■ Onihe double Refraahn of Rod Cryjlal. 307 



it within the reach of ihofe who might have occafion to mea- 

 fure angles with it. 



A defire to accomplifli this objeft induced me to undertake 

 fome refearchos in regard to the catting of rock cryftal, and 

 experiments on its double refradion, founded on thofe made 

 by Beccariaand Rochon. With rock cryltal alone I formed 

 viediums doubly refringent. This is the name which Rochon 

 gives to thefe inftruments cut cylindrically and compofed of 

 two or three prifms of this fubltance, which are perfectly 

 achromatic, and produce verv itrong double refraftion. I 

 conllrudled one with three prifms, in wliich the angle of 

 double refraction is one degree eight minutes: and I find 

 that larger may ftill be obtained. 



I fliall not here indicate the direftion in which I cut my 

 different prifms of rock crydal in order to obtain the maximum 

 of double refraftion, becaufe I have Uill fome experiments to 

 terminate before [ can acquire a thorough knowledge of them. 

 Among thofe which I cut for my experiments, there is one 

 which produces efTects fo fingula'r that I think it my duty to 

 defcribe it. 



This prifm, the feclion of which is an ifofcelcs triangle, 

 has one of its angles obtufe, and of more than 100 degrees. 

 When one looks through the two faces which form the ob- 

 tufe angle, and in a direftion parallel to the face oppofue to 

 it, the objcft appears neither difplaced nor fenfibly coloured, 

 but only inverted in fuch a manner, that what is on the right 

 appears on the left; and vice verfa. If A capital L, for ex- 

 ample, cut out and applied to the pane of a window, be 

 looked at, the horizontal line of that letter, initead of being 

 on the right below the vertical one, appears to be fituated on 

 the left. By continuing to look at this letter, if the prifm be 

 turned on itfclf, and in fuch a manner as if it were traverfed 

 by an axis parallel to the dircAion in which the letter is 

 viewed, the image of the letter turns at the fame time as the 

 prifm ; but it rnoves twice as quick — fo that, if the prilin 

 makes one turn, the image of the letter makes two. 1 made 

 other very fmgular experiments with this prifm ; but as it 

 would be too tedious to detail them here, I fliall referve them 

 for a memoir, in which 1 (hall explaiii the means I em- 

 ployed to afcertain the rules which nnift be followed to cut 

 rock crvftal in the diredion that produces the w^iuw«;w of 

 its double rcfraftion, and to be able to conftrudt, without 

 repealed trials; the inflrumenl invented by Rochon, to mca- 

 fure, with very great prccifion, diltances of every kind. I 

 ihall treat of its application to mines ; and fliall defcribe the 

 U % method 



