methods, in conjunction with 

 the Kofler process, are particularly simple to apply (see Chapter Vll. § 8). 



6. MIXED FUSION — KOFLER'S CONTACT METHOD 



In the contact method, devised by Kofler, the mixing area of two 

 substances in contact between the slide-object and the slip-cover is 

 observed. This method is very suitable for studying the purity of 



