[ 157 ] 
III. On the Atmospheric Lines of the Solar Spectrum , illustrated by a Map drawn on 
the same scale as that adopted by Kirchhoff. By J. B. N. Hennessey, F.R.A.S. 
Communicated by Professor Stokes, Sec. B.S. 
Received January 11,-^Read January 28, 1875. 
The spectroscopic observations hereafter discussed were made with instruments belong- 
ing to the Royal Society, and in accordance with certain suggestions which a Com- 
mittee were good enough to make in connexion with my letter to Sir Edward Sabine, 
President, dated 13th February, 1866. In view of my residence at a considerable 
height, and the exceedingly clear atmosphere prevailing at some periods of the year, it 
was suggested that the locality was peculiarly favourable for comparing the solar 
spectrum when the sun was high with the corresponding spectrum at sunset; any 
differences between these aspects which might appear were to be noted on Kirchhoff’s 
well-known maps. Accordingly I set to work with the spectroscope first supplied to 
me (hereafter distinguished by the prefix old), and during the autumns of 1868 and 
1869 I mapped the differences in question from the extreme red to D: these results 
appeared in the ‘Proceedings of the Royal Society,’ No. 123, 1870, the Map being 
marked vol. xix. pi. 1 ; it is unnecessary, therefore, to dwell on this portion of my 
labours, excepting to add that the definitions and general procedure there adopted 
have been retained in the remarks which follow. 
2. The observations hereafter noticed were always taken in the autumn, when, the 
rainy season having passed away, the atmosphere on these mountains is exceedingly 
clear, so that the sun, the object of inquiry, is bright even to his setting, and a spec- 
trum may therefore be then obtained through a long stretch of terrestrial atmosphere 
corresponding to the height of the station of observation ; on the other hand, with the 
sun about the meridian, the height of station places the observer above a relative amount 
of atmosphere, so that the spectrum obtainable at this time and about sunset are highly 
eligible for the comparison in view. Accordingly the two spectra are given in the accompa- 
nying map (Plate 25) ; and for easy comparison they are placed in juxtaposition. By “ sun 
high” is to be understood any position for the sun within a couple of hours of the meridian; 
by “ sun low ” that the sun was within 3 or 4 diameters of his setting and yet quite bright. 
Indeed it is only when very near sunset that the marked alterations in the lines appear ; 
so that the spectrum required is not only rarely obtainable, but it hardly lasts beyond 
10 minutes of an evening. In this short period (when, moreover, the observer is fatigued 
with previous watching) changes from the sun-high spectrum must first be detected ; 
then their position must be identified, and, failing this, found by measurement ; next, 
MDCCCLXXV. Y 
