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LONDON, EDINBURGH and DUBLIltf^ag**^ 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



NOVEMBER 1846. 



LI. The Astronomer Royal on the Bands formed by the 

 Partial Interception of the Prismatic Spectrum. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



QINCE my return from the continent I have seen a report 

 ^ of the ttansactions of the British Association at its late 

 meeting ; and my attention has been directed to a communi- 

 cation by my friend Prof. Powell, " On the Bands formed by 

 the Partial Interception of the Prismatic Spectrum," which, 

 for care in observation, caution in theory, and courtesy in the 

 expression of doubt, merits well to be taken as a model for 

 papers of a similar character. 



Prof. Powell found that in certain points there is an appa- 

 rent discordance between his observations and the result of my 

 deductions from the Undulatory Theory. It is curious that 

 on these very points there is the most perfect accordance. The 

 discrepance has arisen from this circumstance: that my short 

 paper in the Philosophical Transactions, 1841, part 1, was 

 intended rather to show the possibility of explaining the bands, 

 than to enter into details as to their variation under varying 

 conditions of experiment ; that I therefore investigated closely 

 only that case in which the bands are most conspicuously 

 formed ; and that an expression used by me, with reference 

 only to that one case, has been (not unnaturally) applied to 

 other cases, to which it is totally inapplicable. 



In page 8 of my paper, line 20, I have made this restrictive 



supposition : " And if — j— be equal to the change of k corre- 

 sponding to a change of 2w in R." This limitation, which 

 establishes a relation between the aperture of the eye or telescope 

 Phil. Mag. S. 3. Vol. 29. No. 195. Nov, 1846. 2 A 



