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XXIX. — Description of the Lithoscope, an Instrument for distinguishing Precious 

 Stones and other bodies. By Sir David Brewster, K.H., F.R.S. (Plate XIX.) 



(Read 18tli January 1864.) 



In examining the light reflected from the surface of calcareous spar, when in 

 contact with different fluids, I observed several phenomena, both of light and 

 colour, which led me to make the same experiments on the natural and artificial 

 surfaces of the precious stones and other minerals. In these experiments, the in- 

 tensity of the reflected pencil varied, as might have been expected, with the 

 refractive power of the fluid ; but I was not prepared for the curious fact, that 

 when the reflective power of the surface was reduced almost to nothing, the sur- 

 face was no longer able to reflect white light, but reflected pencils of different 

 colours, depending on the approximation of the refractive power of the fluid to 

 that of the solid. When the crystal had much double refraction, the colour of 

 the reflected light varied with the inclination of the plane of reflection to the 

 plane passing through the axis of the crystal. 



In making these experiments, a drop of fluid is placed upon the surface of the 

 body to be examined, and upon this drop is laid the broad surface of a rectangular 

 prism of glass. A parallel film of fluid will thus be formed between the crystal 

 and the prism, and any luminous image reflected from the two combined surfaces 

 will consist of two images superposed, one reflected from the common surface of 

 the crystal and the fluid, and the other from the common surface of the fluid and 

 the glass. By a slight inclination of the prism, the two images are separated, so 

 that we can examine and compare them, the pencil from the prism and fluid sur- 

 face being a standard light, with which the pencil reflected from the common 

 surface of other crystals and the same fluid may be compared, in reference to 

 colour and intensity of light. 



By the use of fluids, therefore, of known refractive power, we can distinguish 

 precious stones and other minerals, the standard of intensity and of colour being 

 the invariable pencil reflected from the prism and fluid surface. A continuous 

 scale of refractive power, embracing a great variety of minerals and other bodies, 

 may be obtained, by mixing fluids in different proportions. The fixed oils com- 

 bine readily, not only with one another, but with many of the volatile oils ; and 

 the aqueous solutions of alcohol and sugar, will to some extent, be useful auxi- 

 liaries in reducing the refractive power of the surfaces on which they are placed. 



This method of distinguishing bodies is not limited to those which are solid. 

 By standard prisms of crystals, or of different kinds of glass, we may distinguish 

 fluids of all kinds, because the range of refractive powers from tabaheer to 



VOL. XXIII. PART IIT. 5 X 



