InstUute of FruTTCe. aS3 



tvhieh 1^. most confenieiu,) by employing those two methods at 



Oiu-e. 



We sliall also attain the same object, by transmitting in tlie 

 first place I his incident ray through a piece" of sulphated lime of 

 .1 proper thickness, the axis of which forms an an^le of 45'^ with 

 the primitive plan of polarization; for, when a ray is thus pre- 

 pared in order that it may be resolved into coloured fasciculi, k 

 is no longer necessary that the polarizing form of the second 

 plate should be very feeble : it is sufficient for it to combat ajid 

 weaken sufficiently the first irajiressions which it has receiver!, 

 in order that the difference of the numbers of oscillations operated 

 in the two plates may be less than eight. 



We fip.d, for example, that the polarizing power of Icelaiitl' 

 »pav is expressed by lS-t>, if we take that of sulphated lime as 

 luiity; or tliat it requires a thickness of sulphate of lime of lS-6 

 to destroy the moditications impressed on the rays of light by 

 one in thickness of Iceland spar. This relation will also be thiit 

 of Iceland spar, since rock crystal acts precisely like sulphated 

 lime. This report would only be IJ'?, according to other ex- 

 periments of i\l. Mains. the difference is insensible, and 

 IV!. Biot does not aaswer for it : all the other substances v/hich 

 he was enabled to subject to a similar proof, gave him the same 

 equality with the ratio of the polarizing forces : and this ended 

 ])y demonstrating to him, if demonstration was still necessary, 

 that the theory of the oscillations of light attenuates these phe- 

 nomena in their infancy, and brings them back to the cojisidera- 

 tion of tlie true forces by which they are produced. 



In the work which the same author has published on the po- 

 larization of light, M. Biot had been led to conclude, that the 

 luminous molecules, by passing through crystallized bodies, im- 

 dergo not only geometrical deviations in the position of their 

 axes; but that they also acquire real physical properties, which 

 they afterwards carry v.'ith them into space, and the permanent 

 impressions of which are manifested in experiments by affections 

 completely novel. The proofs of this result would depend on a 

 very delicate discussion, and would require an immense number 

 of experiments. The author has endeavoured in his « Memoir 

 on the physical properties acquired by lumuious molecules in 

 passing, through the crystals endowed with double refraction," 

 read to the Institute in IWay 1814, to exhibit less operose ine- 

 tliods to establish a consequence so extraordinary; and the theory 

 itself, which he deduced from it, pointed out to him the most 

 simple raean'3 of establishing it directly. 



He begins by polarizing a white ray by means of reflection on 

 a mirror. He transmits it afterwardb perpendicularly through 



u natural 



