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Art. XXXIV . — Proceedings qf the Astronomical Society of 
London. 
March 10. 1820. A PAPER was read, entitled, “ On the 
doubly refracting power of rock-crystal as a means of microme- 
trical measurement, by the Rev. W. Pearson, LL. D. F. R. S. 
The late Abbe Rochon pointed out a very ingenious mode of 
constructing a micrometer by means of two prisms of rock cry- 
stal, cut and united in a peculiar manner. By placing between 
the object-glass' and eye-piece of a telescope, a double image 
is produced, and these two images may be brought into contact 
by means of a contrivance for bringing the prisms nearer to or 
more remote from the eye-piece. The most prominent disad- 
vantage of this plan is the difficulty and almost impossibility of 
procuring plates of rock-crystal of sufficient size and of uniform 
density. To remove this impediment, Dr Pearson constructs a 
prismatic solid, (a solid consisting of two prisms, united in such 
a manner as to have two parallel faces, and produce two ima- 
^s), and places it before the eye end of the telescope. This se- 
parated the two images of the object examined by a constant 
angle, depending on that of the prism ; and in order to bring 
the images into contact, an eye-piece of variable power is applied 
to the telescope. By separating the lenses of which it consists, a 
gradual increase of power is produced, and the divisions on the 
outside of the eye-tube point out the power *. The constant 
angle of the prisms, divided by the magnifying power of the in- 
strument, is the angular measure of the object examined. In 
the course of this paper, the author notices that the opticians of 
Paris, when directed to cut a prism of rock-crystal which shall se- 
parate the images of an object any given number of minutes^make 
the angle of the prism itself equal to same number of degrees. 
* This variable eye-piece was invented by Dr Brewster in 1805, and a draw- 
ing and description of it was then sent to Mr Carey. The application of it to double 
image micrometers is described in his Treatise on New Philosophical Instruments, 
p. 66. 67. This eye-piece was many years afterwards brought out g,t Paris by 
M. Cauchoix, under the title of Polyalde^ and the invention of it has been ascrib- 
ed to this ingenious artist in the Edinburgh Review., vol. xxxii. p. 374. The same 
eye-piece forms the subject of a paper recently read before the Royal Society of 
London by Dr Kitchener, who is said to have improved it. — Eo, 
