188 Astronomical and Nautical Collections 



consequences, to observe what pencils ought to influence each 

 other, and to produce fringes, we have always seen the 

 results of observation agree with it. Besides, the plates 

 employed in our experiments, being always at least four hun- 

 dredths of an inch in thickness, were capable of having their 

 edges cut obliquely, and producing by these means the sepa- 

 ration of the ordinary and extraordinary pencils, which are 

 then found polarised in directions parallel and perpendicular 

 to the principal section. It i^ not at all probable that this 

 mode of polarisation should be determined by the very 

 slight inclination of the two faces of the crystal, which 

 divides the light into two distinct pencils when this angle is 

 of only ten degrees : in short, a prism of glass of an equal 

 angle gives but a slight degree of polarity to the light by the 

 obliquity of its surfaces, which, even if it were more consi- 

 derable, would only cause a polarisation perpendicular to the 

 plane of incidence. Thus, if we consider the polarising 

 action of the prism of crystal as generally composed of two 

 parts, the one depending on the inclination of its surfaces, 

 and the other on its double refraction, we can only attribute 

 to the latter a polarisation of the two pencils in the directions 

 which are parallel and perpendicular to the principal sec- 

 tion, and we must conclude that they undergo the same kind 

 of polarisation when the parallelism of the faces prevents 

 us from distinguishing them, since this parallelism makes no 

 change in the laws of the double refraction. 



These consequences, however conformable they appear to 

 the rules of analogy, have, however, not been admitted by 

 Mr. BioT, who supposes light to receive, in thin crystallized 

 plates, and even in such as are a tenth of an inch in thick- 

 ness, a form of polarisation wholly different from that which 

 it exhibits when it emerges from a crystal thick enough to 

 separate it into distinct pencils. The opinion of so respect- 

 able a natural philosopher was of sufficient importance to 

 induce me to establish, by some new experiments, the true 

 direction of the polarisation of the ordinary and extraordi- 

 nary rays in crystallized plates : but the results which I have 

 obtained were always conformable to the general analogy of 

 double refraction. 



