98 THE DIFFRACTION SPECTRUM. [MEMOIK VI. 



In a work published in 1844 "On the Forces which 

 Produce the Organization of Plants," the results I ob- 

 tained were illustrated by steel engravings, one of them 

 colored, with a view of comparing the prismatic and dif- 

 fraction spectra, and of showing the photographic ac- 

 tion 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 de- 

 tails may be found in a former number of the Philosoph- 

 ical Magazine (June, 1845), and in that article it is par- 

 ticularly recommended to use wave-lengths for designat- 

 ing rays, instead of titles of color, as red, yellow, etc. 



The chemical action of the diffraction spectrum is, 

 however, only given in part in M. Eisenlohr's publica- 

 tion. He speaks of what occurs in the more refrangible 

 regions, and takes no notice of the action in the centre of 



O / 



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 spectrum the yellow occupies the 

 middle space, and that the light grades off in one direc- 

 tion to the red, and in the other to the violet. These 

 terminate at equal distances from the yellow, so that 

 the wave-lengths for the extreme violet, the centre of 

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



Now from the extreme violet to the yellow the spec- 

 trum blackens silver preparations, and this is what is 

 commonly understood 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 

 with the other half. It can suspend or arrest the black- 

 ening which would be caused by contemporaneously act- 

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

 light may have done some time before. Some remarks 



