422 SIR DAVID BREWSTEE's DESCRIPTION OF THE LITHOSCOPE. 



Gabnet, fi^om Elie> in Fife.— The colour of the pencil is reddish yellow, but 

 the intensity very much less than that from the prism. 



Peridot. — The colour of the pencil is brownish yellow, and its intensity much 

 less than that from the prism. 



EucLASE. — The colour of the pencil is hrillimit yellow, and its intensity less 

 than that from the prism. 



Prehnite. — The colour of the pencil is blue, but brighter than that from topaz. 



Quartz. — The colour of the pencil is 2^ faint blue, both on natural and artificial 

 faces, and its intensity less than that from the prism. 



Amethyst. — The colour of the pencil is a faint blue, as in quartz, and its 

 intensity less than that of the prism. 



DiCHROiTE. — The colour of the pencil is a blue, fainter than in quartz, and its 

 intensity less than that of the prism. 



Black Tourmaline. — The colour of the pencil in a plane perpendicular to the 

 axis is a brownish yellow in candle-light, and its intensity much less than that 

 from the prism. 



AuGiTE. — The colour of the pencil, in candle-light, is yellow, and its intensity 

 Jess than that from the prism. 



Sulphate of Barytes. —The colour of the pencil is brick-red, less faint than 

 in topaz, and the same at all incidences and in all azimuths. The intensity of 

 the pencil is very much less than that from the prism. 



Sulphate of Lead. — The colour of the pencil, in candle-light, is brilliant 

 yellorv, and its intensity t\vice as great as that from the prism. 



Leelite. — The colour of the pencil is greenish blue when the oil is warm, the 

 intensit}^ of the pencil increasing with the incidence. 



Jet. — The colour of the pencil is a very faint yellowish red, and the same at all 

 incidences. 



The following experiments were made with Oil of Anise Seeds, a less refrac- 

 tive oil than Oil of Cassia : — 



Quartz, on a face of the prism. When the plane of reflexion passes through 

 the axis, the image of a candle is just perceptible, and of a brick-red colour. 



In sun-light the colour is blue, of little intensity, hwtpink when the oil is first 

 put on ! 



When the plane of reflexion is perpendicular to the axis, the image of a candle 

 is distinctly visible, and of a brick-red colour. In sun-light the colour is red, but 

 yellow when the oil is first put on. 



On a face perpendicular to the axis of the crystal, the colour in sun-light is 

 blue. 



On a face of the pyramid, the colour is pink in a plane passing through the 

 axis, but light red in a plane perpendicular to the axis. 



