808 
MR. J. NORMAN LOCKYER ON SPECTRUM-ANALYSIS 
photographic records and the solar spectrum are due to causes incident to the 
photographic record itself, or to variations of the intensities of the various molecular 
vibrations under solar and terrestrial conditions, are questions which up to the present 
time I have been unable to discuss. 
Observations on the Maps. 
In the accompanying Map I have placed under each spectrum the spectra mapped 
by Angstrom and Thalen for the sake of comparison. In constructing the Map only 
those lines distinctly referable to the metal have been inserted; minute traces of faint 
lines and the banded structure visible in most of the photographs, and belonging most 
probably to the carbon of the lamp-poles, have been omitted. 
The lines described in the Index as “ nebulous ” are of a peculiar and distinct nature, 
presenting the appearance of being out of focus, and suggesting that they arise from 
the surface of the arc. Whether these lines are due to the metal itself or to some 
compound of the metal stable at the temperature of the electric arc, I am at present 
unable to say. Special experiments will have to be made upon this point. The 
calcium lines 409T8, 4093-3, and 4097*5, and the barium lines 4081, 4084, and 4087, 
are of the nature indicated. 
In certain cases lines have been found of equal thickness, and absolutely coincident 
in two confronted spectra, without being referable to any known impurity common to 
the two metals. In these instances the lines have been assigned to both metals, but 
must be considered as subject to correction. The lines 4282-5 * (common to calcium and 
barium) and 4325 (common to barium, strontium, and iron) are instances of coincidences 
occurring in the present Map. 
The scale of lengths given in the Index is that made use of by Thalen f for indi- 
cating the intensities of the lines, 1 being the longest and 5 the shortest line. In my 
former papers, where eye-observations were made use of, the scale of length ranged 
only from 1 to 4 ; but in the present case we have photographs to decide from, and 
these permit a more satisfactory determination. 
The Map and accompanying Tables have been constructed by my assistant, Mr. R. 
Meldola, to whom I am much indebted for the skill and patience he has brought to 
bear upon the inquiry. 
The photographs which accompany this paper have been taken in the room in Dr. 
Frankland’s private laboratory, to which I made reference in my former communication. 
In again tendering my thanks to Dr. Frankland for having placed it at my disposal, I 
cannot refrain from pointing out that without such aid my present researches would 
have been impossible. 
* It has since been found by using a camera of 6 feet focus that this line is not absolutely coincident in both 
spectra, the calcium line being very slightly more refrangible. The Map and Index have been altered accordingly. 
— J. N. L., September 26, 1874. 
f “ Memoire sur la determination des longueurs d’onde des raies metalliques,” Nova Acta, Upsala, 1868. 
