LINES OF THE SOLAE SPECTEUM. 
159 
the prisms are beauties: an object-glass, about 1 inch in diameter, is fitted at the end of 
a rod, and can be adjusted so as to throw an image of the object on the slit ; this pro- 
vision was exceedingly useful to me. Further description of the instrument appears 
unnecessary, excepting to state generally that I am much pleased with its good qualities. 
5. I now proceed to add a few words as to my reasons for ascribing the differences in 
certain parts of the solar spectrum, sun high and sun set, in all cases to the influence 
of the earth's atmosphere, believing that I can definitely show the relation between this 
effect and this cause. I will premise that I now have access to the Philosophical 
Transactions for 1860, in which the paper by Sir David Brewster and Dr. Gladstone 
on the lines of the solar spectrum is given, together with an illustrating map ; and I here 
make allusion to these documents, because, though the Committee were good enough to 
call my attention to them, I was unable when writing in 1870 to get possession of a 
copy. I have also access to other volumes of the Philosophical Transactions, including 
Professor Stores’s drawings in the volume for 1852, besides various documents on the 
subject of air-lines, as Report on a Mission in Italy by M. Janssen, &c. All these 
papers contain descriptions or allusions to experiments showing the effect of reflections 
from various surfaces, and of the passage of light through strata of variable lengths, &c. 
And in turn I also (in keeping with suggestions by Professor Stores, for which I am very 
much obliged) tried certain experiments which I will now briefly allude to. Selecting a 
bright clear day, I first turned the collimator to the sun about the meridian, and set 
the slit for good definition of the lines ; after this, with the slit as before, I admitted 
the sun’s light reflected from blue or white glass backed with velvet, from ink of various 
degrees of blackness, from coloured solutions, &c. ; and finally I got a reflection from a 
distant muddy river; but none of these, or other experiments which need not be 
detailed, produced the smallest approach to the variable lines which were the especial 
aim of these experiments, nor yet, as a matter of fact, to those seen only at sunset that 
are plainly air-lines. Some of the belts are specially deserving of attention — for instance 
the huge shadow 1073 to 1155 of Kirchhofe’s scale on my Map; this shadow or belt 
stands out like a wall at sunset, and then not only comes into existence itself, but 
with it come 1108, 1114, and 1121, which I could not see sun high, nor has Kirch- 
hoff shown. 
6. I now turn to another fact. When the autumn has well advanced here, there springs 
up from the plain country, stretching away S.E. and S.W. by W., a kind of haze which 
becomes visible at sunset, and which grows day by day in height until it attains to 
perhaps 3° or more above the horizon ; this haze, moreover, grows denser daily, until at 
last it is sufficiently opaque to obscure the sun’s rays. I need not in this place enter 
into the causes which produce this haze ; it is sufficient to remark here that I have 
noticed it year after year, and from its opacity and its formation occurring just before 
winter, I always call it “ the winter bank ;” indeed I remember talking about it one 
evening with the late Archdeacon Pratt, who also had noticed it, in connexion with 
some other fact. N ow this haze bank practically compelled the sun to set whenever 
