32 GENERAL CHAKACTERS OF PRECIOUS STONES 



The fact that doubly refracting crystals are singly refracting along their optic axes 

 must not be forgotten in making observations with the polariscope. A doubly refracting 

 stone, placed in the instrument so that its optic axis coincides with the line of vision, will 

 behave as if it were singly refracting, and will remain dark during a complete rotation of the 

 carrier. A single observation of this kind is therefore not sufficient to prove the singly 

 refracting character of the stone. The probability of a stone being placed in the instrument 

 in this position will, as a rule, be small ; when a stone gives the indications of single refraction, 

 explained above, it should be placed on the carrier in another position and re-examined. A 

 second indication of the same kind may be regarded as conclusive, though in case this 

 second indication should be due to the second optic axis coinciding with the line of vision — 

 a very improbable chance — the stone may be examined in a third position. As a rule, an 

 examination of the stone in two positions will be sufficient to establish its singl)' refracting 

 character. If, on the other hand, a stone should at the first trial give the indications of a 

 doubly refracting substance, the observation may be regarded as conclusive and further 

 examination is superfluous. 



When a stone in the polariscope gives the appearances peculiar to singly refracting 

 substances after examination in two, or even in three, positions, it has been, stated above 

 that it may legitimately be concluded that the stone is really singly refracting. We have 

 now to show that, under certain circumstances, such is not the case, and that the stone may 

 be in reality doubly refracting. When a cut stone is examined in the polariscope, the 

 facets on the side turned towards the observer will not be parallel to the facet upon which 

 the stone lies, but they may be very steeply inclined to it. Light travelling vertically 

 upwards from beneath will always be able to enter the stone by the facet on which it lies ; 

 it may, however, strike the upper, steeply inclined facets so obliquely as to be totally 

 reflected and pass out at the sides of the stone, thus never reaching the eye of the observer. 

 The error which this may lead to may be avoided in several ways. 



The majority of cut stones have, as shown in Plates II.-IV., a large facet, the table, on 

 one side, and a small facet, the culet, parallel to the table, on the opposite side. If the 

 stone be examined through these two parallel faces, there will be no possibility of internal 

 total reflection. With this object in view, the stone should be placed on the object- 

 carrier so as to rest upon the culet, the table being uppermost ; should the culet be very 

 small the stone may be supported by pieces of wax. With the stone in this position, the 

 light entering it will strike the table perpendicularly and there will thus be no chance 

 of reflection from this facet. Moreover, the position in which the stone rests, upon the 

 culet has the further advantage that the whole area of the table is available for the egress 

 of the light which enters the stone by the culet ; whereas, in the reverse position, much of 

 the light which enters by the table will fail to escape by the culet, but will be totally 

 reflected from the side facets. 



When a stone, examined in the polariscope in the position just described, gives the 

 indications of a singly refracting substance, this observation, as explained above, cannot be 

 regarded as conclusive, and the stone must be re-examined in another position. In any 

 other position, however, there is a possibility of a doubly refracting stone appearing to be 

 singly refracting owing to total reflection of the light within it. This possibility may be 

 avoided by the following simple device : 



The stone is completely immersed in a strongly refracting liquid contained in a small 

 glass vessel placed on the object-carrier. The difference between the index of refraction of 

 the stone and of the sun'ounding medium will be much less than when the stone was in air, 

 and the result will be, as has already been explained, that a larger proportion of the light 



