DOUBLE REFRACTION OF LIGHT 53 



will escape total reflection and will pass out of the stone. The amount of light totally 

 reflected will in any case be considerably diminished, but total reflection will not be entirely 

 absent, unless the refractive index of the liquid is exactly the same as that of the stone. In 

 the case of diamond, however, this state of affairs will not exist, since there is no liquid with 

 so high a refractive index. 



Liquids used for this purpose should be transparent, not deeply coloured, and of high 

 refractive index. One which fulfils these conditions, and has been already mentioned, is 

 methylene iodide. It is one of the most strongly refracting liquids known, having at bhe 

 ordinary temperature of ]5° to 20° C. an index of refraction of 1-75 for the middle rays 

 of the spectrum ; this value for the refractive index is exceeded by only few precious stones, 

 notably the diamond, the index of refraction of which is 2'4<3. From the upper faces of a 

 feebly refracting stone immersed in methylene iodide there will be no total reflection of 

 light, but this will still take place if a diamond is substituted for the feebly refracting stone. 

 Ail rays of light, forming with the normal to the surface from which they emerge an angle 

 greater than 46° 19' (Fig. 13) ; in other words, all rays travelling vertically upwards will be 

 totally reflected from facets inclined to the horizontal object-carrier of the polariscope 

 at an angle greater than 46° 19'. Total reflection in diamond is thus not eliminated but 

 considerably diminished in amount, the corresponding critical angle for diamond in air 

 being 24° 24'. 



A drawback to the use of methylene iodide for this purpose is its high price; 

 nionobromonaphthalene is a much cheaper liquid, with a refractive index almost as high as 

 that of methylene iodide ; it can therefore be used as a substitute for the latter in 

 optical determinations, but not in determinations of specific gravity, being too light a liquid 

 for this purpose. 



It will be well at this point to review the method of using the polariscope for 

 determining the singly or doubly refracting character of a precious stone. The stone is 

 placed on the object-carrier in the dark field of the polariscope and the carrier rotated. If 

 now, all side light being carefully screened off", the field shows alternations of lightness and 

 darkness the stone may be considered to be without doubt doubly refracting. Should the 

 field remain dark during the rotation, the stone must be placed in another position on the 

 object-carrier and again rotated. If this rotation results in alternations of lightness and 

 darkness in the field, the stone is certainly doubly refracting ; but should the whole of the 

 field still remain dark we cannot conclude that the stone is singly refracting until it has 

 been proved that the absence of light has not been due to total reflection within the stone. 

 ^^'^ith this object the stone must be examined for the third time, either in the position 

 described above, resting upon the culet and with the table horizontal and uppermost, or 

 immersed in a strongly refracting liquid to diminish or eliminate total reflection. If on 

 rotation the whole of the field still remains dark the stone must be singly refracting. For 

 the examination of stones cut in a spherical form, or those with a rough and irregular 

 surface, it will often be necessary to immerse them in liquid at first. Observations made 

 with the polariscope require no special skill, and with practice and attention to the necessary 

 precautions are very reliable. 



Before leaving the subject of refraction certain anomalous cases must be considered. 

 Many singly refracting substances, such, for example, as diamond, occasionally show the 

 appearances peculiar to doubly refracting substances. When this is the case, such substances 

 arc said to possess anomalous double refraction. The phenomenon is frequently 

 due to internal strains set up on the solidification of the substance or brought about by 

 subsequent causes. These internal strains may be so great in certain crystals, for example 



