360 De la Rue and Celestial Photography. [July, 
fact at the same time, by two distinct observers,* that the chemical 
action produced by the rays coming from the edge of the sun were 
less active than those proceeding from its central regions. This 
fact has been, strangely enough, recently put forward as a discovery 
by Professor Roscoe,t without the mention of any previous observer, 
excepting Secchi, whose observations had reference to the calorific, 
and not to the chemical radiations. It is true that Professor 
Roscoe has made a series of excellent experimental observations, and 
that he has proved “that the intensity of the chemically active 
rays at the centre is from three to five times as great as that at the 
edge of the disc;” but in doing this he has only confirmed the 
results already published.{ Tor example, in 1840, Sir John Her- 
schel, in the ‘ Philosophic Transactions’ (Part I., p. 48), distinctly 
stated that he had detected “a real difference between the chemical 
agencies of those rays which issue from the central portion of the 
sun’s disc, and those which, emanating from its borders, have undergone 
the absorptive action of a much greater depth of its atmosphere, and 
yet I confess myself somewhat at a loss what other cause to assign 
for it. It must suffice, “however, to have thrown out the hint, 
remarking only that I have other, and, 1 am disposed to think, 
decisive evidence of the existence of an absorptive solar atmo- 
sphere extending beyond the luminous one. The breadth of the 
border, I should observe, is small, not exceeding 0°5, or one-seventh 
part of the sun’s radius; and this, from the circumstance of the 
experiment, must necessarily err in excess.” 
Mr. Robert Hunt, in the ‘ Philosophical Magazine’ already quoted, 
noticed the same phenomenon, and gave the same, as being the most 
familiar explanation of it; and subsequently M. Arago, in his ‘Memoirs 
on Photometry,’ again drew attention to this important fact. 
The results which have been obtained since 1840 appear to 
show, not merely that the chemical radiations generated near the 
edge of the solar disc are absorbed in passing through a greater 
depth of the sun’s atmosphere, but that there is an actual inter- 
ference (using this term in its ordinary acceptation rather than its 
scientific sense) exerted by the luminous radiations, and that the 
chemical radiations haye their orig in a lower zone, that which 
produces Light-energy. The protected band which is seen to 
surround the prismatic image of the sun is not due to a lowering 
* *Philosophical Magazine,’ vol. xvi., 3rd series, contains an abstract of the 
memoir read before the Royal Society by Sir John Herschel; and also a paper in 
the same monthly part of this magazine by Robert Hunt, on “ Experiments and 
Observations on Light which has permeated coloured Media, and on the Chemical 
Action of the Solar Spectrum,” in both of which this fact was, for the first time, stated. 
+ “On the Measurement of the Chemical Brightness of various Portions of the 
Sun’s Dic.” By Henry Enfield Roscoe, B.A., F.R.S. Received June 12, 1863. 
{ See “On the Present State of our Knowledge of the Chemical Action of the 
Solar Radiations.” A report to the British Association, in 1850, by Robert Hunt. 
