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XVII. The Bakerian Lecture.— On the Photographic Method of Mapping 
the least Refrangible End of the Solar Spectrum. 
By Captain W. de W. Abney, R.E., F.R.S. 
Received December 17, 1879,—Read January 8, 1880. 
[Plates 30-32.] 
The research which I commenced some five years ago on a method of photography by 
which the least refrangible end of the solar spectrum could be mapped has reached 
such a stage that it seems desirable that I should put on record the results I have 
obtained, and with it to present a map of the solar spectrum between wave lengths 
7600 and 10,750 based upon measurements from a series of photographs which appear 
to be satisfactory for the purpose. 
Action of dyes on silver bromide. 
In December, 1873,'"' Dr. H. C. Vogel, of Berlin, announced that by means of dyed 
collodion films which contained silver bromide he had been enabled to photograph with 
the yellow and green rays of the solar spectrum, which had hitherto been supposed to 
be possessed of but little chemical effect. About the same time I had set myself the 
task of mapping the ultra-red region of the spectrum, and I was naturally led to 
examine the method advocated by Dr. H. C. Vogel. 
If a spectrum be thrown on an ordinarily prepared photographic plate containing 
only silver bromide, it is well known that the length of the spectrum impressed varies 
considerably from that obtained by a plate containing silver iodide or silver bromo- 
iodide. The commencement of the photographic action in the first case is somewhere 
near the line B or slightly below, and in the last two near E in the green; in all the 
three the action extends to the limit of the solar spectrum in the ultra-violet. The 
relative chemical effects produced by the different rays show themselves by a varying 
thickness, or what is usually called density, of the metallic silver reduced or precipi¬ 
tated by the action of developing solutions. For the above-named silver compounds 
the maximum effect is somewhere about the line G; and if we represent the density of 
the metallic silver at any point by ordinates, it will be found that the area of the curve 
MDCCCLXXX. 
* Photographic News, Dec. 12, 1874. 
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