420 SIR DAVID Brewster's description of the lithoscope. 



diamond and chromate of lead, is much greater than from water to oil of cassia. 

 In this case, a standard pencil will be obtained from a standard fluid of a known 

 refractive power, placed on a surface of the same glass as the prism, as will be 

 presently explained. Impurities in oils and other substances may be thus 

 detected, and specific gravities approximately ascertained. 



In order to make these experiments easily and correctly, I contrived the 

 instrument now on the table, which was constructed for me many years ago by 

 the late Mr Dollond, and which may be called a LitJioscope, from its application 

 to the discrimination of precious stones. 



It consists of a rectangular prism PR, mounted as shown in Plate XIX., 

 where AB is a pillar, carrying the horizontal and vertical branches CD, CE. The 

 prism turns vertically round a joint at E, and may be lifted round that joint by 

 the hand, from its horizontal position, or raised slowly by the milled-head F of a 

 screw EG, resting at G upon the branch CD. A small circular plate H, at the top 

 of a screwed rod I, is made to rise or fall by means of the milled head KL ; and 

 has also a motion of rotation round the top of the rod. Two steel rods de, of, 

 carry two small aperture tubes <?,/, through one of which, /, the incident pencil 

 falls nearly perpendicularly upon one side of the prism, and at an angle of 45" 

 upon its base. These rods, with their tubular apertures, slide through short tubes 

 at m and n, in order that they may receive the incident and reflected pencils. 

 Bottles containing oil of cassia and other standard oils are placed at a, b, c. 



In using the Lithoscope, the mineral upon whose surface the observation is to 

 be made is attached, if necessary, by cement or otherwise to the plate II. A drop 

 of the proper oil is then put upon the surface of the mineral, and tlie prism 

 brought into a horizontal position. If it does not touch the oil, it is brought into 

 contact with it by the milled head KL, which raises the plate H. A plate or film 

 of oil, with parallel surfaces, is thus formed between the mineral and the prism ; 

 and if we now view the image of the sun or of a small bright flame, transmitted 

 through the aperture/, reflected from the film, and reaching the eye through the 

 aperture e, we shall see one image consisting of two coincident images, the one 

 reflected from the surface of the prism and the oil, and the other from the surface 

 of the oil and the mineral. In order to separate these images, the prism is 

 slightly raised round the joint at E, by turning the milled head F. The prism 

 image will then appear at the right hand of the other image ; and by a compari- 

 son of the colour and intensity of these images, we obtain the information we 

 desire. 



It was by an apparatus of this kind, furnished with a graduated circle, that I 

 discovered the influence of the doubly refracting force in polarising common light 

 in planes inclined to the plane of reflexion. These experiments were published 

 in the " Philosophical Transactions" for 1819, and have led several distinguished 

 mathematicians — Professor Maccullagii of Dublin, M. Seebeck of Berlin, and 



