178 Astronomical and Nautical Collections. 



probable, by other remarkable properties of polarised light 

 which remain to be explained. 



Mr. Arago and myself, in studying the interference of 

 polarised rays, discovered that they exert no influence on 

 each other when their planes of polarisation are perpendicular 

 to each other, that is to say, that in this case they produce no 

 fringes, although all the conditions, which are commonly 

 necessary for their appearance, are scrupulously fulfilled. I 

 shall mention the three principal experiments which served 

 to establish this fact ; beginning with that which was made 

 by Mr. Arago. It consists in causing the two pencils, 

 emitted by the same luminous point, and introduced through 

 two parallel slits, to pass through two very thin piles of 

 transparent plates, such as those of mica, or of blown glass, 

 sufficiently inclined to polarise almost completely each of the 

 two pencils, taking care that the two planes, in which they 

 are inclined, should be perpendicular to each other : in this 

 case no fringes are observable, whatever pains we may take 

 to compensate the difference in the paths, by causing the 

 inclination of one of the piles to vary very slowly ; although, 

 when the planes of incidence of the piles are no longer per- 

 pendicular to each other, we always succeed in this manner 

 in obtaining the fringes ; and the same result is obtained 

 with much thicker plates of glass, provided that proper care 

 be taken to form and polish them very correctly ; and to 

 vary their inclination very slowly, in order that the fringes 

 may not pass unperceived. In proportion as the planes of 

 the two piles are further removed from parallelism, the 

 fringes are weakened, and they wholly disappear when they 

 are at right angles, provided that the polarisation of the 

 rays have been tolerably perfect. It follows, from this expe- 

 riment, that the rays of light, polarised in the same plane, 

 interfere with each other in the same manner as rays not 

 modified ; but that this influence diminishes as the planes are 

 separated, and disappears when they are at right angles. 



A similar conclusion may be inferred from the following 

 experiment. We take a plate of sulfate of lime, which, 

 though Dr. Brewster has shown that it has two axes, yet, 

 when divided into plates parallel to their common plane, 



