ON THE ABSORPTIOH OF LIGHT, ETC. 477 



into unproductive channels. I have shown, I think 

 satisfactorily, however, in my Article on Light,* that the 

 applicability of the analogy of the colours of thin plates 

 to those of natural bodies is limited to a comparatively 

 narrow range, while the phenomena of absorption, to 

 which I consider the great majority of natural colours 

 to be referrible, have always appeared to me to consti- 

 tute a branch of photology sui generis to be studied in 

 itself by the way of inductive inquiry, and by constant 

 reference to facts as nature offers them. 



(2.) The most remarkable feature ih this class of 

 facts consists in the unequal absorbability of the. several 

 prismatic rays, and the total abandonment of anything 

 like regularity of progress in this respect as we proceed 

 from one end of the spectrum to the other. When we 

 contemplate the subject in this point of view, all idea of 

 regular functional gradation is at an end. We seem to 

 lose sight of the great law of continuity, and to find our- 

 selves involved among desultory and seemingly capri- 

 cious relations, quite unlike any which occur in other 

 branches of optical science. It is, perhaps, as much 

 owing to this as to anything, that the phenomena of 

 absorption in some recently-published speculations, and 

 in the view which Mr Whewell has taken in his Report 

 of the progress and actual condition of this department 

 of natural philosophy, read to this meeting, have been 

 characterized as peculiarly difficult to reconcile with the 

 undulatory theory of light. In so far as I have above 



*'This refers to the Article on -Light published in the " Encyclo- 

 paedia MetropoVitana '' tn 1826-7. 



