of the Chemical Rays of Light. 57 



tion, and shall next proceed to detail experiments which go 

 to prove that these rays can be polarized by reflexion. The 

 apparatus made use of consists of a mirror composed of nine 

 plates of parallel glass, by which a beam of polarized light 

 can be thrown upon an analyzing plate of thick flint glass, so 

 mounted that its angle of position can be changed, and its 

 plane of reflexion made to revolve round the polarized beam. 

 The image of the sun, after having been reflected from the 

 mirror at the polarizing angle, was thrown upon the analyzing 

 plate, the plane of which had previously been turned at right 

 angles to the plane of primitive polarization. From this plate 

 the ray was received upon a piece of photogenic paper, and 

 in three minutes a very faint impression was obtained. The 

 apparatus being still in the same position, another piece of 

 paper, cut from the same sheet, was substituted for that used 

 in the last experiment, and the ray was depolarized by inter- 

 posing a plate of mica between the mirrors ; in three minutes 

 the paper received a dark impression, thus affording another 

 illustration of the effect of crystallized plates on the polarized 

 chemical rays. 



Another piece of paper, also cut from the same sheet, was 

 made use of, but the analyzing plate was turned 90° upon its 

 axis before the ray reflected from it was allowed to fall on 

 the paper, and in three minutes an impression was obtained 

 equal in intensity to that produced in the last experiment. 



These three experiments, which occupied little more than 

 ten minutes in their performance, were executed at a time 

 when the sun's rays were of equal intensity, and the paper 

 used was also of equal sensibility ; and they afford an addi- 

 tional proof of the similarity of effect produced by polarizing 

 forces on the chemical and luminous rays. 



3. Polarization of the Chemical Rays by repeated single 

 Refraction. . 



To exhibit this phenomenon I prepared two bundles of 

 mica plates, nine in each bundle. These were arranged dia- 

 gonally in a tube, one half of which could be turned round 

 within the other. The tube was turned so that the planes of 

 both bundles were at right angles, and the sun's rays were 

 transmitted through it so as to fall on sensitive paper. In a 

 few minutes little or no effect was produced, but on turning 

 the planes round so as to coincide, an immediate darkening 

 of the paper took place. 



I next employed this method to polarize the chemical ema- 

 nations proceeding from the sky alone; but as it was neces- 

 sary in this case to have simultaneous results to obviate the 



