158 
MR. J. B. N. HENNESSEY ON THE ATMOSPHERIC 
the appearance should be drawn, and finally the drawing should be compared with the 
original : under these conditions a week may be easily absorbed by a single group. It 
is also to be borne in mind that no human eye will endure, without at least temporary 
injury, protracted watching of the bright solar spectrum for more than four or five 
weeks at a time ; indeed, though I habitually used both eyes as a relief to one another, 
they both invariably suffered, and continued to do so for several weeks after every 
autumn. The following facts may be here mentioned : — 
In 1870, commencing October 17, I observed 17 sunsets. 
„ 1871, „ „ 5, „ 20 „ 
„ 1872, „ „ 10 (about), „ 20 „ 
„ 1873, „ „ 6, „ 35 „ 
3. In the autumns of 1870 and 1871 1 continued to work with the old spectroscope, 
mapping from D to E, in extension of the Map already published ; but all desire for 
publication of these results was naturally suppressed when Professor Stokes gratified 
me by announcing that the Royal Society had ordered a new spectroscope for my use. 
This instrument reached my residence at Dehra, together with two actinometers, 
when I was absent with the eclipse expedition in December 1871 ; and I need hardly 
add that after my return I lost no time in examining the contents of the package. It 
appears inevitable that instruments should suffer in travelling ; this one did, and the 
injuries took weeks to repair; but once the spectroscope was fit for use and I was 
able to judge of its capabilities, the idea of not superseding the map already published, 
based on my work of 1868 and 1869, or of not suppressing the map in hand from obser- 
vations 1870 and 1871, was at once relinquished; thus the map now submitted was 
obtained entirely with the new spectroscope. However, I had my old maps as skeletons 
to begin with ; and adopting Professor Stokes’s suggestion to compare, in the first 
instance, the spectra by the two instruments, I set to work de novo from the extreme 
red in the autumn of 1872, and finished the work in November 1873 ; it was not, how- 
ever, until the following summer that I was able to forward the map appended, nor 
have I had it in my power until now to attempt this explanatory paper. 
4. As regards my station of observation, it is best known locally by Vincent’s Hill *, 
being a knoll on some property once owned by the late General Vincent: the site is in 
N. lat. 30° 27', E. long. 78° 3' ; height above sea 7100 feet ; and it commands a complete 
view of the horizon from S.E. to S.W. by W. The site in question was made available 
for my purposes through the courtesy of Surgeon-Major R. Whittall. Next, of the 
new spectroscope by Grubb of Dublin : it mounts three (compound) prisms, which are 
moved with the telescope by an automatical contrivance for maintaining minimum de- 
viation ; the eye-end of the telescope is fitted with a micrometer, and the highest power 
eyepiece which may be generally employed gives an image of the dispersion about 
3-g- fifths of that delineated in Kirchhoff’s maps at the usual distance of reading: 
* On the Himalaya Mountains, N. AV. Provinces, India. 
