ARC SPECTRUM OF ELECTROLYTIC IRON. 
987 
and the preliminary reductions indicate that, at the period of minimum sun spots 
many of the most widened lines are due to iron, while at maximum the lines are 
chiefly of unknown origin. A table of iron lines is therefore essential in an inquiry 
of this nature. The series of photographs of stellar spectra, which have been taken 
at Kensington, include some stars which resemble the sun, some which differ slightly 
from it, and others which differ greatly. A comparison of these in terms of iron is 
very important, and is a natural first step in their study when we have a terrestrial 
iron spectrum about which there is no doubt. 
The necessity therefore got stronger and stronger to get the true spectrum of 
iron. At this juncture in 1887, in a conversation with my colleague, Professor W. C, 
Roberts-Austen, he informed me that he was preparing some iron of exceptional 
purity by electrolytic deposition, and that there was a certain quantity of this 
available for research purposes, which he placed at my disposal. 
I at once determined to obtain photographs of the spectrum of this material, 
using it for both the poles of an electric arc, so that all carbon joole impurities might 
be avoided. 
This paper is the result. Owing to the small quantity of iron available, the 
exposures were necessarily short, so that in some parts of the spectrum the number 
of lines is not so complete as is desirable. 
Thalen’s memoir is practically complete, so far as the visible arc spectrum of iron 
is concerned. The photographic arc spectrum of iron over the same region has not, 
however, hitherto received such minute attention. I have therefore taken up the 
subject by photographically comparing the spectrum of iron with the solar spectrum 
between about K and A, using the electrolytic iron previously referred to. The 
main advantage gained by photographic comparisons of this character is that a 
permanent record of the positions of lines relatively to Fraunhofer lines is obtained, 
which can be referred to at any time, and that the coincidence or non-coincidence of 
iron lines with solar lines can be easily and exactly determined at leisure by a 
microscopical examination of the negatives. 
Method Emi^loyed. 
Portions of the electrolytic iron were arranged to form the poles of an electric arc 
lamp placed about four feet from the slit of a Steinheil spectroscope, having three 
prisms of 45°, and one of 60°; an image of the arc being formed upon the slit bj;' 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 between K and A has been photographed on four plates. The first 
plate takes in the spectrum from about X3900 to X4220, the next from about X4220 to 
X4700, a third extends from X4700 to X5900, and a fourth from X5900 to X7600. 
The steps are approximately from K to G, G to F, F to D, and D to A of the solar 
spectrum. 
6 K 2 
