386 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII. 



the other in the violet region, it is clear that in the same 

 spectrum, from the very circumstance of their greater re- 

 frangibility, those in the violet will be relatively more 

 separated from each other than those in the red. The 

 result of this increased separation in the more refrangi- 

 ble regions is to give an apparent dilatation to them, 

 while the less refrangible are concentrated. The relative 

 position of the colors must also vary : the fixed lines 

 must be placed at distances greater than their true dis- 

 tances as the violet end is approached." I am quoting 

 from the fifth chapter of a work " On the Forces which 

 Produce the Organization of Plants," published by me in 

 1844. In this chapter, one of the chief points insisted on 

 is the necessity of using wave-lengths in the measure- 

 ment and discussion of spectrum results a suggestion 

 which, I believe, I was the first to make, and which I re- 

 newed in a memoir in the Philosophical Magazine (June, 

 1845). 



The importance of these remarks respecting the pecul- 

 iarities of the prismatic or dispersion spectrum may per- 

 haps be most satisfactorily recognized on examining such 

 a spectrum by the side of a diffraction or interference 

 one. By the aid of Fig. 86 this may be done. 



Regarding the space between the fixed lines D and E 

 as representing the central region, in each the fixed lines 



D and E are made 



A BC D E F G H 



coincident. The 



GH 



other lines are 

 laid off in the 

 prismatic as they 



Fig. 86. , -i 



appear through 



the flint-glass prism of the spectroscope; those of the 

 diffraction are arranged according to their wave-lengths. 

 It thus appears that in the prismatic, from the fixed line 

 D to A, the yellow, orange, and red regions occupy but 



