On the Polarization of the Chemical Rays of Light. 53 



of Professor Forbes, after my paper had been read. In a 

 communication which I have recently received from him, he 

 also says, " In spring 1839 I tried the experiment of letting 

 the picture formed by polarized light passing through calc 

 spar fall upon sensitive paper, then newly discovered : — whe- 

 ther from the fault of the paper, 1 know not, but on my first 

 trial I obtained no kind of effect, and my attention being oc- 

 cupied with other matters, I never repeated it : but at Bir- 

 mingham, in August 1839, being requested to give some ac- 

 count of the Daguerreotype, which I had seen in Paris, I men- 

 tioned the experiment, and pointed out its valuable application 

 to fix with unerring accuracy phsenomena of diffraction and 

 polarization, which different eyes have seen differently, and 

 which, regarded as the test of theories, would thus be pre- 

 served with unimpeachable fidelity for examination at leisure 

 by every eye." 



With these few preliminary remarks I shall proceed to the 

 paper itself. 



Liverpool, 22d March, 1841. 



In the course of last summer it occurred to me that the in- 

 visible chemical rays of light might be subject to the laws of 

 polarization, and early in the month of July I instituted a se- 

 ries of experiments to determine the point. In all investiga- 

 tions of this nature it is of importance that the solar light 

 should continue for a certain time of nearly the same inten- 

 sity; but during the summer the sky was so frequently over- 

 cast that a very few days only could be devoted to the subject, 

 and for a considerable part of the autumn the sun's altitude 

 has been too low. For these reasons I have been unable to 

 pursue the investigation so far as I could have wished, but I 

 have nevertheless obtained a sufficient number of results to 

 establish the principal facts, and I have thought it better to 

 bring these forward at the present time than to allow the sub- 

 ject to lie over for the several months which must intervene 

 before I can again resume it. 



I have succeeded in polarizing the chemical rays: — 1st, as 

 they proceed directly from the sun ; 2nd, as they exist at the 

 extreme violet end of the spectrum ; 3rd, as they fall from the 

 sky; and by three different processes, — double refraction, 

 reflexion, and repeated single refraction. I shall describe in 

 succession these processes, with the apparatus used. 



1. Polarization of the Chemical Rays by double Refraction. 

 The first important fact in regard to the chemical rays is, 

 that they are susceptible of double refraction, in the same man- 

 ner as the luminous rays are. To prove this, a prism of cah. 



