220 ANNUAL REPOBT SMITHSONIAN INSTITUTION, 1911. 



practice, will also enable one to determine with similar ease the 

 amount and kind of double refraction and the degree of dispersion. 



As will be seen from the diagram (pi. 1, fig. 2), the main principle 

 of the instrument is the same as that of the reflectometer, the refrac- 

 tive index being measured against a standard of highly refracting 

 glass by means of the angle of total reflection, which of course 

 diminishes the nearer the index of the stone approaches that of the 

 standard. It is, however, in the details of construction that such 

 a marked advance has been made, and it is these details winch make 

 all the difference in practical work. To use this instrument all that 

 has to be done is to place the stone under examination in optical 

 contact with the flat surface of the dense glass, and arrange it so that 

 a good light (preferably monochromatic) enters the instrument 

 through the lower lenticular opening, when the refractive index 

 is read off directly on a scale, without calculation. 1 



Some little advance has also been made in the construction of the 

 dichroscope for determining pleochroism. As will be seen from the 

 illustration (pi. 1, fig. 3), the instrument in use to-day is provided 

 with a revolving holder tipped with wax, to which the stone is readily 

 fixed, leaving both hands free — a detail, but again it is such details 

 that count in practice. 



Taking the properties of precious stones as a whole, the great point 

 about them is the remarkable combination of qualities; it is not so 

 much that they have optical properties which make them extraordi- 

 narily beautiful, or that they have remarkable hardness and dura- 

 bility, but they have both, and it is the impossibility of reproducing 

 this combination in any other material that renders the detection of 

 imitations a matter of ease in the hands of anyone familiar with the 

 facts. 



Of course, glass is the obvious material to use in the production 

 of imitation gems, and, as I have indicated, it has been so used from 

 time immemorial. And, in later times, while science was equipping 

 the expert in precious stones with the means of identifying them 

 with certainty, the maker of imitations was also invoking its aid 

 in the production of more successful imitations. 



In modern times the manufacture of imitation gems on scientific 

 lines was introduced by Strasser in Vienna; hence the name "strass," 

 although "paste" is the more commonly used term. 



The finest of such modern paste bears little relation to the clumsy 

 imitations of early times; the glass is specially prepared in order to 

 combine, as far as possible, the necessary optical qualities with a 

 fair amount of durability. It is well known that by using lead 



1 It is impossible here to give any detailed account of the construction and use of this instrument. Full 

 particulars will be found in The Herbert Smith Refractometer, published by J. H. Steward, 400 Strand. 



