126 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIK VII. 



Several years before the announcement of the discov- 

 ery of photography by Daguerre and Talbot (1839) I 

 had made use of that process for the purpose of ascer- 

 taining whether the so-called chemical rays exhibited 

 interference, and in 1837 published the results in the 

 Journal of the Franklin Institute, Philadelphia (July, 

 1837, p. 45). In this, as will be seen by consulting that 

 publication, I was successful. 



Encouraged by this result, I some years subsequently 

 attempted to photograph the diffraction spectrum itself. 



The following is an extract from the publication I 

 made of this experiment in 1844: "Through a narrow 

 fissure or slit, #, Fig. 13, I direct a beam of light hori- 

 zontally, and at a distance of twelve feet receive it on a 

 grating, b c, the lines of which are parallel to the slit. 

 Having found that there are advantages in using a re- 

 flecting grating, I silvered this with tin amalgam, which 

 copies the ruling perfectly. There is no difficulty in 

 placing b c so that the ray coming from a falls perpen- 

 dicularly on it, for all that is required is to move the 

 grating into such a position that the light, after reflection 

 from it, goes back through the fissure a. At a distance 

 from b c of 

 six inches I 



place an ach- L _ m _ J 

 romatic ob- ::: *^ c 



ject- glass, dj 

 in such a po- 

 sition that it 



shall receive perpendicularly the reflected rays of the 

 spectrum of the first order. The lens is brought as near 

 to the grating as possible without its edge intercepting 

 the ray coming from a. In the focus of this lens, at <?/, 

 a ground glass is placed. This portion of the apparatus 

 is, however, nothing more than the sliding part of a com- 



