474? On the Bands formed in the Prismatic Spectrum. 



might furnish the means of proving either the correctness or 

 the fallacious nature of the temperature theory, to which ob- 

 jections have been advanced in this paper. 



LXVI. Note on the Bands formed by Partial Interception in 

 the Prismatic Spectrum. By Professor Powell.* 



IN a communication to the Physical Section of the British 

 Association at its last meeting, I offered some remarks on 

 the question relative to the above-named singular phaenomena; 

 in particular stating one experimental result, originally an- 

 nounced by Sir D. Brewster, which I had fully verified, and 

 which had appeared to him at variance with the theory of 

 Mr. Airy, along with another fact of my own detection, viz. 

 that when the aperture was contracted the bands became more 

 vivid. No one, I trust, was led, from the manner in which my 

 remarks were reported, to imagine that I brought this forward 

 in any spirit of hostility to the theory. On the contrary, I never 

 entertained a doubt (and I believe I expressed this at the 

 meeting) that upon every ground of analogy and probability 

 the theory would still be found adequate to the solution of the 

 difficulty : I fully expected that it wanted only the introduc- 

 tion of some further condition or modification, though, as it 

 appeared, no such condition bad yet been pointed out. It 

 seems that this essential condition, now first brought to notice 

 by the Astronomer Royal in his paper in the last Number of 

 this Journal, had in some way escaped the attention of all 

 those who had read his previous papers in the Philosophical 

 Transactions, ever since the first announcement of Sir David 

 Brewster's objection at the British Association at Manchester, 

 1842f, and notwithstanding that his detailed repetition of that 

 objection was fully discussed at the Cambridge meeting in 

 1845. And I am anxious to seize this opportunity of ac- 

 knowledging and recording my conviction, that it entirely 

 supplies what was wanting to relieve the theory of the objec- 

 tion which seemed to beset it; and that this has proved but 

 one more of those already numerous and striking instances, 

 in which what seemed a most formidable difficulty in the un- 

 dulatory theory of light has been converted into one of its 

 strongest points of support. 

 Oxford, November 2, 1846. 



* Communicated by the Author. 



f See Report, Sectional Proceedings, p. 12. 



