491 



In this communication the author again notices certain coloured 

 images, seen on each side of the flame of a candle, or other luminous 

 object seen through the agate, one at 10^, and a second at 21, 

 hut which he is not yet able to explain, and supposes to be a new 

 case of production of colours. 



Dr. Brewster next gives the result of experiments on the trans- 

 mission of light previously polarized, through various substances, and 

 notices those positions of crystallized bodies in which the polarization 

 continues unchanged, and those intermediate positions at which com- 

 plete depolarization takes place ; and adds, that such effects are also 

 occasioned by plates of horn, gum-arabic, glue, tortoise-shell, and 

 even plate glass. 



In addition to these properties, which mica, topaz, and rock crystal 

 possess in common with other crystallized bodies, Dr. Brewster ob- 

 serves, that they have the power of depolarizing in certain oblique 

 positions, which he considers peculiar to them. And at the same time 

 these bodies have certain oblique positions in which they do not de- 

 polarize, and which he terms neutral. 



In the preceding experiments depolarization has been effected by 

 the interposition of a second body, through which the rays are trans- 

 mitted after having been previously modified by some polarizing sub- 

 stance ; but the author observes that these effects may both be pro- 

 duced by the same crystals, if the direction of the light be such, that 

 after reflection from the posterior surface it will coincide with the 

 oblique depolarizing axis. 



It was in attending to the affections of light thus polarized and 

 depolarized by a plate of topaz iVVth of an inch in thickness, that 

 the author observed certain elliptical coloured rings, which he con- 

 siders entirely new ; and as he thinks them important, he takes much 

 pains to describe their various dimensions and successions of colours, 

 and represents them in coloured drawings. 



When a doubly-refracting substance is employed to view these 

 rings, the two images seen of them are differently coloured, the co- 

 lours of one set being complementary to those of the other. 



When a plate of agate, or a plane reflector at a specific angle of 

 inclination are employed, then only one or the other set is seen, ac- 

 cording to the relative position of the planes of incidence ; and it is 

 in the instance of using the plane reflector, that these rings appear 

 with such peculiar brilliancy on account of the absence of all foreign 

 light, which can, in this mode of making the experiment, be com- 

 pletely avoided. 



In addition to the above experiments, of which the author gives a 

 detailed account, he remarks, that light reflected at a particular angle 

 from the surface of blue steel is polarized, and thence infers that the 

 oxide is a thin transparent film ; that light is partially polarized by 

 reflection from all metallic surfaces. 



That light from white clouds or blue sky is partially polarized, but 

 that no part of the moon's light has suffered any degree of polari- 

 zation. 



