424 Prof. PowelPs Remarks on Mr. Barton's Paper 



laws were reduced to a simple form; yet by the consent of 

 the world, the theory of gravitation, though imperfect as a 

 theory of attraction, though sometimes completely disguised 

 by the forces which Newton left unexplained, was adopted as 

 a true system. That the existing theory of undulations stands 

 in the same relation to the complete theory of light as New- 

 ton's universal gravitation to the complete theory of attrac- 

 tions, I have not the slightest doubt. 



With regard to the importance, as a difficulty, of the num- 

 ber of interruptions in the spectrum produced by nitrous gas, 

 I do not entirely agree with you. If a plausible reason can be 

 found, on either theory, for a single interruption, I have no 

 doubt that good conjectural reasons will very soon be found 

 for a thousand interruptions. And with regard to the atten- 

 tion which, in shaping an optical theory, these interruptions 

 at present deserve, I may perhaps not quite agree with you. 

 They are not yet disciplined under laws: they stand a mere 

 " mob of facts ;" and no one can tell what they seem to indi- 

 cate. But every attention ought to be given to reduce them 

 to rules; and the apparent uniformity of the lines of nitrous 

 gas, at least as compared with the solar lines or the interrup- 

 tions by different kinds of glass, seem to make it probable that 

 this beautiful and important discovery (setting apart its prac- 

 tical uses,) may assist us in discovering the laws which govern 

 the most obscure and most difficult part of optics. The theory 

 of crystalline absorption cannot then, I think, remain long 

 without explanation. With sincere respect, I remain, 



My dear Sir David, your faithful Servant, 



Observatory, Cambridge, May 7, 1833. G. B. AlRY. 



LXX. Remarks on Mr. Barton's Paper " On the Inflexion of 

 Light" in the London and Edinburgh Journal of Science, fyc. 

 No. X. By the Rev. B. Powell, M.A. F.R.S. Savilian 

 Professor of Geometry, Oxford*. 



f T , HE question respecting the truth of the undulatory theory 

 ■*■ of light is at the present time exciting more discussion 

 than we might have anticipated so abstract a topic would be 

 likely to call forth : this circumstance alone is a favourable in- 

 dication of the increasing interest taken in matters of pure 

 science ; and whether we consider the importance of the sub- 

 ject, the beautiful and refined nature of the inquiry, or the 

 value of fair and well-urged objections and dispassionate con- 

 troversy for eliciting truth, and promoting the real advance 



Communicated by the Author. 



