GENERAL METHODS 563 



proof of the fact, for doubly refracting crystals when viewed in the direction of an optic 

 axis present the same appearances. The stone must, therefore, be placed upon the object- 

 carrier in another position, and if on rotation it appears alternately light and dark it must 

 be doubly refracting, while if, on the contrary, it remains dark, we may conclude that it is 

 singly refracting. Absolutely conclusive evidence as to the refraction of a stone is only 

 obtained, however, when the appearance of an alternately light and dark field shows that 

 it is doubly refracting. Cut stones should be examined in two positions, but the two facets 

 on which they successively lie should not be parallel. It should always be borne in mind 

 that the rays of light which pass into a strongly refracting stone may undergo total internal 

 reflection and not emerge from the stone in the desired direction. When this is the case 

 the stone will remain dark during the rotation of the carrier, even though it be strongly 

 doubly refracting. In order to avoid this ambiguity, the gem may be attached to the 

 carrier with wax in such a position that the largest facet, for example, the table in brilliants 

 or the base in rosettes, shall be directed towards the observer and parallel to the object- 

 carrier. Another device which serves the same purpose is to immerse the stone in 

 methylene iodide or monobromonaphthalene or other strongly refracting liquid contained 

 in a vessel with a plane transparent bottom. This device makes it possible to examine in 

 the polariscope not only gems with plane facets but also stones cut en cahochon and quite 

 irregular fragments. For the examination of a stone in the polariscope perfect transparency 

 is by no means a sine qud nan ,■ a considerable degree of transparency is quite sufficient to 

 admit of the different degrees of illumination at different stages of the rotation being 

 distinctly observed. 



The existence of anomalous double refraction in certain stones must be borne in mind, 

 but is not likely to lead to error. In all these observations with the polariscope it is 

 necessary to cut off all light incident upon the stone from the sides by placing upon it an 

 opaque tube of paper or by using the hand in a similar manner. 



An absolute proof of the double refraction of a stone is obtained when it is shown to 

 be dichroic. When any transparent and not too faintly coloured stone is placed in front of 

 the dichroscope, Fig. 28, and held towards the light, two coloured images of the aperture 

 of the instrument are seen. If these images be differently coloured the stone is indisputably 

 dichroic, and consequently doubly refracting. If, on the other hand, both images appear of 

 the same colour and remain so during a complete rotation of the stone relative to the 

 dichroscope, then the stone may be characterised by the absence of dichroism, and it may be 

 singly refracting. It is possible, however, in such a case that the stone is really dichroic, 

 though so feebly so that the difference in colour of the images is not noticeable. Moreover, 

 the light may have passed through the stone along an optic axis, which also would account 

 for the images being of the same colour. It is necessary, therefore, to make a second 

 observation with the stone in a different position ; and if now the two images differ in colour 

 the stone is indisputably dichroic and doubly refracting. If, however, they are still of the 

 same colour the stone may be devoid of dichroism and singly refracting ; but the evidence 

 cannot be regarded as conclusive. In making observations with the dichroscope it is necessary 

 to avoid the possibility of internal total reflection by placing the stone with its largest facet 

 over the opening of the instrument. 



The dichroism of some precious stones is strong enough to be apparent to the naked eye. 

 The use of the dichroscope in such cases is superfluous, except to confirm one's direct 

 observation or to determine the precise shades of colour shown by the two images. These 

 shades are in a measure characteristic of each stone, and are therefore given a place in the 

 tables which follow. As a means of establishing the identity of a stone, the observation of 



