SIR DAVID Brewster's description of the lithoscope. 423 



Amethyst. — The colour of the pencil is 2ijine blue at all incidences and in all 

 azimuths. 



DiCHROiTE. — The colour of the pencil varies from reddish yellow, when the oil 

 is warm, to a Uuish colour when the oil is cold, the intensity being the same at 

 all incidences. 



Opals, Common and Precious. — The colour of the pencil a pale yellow, but 

 bright. 



Bloodstone. — Colour of the pencil lemon yellow. 



Leelite. — Colour of the pencil purple, becoming bluish at great incidences. 



Labrador Feldspar. — Colour of the pencil sky blue. 



Rock Salt. — The colour of the pencil blue, at all incidences. 



The following experiment was made with Sulphate of Lime: — 



With Balsam of Capivi the pencil was colourless, and faint at all incidences. 



With Castor Oil, the pencil was colourless at all incidences. 



The following experiments were made on Quartz with pencils of polarised 

 light, and E, polarised in planes + and — 45*^ to the plane of reflexion : — 



On the face of the prism, and in a plane passing through the axis of the crystal, 

 with Canada Balsam, E was orange yellow, and not so bright as 0, E being polar- 

 ised at a greater incidence than 0. 



In a plane at right angles to this = E, and both vanish together. 



On the face of the pyramid, and in a plane passing through the axis of the 

 crystal, is bluish and E a straiv yellow, being much brighter than E, and both 

 vanishing together. 



In order to reduce the reflective power of the quartz, I mixed Oil of Sassafras 

 and Oil of Anise Seeds, in such proportions that the combination had nearly the 

 same refractive power as the mineral. The image of a candle placed a few inches 

 from the reflecting surface was scarcely visible. 



In sun-light, however, and on a face of the prism in a plane passing through 

 the axis, the pencil E was a bright greenish blue at incidences below the polarising 

 angle, and it consisted of two pencils of different colours, the one a bright blue, 

 polarised in the plane of reflexion, and the other a yellow, polarised in a plane 

 perpendicular to it. 



In a plane perpendicular to that passing through the axis, the colour of E 

 was a bright purple at various incidences. 



On the face of the pyramid, the colour was blue in the one plane, and purple 

 in the other. 



The use of the Lithoscope, in distinguishing both solids and liquids, may be 

 extended in two ways — 



vol. XXIIL part III. 5 Y 



