IN CONNEXION WITH THE SPECTKUM OE THE SUN. 
489 
those cases are much more common in which they are long in one spectrum and shorter 
in the others. 
2nd. As a rule, in the instances of those lines of iron, cobalt, nickel, chromium, and 
manganese which are coincident with lines of calcium, the calcium lines are long, while 
the lines as they appear in the spectra of the other metals are shorter than the longest 
lines of those metals. Hence we are justified in assuming that short lines of iron, cobalt, 
nickel, ehromium, and manganese, coincident with long and strong lines of calcium, are 
really due to traces of the latter metal occurring in the former as an impurity. 
3rd. In cases of coincidences of lines found between the lines of various spectra the 
line may be fairly assumed to belong to that one in which it is longest and brightest. 
In order to show what a fair promise there is that all these questions will in time be set 
at rest by photography, and set at rest in the direction I have indicated, I beg permission 
to refer to one of the very earliest photographs taken by the method which has been 
described in a former part of this paper. It is a confronting of the spectra of the 
metals calcium and strontium. 
A simple comparison of the two spectra shows that there are three strong and thick 
lines common to the two at w. 1. 3968, 3943, the two H lines, and 4226-3, the large 
calcium line near G. The spectrum of strontium shows three lines which are very 
much thicker than the corresponding lines in the (calcium) spectrum ; they are situated 
near wave-lengths 4029 and 4077 and at 4215-3. The latter line has been ascribed by 
Thal£n to calcium, and is coincident with a strong solar line. An inspection of the 
photograph, however, at once showed me that this line is really a strontium line, since 
it is thickest in the spectrum of that metal ; so that this single plate was at once sufficient, 
in the light of these researches, to establish the presence of strontium in the reversing 
layer of the sun. 
It is seen, then, that a comparison of a photograph of any spectrum with the photo- 
graphs of the other spectra in which coincident lines occur, will be sufficient to show to 
which spectrum a disputed line belongs. It will be also noticed that the three calcium 
lines first mentioned are nearly as thick in the lower (strontium) spectrum as in that of 
calcium itself, while the difference between the thick lines of strontium and the corre- 
sponding lines also visible in the calcium spectrum is very great. All these facts are 
easily explained on the supposition that the calcium was very much less impregnated 
with strontium than the strontium with calcium. In fact I had such faith in the 
efficacy of the method, and in the opinion that coincidences are merely due to impuri- 
ties, that I did not even consider it necessary to change the poles, but proceeded at once 
to place the strontium salt on that which had just before served for the ignition of the 
calcium. This at once accounts for the greater impurity of the former. In nearly pure 
strontium the same lines are seen, but they are then thinner and shorter. 
I may remark that the lines at 4045, 4063, and 4071 are due to an iron impurity ; 
these are the longest lines of iron in that portion of the spectrum photographed. 
I may conclude f his statement by remarking that the above facts have been given as 
