384 Institute of France. 



a natural plate of sulphated lime of a thickness which exceeds 

 tW of a millimetre, and of which the axis forms an aiigle of 

 45° with the primitive plan of polarization. The two bundles, 

 ordinary and extraordinary, which result, both issue in the same 

 direction: besides, according to the theory formerly established, 

 tlicse two fasciculi come out white ; and if the thickness is onh 

 a few centimetres, they act as being polarized at right angles, 

 one in the direction of the primitive polarization, and the other 

 in a rectangular direction. 



He excludes this second fasciculus by transmission through a 

 j)ile of pieces of glass arranged so as to reflect it in totality, with- 

 out acting in nny way on the first fasciculus, which alone remains 

 visible through the pile. 



Then, if we compare the latter with a ray polarized in the 

 same direction by simple reflection on a glass, we see that they 

 appear perfectly similar as to the geometrical arrangement of the 

 particles, and in the direction of the polarization ; for they act 

 absolutely in the same way, when we prove them by a prism of 

 Iceland spar, or bv reflection on an inclined glass. In the first 

 rase they are also resolved into two white images, which disap- 

 pear and reappear at the same limits ; in the second they are 

 reflected in the same way, and escape both together from re- 

 flection. Also, if we make them pass through thin laminae of 

 sulphated lime, rock crystal for instance, they also give images 

 coloured with the same tints ; and both of them cease to give 

 any when these laminae have attained certain limits of thickness. 

 But to so close a resemblance a capital difference is added : it 

 is, that beyond these limits, the thickness always increasing, the 

 ray polarized by simple reflection never gives colours ; whereas 

 the fasciculus which has in the first place passed through the 

 thickness e of sulphated lime begins to give them over again, 

 when the thickness of the second lamina of this subtance enters 

 into the limits e + -^-^^ of a millimetre. It preserves, there- 

 fore, in this respect the durable trace of the physical impressions 

 which it had at first undergone on passing through the first cry- 

 stallized plate, and these impressions are relative to the thick- 

 ness e of this plate ; whereas the ray polarized by simple reflec- 

 tion is modified completely, as if it had passed through a plate 

 crystallized of an infinite thickness. The difference of the two 

 rays is also manifested in several other phaenomena, indicated 

 by theory, and which it would have been difficult if not impossi- 

 ble to account for otherwise. 



In his previous inquiries respecting crystals endowed with 

 double refractioi, the author has shown that we may obtain 

 fasciculi coloiuedj both ordinary and extraordinary, with thick as 



well 



