CHAPTER X. 
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, upon the color and upon the 
method of illumination 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 mil 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 refract¬ 
ive 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 this solid. 
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