154 Prof. Draper on the Diffraction Spectrum. 



thus employ a reflected spectnim. It is far more brilliant 

 than the transmitted one, and the silvering acts so perfectly that 

 the most minute fixed lines may be seen. 



In the work above referred to, the results were illustrated by 

 steel engravings, one of them coloured, with a view of comparing 

 the prismatic and interference spectrum, and of showing the 

 photographic action on iodide, chloride, and bromide of silver. 

 These results are substantially the same as those of M. Eisen- 

 lohr. They give the wave-lengths at which action begins and 

 ceases on each of those substances. Some of the details may 

 be found in a former Number of this Journal (June 1845), 

 and in that article it is particularly recommended to use wave- 

 lengths for designating rays, instead of titles of colour, as red, 

 yellow, &c. 



The chemical action of the diffraction spectrum is, however, 

 only given in part in M. Eisenlohr's publication. He speaks 

 of what occurs in the more refrangible regions, and takes no 

 notice of the action of the centre of the spectrum, and in its less 

 refrangible space. 



I will state in detail what I here mean. It is well known that 

 in such a spectrura the yellow occupies the middle space, and 

 that the light grades off in one direction to the red and in the 

 other to the violet, which terminate at equal distances from the 

 yellow, so that the wave-lengths for the extreme violet, the ceuti'e 

 of the yellow, and the extreme red, are as 1, 1^, 2. 



Now from the extreme violet to the yellow the spectrum 

 blackens silver preparations, and this is what is commonly un- 

 derstood by its chemical action by those who have written on 

 photographic subjects. But from the yellow to the extreme red 

 the spectrum is also active, though in a different way. This half 

 is in antagonism to the other half. It can suspend or arrest the 

 blackening which would be caused by contemporaneously-acting 

 diffused light ; nay, even more, it can undo what such light may 

 have done some time before. Some remarks on this topic may 

 be seen in the Philosophical Magazine, February 1847. 



The centre of the yellow space is therefore the point of a 

 change in photographic action. From the commencement of the 

 red to that point the action is negative ; from that point to the 

 violet extreme it is positive. The phase of action changes in the 

 centre of the yellow. 



M. Eisenlohr has made an allusion to the heating power of 

 the spectrum, and to that point my attention has also been 

 directed. This is the result to which I eventually arrived, — that 

 the centre of the yellow is the hottest space, and that the beat 

 declines equally to the two ends of the spectrum. I attempted 

 to form a diffraction spectrum without the use of any dioptric 



