1893.] Photographic Spectrum of Electrolytic Iron. 359 



Mven, M.A. ; William Henry Perkin, LL.D. ; the Marquis of 

 Salisbury, K.G., M.A. ; Professor J. S. Burdon Sanderson, M.D. ; 

 Adam Sedgwick, M.A. ; Professor Thomas Edward Thorpe, Sc.D. ; 

 Professor William Augustus Tilden, D.Sc. ; Professor W. Cawthorne 

 Unwin, B.Sc. 



The following Papers were read : 



I. "The Photographic Spectrum of Electrolytic Iron." By 

 J. NORMAN LOCKYER, F.R.S. Received October 27, 1893. 



(Abstract.) 



This investigation is an extension of the author's earlier researches 

 on the spectrum of iron, and supplements the important paper pub- 

 lished by Thalen in 1884. Since the work was commenced, two 

 other papers on the spectrum of iron have been published, one by 

 Professors Kayser and Runge, of the Hanover Technical High School, 

 and the other by Mr. F. McClean. In the former, the arc spectrum 

 of ordinary commercial iron was in question. The wave-lengths 

 were determined by micrometric measures, a number of standard 

 lines being used to construct the interpolation curves. Mr. McClean's 

 paper consists of a series of photographic comparisons of the spark 

 spectrum of iron and the solar spectrum. 



A knowledge of the true spectrum of iron is of the utmost import- 

 ance, both for solar and stellar work. Comparisons in terms of 

 iron are very important, and such a comparison is a natural first step 

 in their study when we have a terrestrial iron spectrum about which 

 there is no doubt. 



Thai en's memoir is practically complete so far as the visible arc 

 spectrum of iron is concerned. The photographic arc spectrum over 

 the same region has not, however, hitherto received such minute 

 attention. This subject has been taken up at Kensington by photo- 

 graphically comparing the spectrum of iron with the solar spectrum, 

 between Fraunhofer's lines K and C. For this purpose, a small por- 

 tion of some iron of exceptional purity, prepared by electrolytic 

 deposition by Professor W. C. Roberts- Austen, was placed by him at 

 the author's disposal. A part was arranged to form the poles of an 

 electric arc lamp, which was placed about 4 ft. from the slit of a 

 Steinheil spectroscope, having three prisms of 45, and one of 60, 

 an image of the arc being formed on the slit plate by a suitable lens. 

 The current employed was from a "Victoria-" Brush dynamo, driven 

 by an " Otto " gas engine, and making about 850 revolutions per 

 minute. 



The region of the spectrum between K and C was photographed 

 on four plates. For the region between K and F ordinary photo- 



