SIR DAVID Brewster's description of the lithoscope. 421 



Professor Neumann of Konigsberg — to important extensions of the Undulatory 

 Theory. I shall have occasion, in another paper, to submit to the Society a series 

 of experiments on the influence of the doubly refracting force in turning the 

 planes of polarised light out of the plane of incidence and reflexion ; but at 

 present I confine myself to the consideration of the intensity and colour of the 

 reflected pencils. 



The following observations were made chiefly with oil of cassia and oil of 

 anise seeds, on account of their great refractive power, and will be sufficient to 

 show the use and application of the Lithoscope : — 



Wit?i Oil of Cassia. 



Diamond. — The colour from the crystal is yellow, and the intensity of the 

 pencil four or Jive times greater than that of the pencil from the prism. The 

 Diamond may therefore be easily distinguished from Quartz and from Glass of 

 very high refractive power, for which it is often mistaken. 



Zircon. — The colour from the crystal is white, and the intensity of the pencil 

 two or three times greater than that of the pencil from the prism. 



Ruby, Oriental. — The colour from the crystal is yellow in artificial light, and 

 the intensity of the pencil nearly double that from the prism. 



Chrysoberyl, Cymophane. — The colour from an artificial face in candle-light 

 is yellowish, and the intensity of the pencil greater than that from the prism. 



Beryl. — The colour from the crystal is slightly blue, and fainter than the 

 pencil from the prism. 



Blue Topaz. — The colour from the crystal is a brilliant lilac, approaching to 

 Hue, when the plane of reflexion passes through the axis of the prism, and the 

 surface is artificially polished. On the same face, and in a plane at right angles 

 to this, the colour of the pencil is bluish pink. The intensity of the pencil is two 

 or three times less than that of the pencil from the prism. 



Upon a natural face, the colour of the pencil is bright blue in every azimuth. 



Above the polarising angle, the reflected pencil consists of polarised red light 

 and of unpolarised blue light. 



TovKzfrom Brazil, colourless. — The colour from a cleavage plane is a brilliant 

 hlue^ in every direction. 



Topaz, yellow, .from Brazil. — Upon natural and artificial faces, the colour is a 

 faint red, and the image scarcely perceptible. On another specimen, I found the 

 colour to be a bright pink, the intensity being much less than that of the pencil 

 from the prism. 



Cinnamon Stone. — On an artificial surface of this variety of garnet, the colour 

 of the pencil was yellowish red, and its intensity less than that from the prism.. 



Garnet, precious. — The colour of the pencil is reddish yellow., and its intensity 

 a little greater than that from the prism. 



