986 
PROFESSOR J. N. LOCKTER ON THE PHOTOGRAPHIC 
of a sun spot or a prominence, of determining the spectrum of a practically isolated 
mass of vapours in the hottest region open to our inquiries, and seeing whether it is 
like or unlike the general spectrum of the sun. What then are the facts ?. 
The whole character of the spectrum of iron, for instance, is changed when we pass 
from the iron lines seen among the Fraunhofer lines to those seen among the spot and 
jorominence lines; a complex spectrum is turned into a simple one, the feeble lines 
are exalted, the stronger ones suppressed almost altogether.”* * * § 
One of the best examples of the changes of intensity of the iron spectrum brought 
about by changes of temperature is afforded by the group of three lines at wave¬ 
lengths 4918, 4919‘8, and 4923'2 (Angstrom’s scale). In the solar spectrum, 4919’8 
is thickest, in the oxyhydrogen flame none of them is visible, in the electric spark 
with jart, 4923'2 is thichest, while it is almost invisible in the electric arc; under no 
conditions are all intensified at once, each one seems intensified at the expense of the 
other. Observations made at Kensington, of the most widened lines in the spectra 
of spots, show that the lines at wave-lengths 4918 and 4919'8, which are seen almost 
alone in some photographs of the arc spectrum, are seen alone in the spots, or, at all 
events, in 73 spots out of 100, and the other line which is enormously expanded when 
we use the highest temperature, is seen alone in 52 out of 100 prominences by 
Tacchini. “ Then, we finally learn, that in several cases when a change of 
refrangibility has been observed in the iron lines in the spots visible on the sun, that 
the two lines 4918 and 4919’8 have been affected, while 4923'2 has remained at 
rest.”I These variations are, I hold, therefore, the result of temperature changes. 
Messrs. Liveing and Dewar, however, deny that the line of the triplet seen in the 
prominences, and most brilliant at the highest temperature available in our labora¬ 
tories, is due to iron, although it has been recorded as an iron line, as shown by 
Watts, Kirchhoep, Huggins, Thalen, Lecoq de Boisbaudran, and myself. Its 
quality as an iron line, therefore, is as established as that of any other lines seen in 
the spectrum. Quod uhique quod ah oninihus. In their words, “ The line at wave¬ 
length 4923, which occurs so often in the chromosphere, according to Young and 
Tacchini, and is assumed to be due to iron, is so near to lines which come out in our 
crucibles on the introduction of other metals, that we cannot help feeling some doubt 
as to its absolute identification with the iron line.”§ 
Farther, a knowledge of the true spectrum of iron is of the utmost importance for 
the solar and stellar work which is in progress at Kensington. Observations of the 
lines which are most widened in the spectra of sun spots have been made since 1879, 
* ‘ Cliemistry of tte Sun,’ p. 253. 
t Tlie quantity spark employed by Mr. McClean to obtain the pliotogi’aphs, wbicli are referred to 
later, apjoroacbes almost the conditions of tbe electric arc. The changes here mentioned, however, 
depend upon experiments with a high tempei’atnre iar spark. 
t Ibid., p. 351. 
§ ‘Roy. Soc. Proc.,’ vol. 33, p. 432, 1882. 
