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XLI. On Lines in the Infra-red Region of the Solar Spectrum. 

 By Captain Abney, R.E., F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IN the last number of the Philosophical Magazine Dr. J. 

 W. Draper describes his phosphorographs of the solar 

 spectrum, and incidentally brings my map of the infra-red 

 region of the solar spectrum under review. From the date of 

 his communication (Dec. 1, 1880), I am under the impression 

 that his observations were based on a photograph of the map 

 which I had the pleasure of sending him, and not after a 

 perusal of the paper in the Phil. Trans, for 1880, a copy of 

 which I also sent him more recently. Since Dr. Draper's 

 remarks may cause a false impression in some respects, I wish 

 to make a comment on one or two points which he has raised. 

 Dr. Draper says that the investigations which I have carried 

 on " are not to be regarded as an extension of exploration in 

 the infra-red region (for they really do not carry us beyond 

 my own observations in 1843), but as securing the resolution 

 of these lines* or bands into three constituent elements." In 

 the paper already referred to, it will be seen that the know- 

 ledge of that region of the prismatic solar spectrum has been 

 increased in several respects. The accompanying figure will 

 show in some degree the extension. No. I. is the spectrum 



of the infra-red region as measured from a photograph taken 

 with three prisms. No. II. are the coincidences of a, /?, 7 as 

 given by Dr, Draper in his communication. No. III. are the 

 coincidences which I have obtained from reversed images. 

 Up to W.L. 10,750 I have mapped with a grating ; and 

 taking Dr. Draper's coincidences of a, ft, and 7 as correct, 

 instead of three lines or bands, we have some 280. 

 * cc, /3 ? y, which Dr, Draper first showed to exist. 



