Astronomical and Nautical Collections. 191 



which give colours to polarised light, when it is analysed at 

 its emersion, by means of a rhomboid of calcarlous spar : for 

 it is this production of colours that led Mr. Biot to a con- 

 trary supposition. For this purpose, I took a plate of sul- 

 fate of lime, about one hundredth of an inch in thickness, 

 which exhibited strong colours, and yet was in no danger of 

 having the different groups confounded : and having divided 

 it into two pieces, I placed them in the manner already 

 described. The two groups of fringes, instead of being 

 entirely separated, as they had been when the plates were 

 three or four times as thick, were mixed a little in the inter- 

 mediate space ; tut it was easy, nevertheless, to distinguish 

 in each of them the stripes of the three first orders, and to 

 ascertain that the right hand group, for example, was pola- 

 rized perpendicularly to the axis of the right hand plate ; 

 for when the principal section of the rhomboid was turned in 

 this direction, it disappeared entirely from the extraordinary 

 image ; and when, instead of the rhomboid, a pile of glass, 

 sufficiently inclined in its direction, was placed before the 

 lens, the left hand group only was discernible, and was in 

 this case perfectly free from the niixture of the colours of the 

 right hand group, exhibiting the usual appearance of a single 

 group. And when the experiment was made with two me- 

 tallic mirrors, the slight polarisation which they occasion in 

 the reflected rays, being destroyed by a pile of three or four 

 pieces of glass, properly inclined, before their passage 

 through the plates, the same direction of the polarisation is 

 still found for each of the groups of fringes. It is therefore 

 fully proved, that in one of these cases, as well as in the other, 

 the thin plates polarise the ordinary and extraordinary rays 

 in directions parallel and perpendicular to their axes. 



Having shown that the hypothesis of moveable polarisation 

 is contradicted by facts, whenever it is possible to distinguish 

 the ordinary from the extraordinary rays, I shall now pro- 

 ceed to a particular description of the phenomena of the 

 colours of crystallized plates, which led Mr. Biot to this 

 hypothesis, and I shall show that it is by no means neces- 

 sary to their explanation. 



[To be continued in our next Number.] 



