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LIII. On the Methods of cutting Rock Crystal/or Micrometers. 

 By William Hyde VVollaston, M.Z>. F.R.S.* 



F OR the mere purpose of examining the phtenomena of double 

 refraction, it is extremely easy for any skilful workman to com- 

 bine a wedge of rock crystal, or any other doubly refracting sub- 

 stance, with anotlier wedge of crown glass opposed to it, in such 

 a manner that a luminous object seen through them shall ap- 

 pear in its true place by ordinary refraction, accompanied by a 

 second image at a small distance, produced by the extraordinary 

 refraction of the crystal. 



In consequence of the dispersion of colours which occurs in 

 employing different substances, the above combination is not 

 suited for the purpose of the micrometer invented by the Ahh6 

 Rochon ; but it is not difficult to obtain such a section of rock 

 crystal as may be substituted for the wedge of glass, so that the 

 pencil of light shall be restored to its original direction void oi 

 colour, without diminishing the separation of the images occa- 

 sioned by the first wedge. 



But since the degree to which the double refraction of rock 

 crystal separates the two portions of a beam of light transmitted 

 through it, is not so great as may frequently be wished, it be- 

 comes desirable to increase this effect beyond what can be pro- 

 duced by the most obvious method of employing that substance; 

 and it does appear from M. Rochon's own account of his con- 

 trivance f, that he fully succeeded in accomplishing this end. 

 But although he informs us that the means employed, as best 

 suited to his views, had exactly the effect of doubling the amount 

 of deviation produced by ordinary means, he has not chosen to 

 explain the mode of construction he adopted, and has merely 

 referred to a certain artist living at that time in Paris, who was 

 in possession of his secret, and skilful in applying it to the con- 

 fctruction of micrometers. 



As I have reason to think that the method to which he alludes 

 in his memoir has never yet been described, I design, in the 

 present communication, to explain a combination which I have 

 found advantageous, and which I think must be the same as that 

 of M. Rochon. 



I shall hope to render the principles of this construction in- 

 telligible to every one actjuainted with the original observation 

 of Huygens on the properties of polarised light, and to enable 

 3n3f competent artist to cut wedges from hexagonal prisms of 



• From Ttansiictions of the Royal Society for 1820, Fart I. 

 f Journal de Phj/ii'jHc, An, 'J, 



rock 



