CHAPTER IX. 



THE DETERMINATION OF REFRACTIVE INDEX BY MEANS 

 OF THE MICROSCOPE. 



All transparent and translucent objects when immersed in 

 liquids yield images in the microscope which are bounded by 

 dark lines or bands or which appear to be surrounded by a colored 

 fringe or halo. The width or thickness of these dark or colored 

 contours depends upon the magnitude of the difference between 

 the refractive indices of the two phases (the solid and the liquid), 

 upon the dispersive power of each and upon the method of illumi- 

 nation employed. 



Contour bands appear when the refractive index of the solid 

 is either greater or less than that of the liquid in which the solid 

 is immersed. As the index of refraction of the solid approaches 

 closer and closer to that of the liquid the dark bands decrease in 

 prominence, and finally vanish when both object and liquid have 

 the same refractive index. If both have also the same dispersive 

 power, the same light-absorbing power and the same color, the 

 object will be invisible in the liquid. But complete disappear- 

 ance is impossible in practice since these conditions can never 

 be all fulfilled and since moreover it is next to impossible to obtain 

 crystals or other solids which are so perfect as to be free from air 

 bubbles, fractures or cleavage planes or which contain no occlu- 

 sions of dirt, of mother liquor or of foreign salts. The vanishing 

 of the black lines is therefore the criterion upon which we must 

 depend for an indication that the solid and the liquid have the 

 same index of refraction. 



It is evident, that, given a series of liquids of known refrac- 

 tive index, if a solid of unknown index be immersed in these, 

 one after another, until the black contours bounding the image 

 just disappear, the index of this particular liquid is the index 

 sought of the solid. 



In like manner if we have a series of crystals, or fragments of 



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