360 Photographic Spectrum of Electrolytic Iron. [Nov. 23, 



graphic plates were used, but for the spectrum from F to specially 

 prepared plates were employed. 



The wave-lengths of the lines have been determined by reference to 

 Professor Rowland's second series of photographic prints of the solar 

 spectrum, and, consequently, are all expressed on Rowland's scale. 

 Tables of the wave-lengths and intensities of all the lines in the 

 photographs have been compiled, and a comparison instituted between 

 the Kensington results and those of Thalen, Messrs. Kayser and 

 Runge, and McClean. Though the iron used was the purest obtain- 

 able, a few of the lines are probably due to impurities, and these have 

 been indicated in the tables. 



Many of the faint lines mapped by Messrs. Kayser and Runge are 

 evidently due to impurities in the iron used in their researches, and 

 the origin of these lines has been traced, as far as possible, by 

 reference to the, as yet, unpublished Kensington maps of metallic arc 

 spectra, and the results are shown in the tables. 



The comparisons have led to the following conclusions : 



1. The visual spectrum, as mapped by Thalen, differs but slightly 



in essential points from that which has been photographed at 

 Kensington. The principal difference is the greater number 

 of lines mapped by Thalen in all regions except that between 

 wave-lengths 4000 and 4300. This is probably to be accounted 

 for by the insufficient exposure of the photographs, which was 

 necessitated by the limited amount of material available for the 

 experiments. 



2. The comparison with the spectrum photographed by Mr. 



McClean, indicates that the experimental conditions employed 

 by him produced a temperature not greatly differing from 

 that of the arc employed at Kensington. There are only a few 

 lines which are not common to the two series of photographs, 

 and these, in many cases, can, with certainty, be ascribed to 

 impurities present in one case and not in the other. The 

 apparent differences in intensity between some of the lines 

 which are common to both are mostly due to superposition of 

 the spectrum of air upon that of iron in Mr. McClean's photo- 

 graphs. In some cases, however, there appears to be a real 

 difference in the intensities of the lines, and this may prob- 

 ably be ascribed to the difference between the temperatures 

 employed. 



3. The number of lines mapped by Messrs. Kayser and Runge is 



considerably in excess of that mapped by the author in corre- 

 sponding regions of the spectrum. The comparison indicates 

 that this is partly due to the fact that the iron employed in 

 their experiments contained a greater number of impurities 

 than that employed at Kensington. No origins have been 



