so 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
The quality of its papers however varies much with the doses of the ingredients, and, 
in fact, I have never succeeded in hitting again that precise proportion of the argentine 
and saline solutions which afforded the results I am about to state ; and but that 
every portion of a pretty large sheet of paper agreed in giving the same, and that I 
have some of them now before me, I should hesitate to do so. The preparation of 
this particular specimen of paper, (marked 573-574 in my list of papers put on trial,) 
was, 1st, nitrate of silver S. G. IT 32 ; 2nd, saturated solution of Rochelle salt ; 3rdly, 
nitrate of silver IT 32, which proving but little sensitive, a third wash of the nitrate 
was added, by which its sensibility was materially increased, its other qualities re- 
maining unaffected. The paper was of the kind called “ smooth wove demy.” 
81. The spectrum impressed on this paper commenced at, ora very little below, the 
fiducial point, of a delicate lead-colour, but faint and dilute, and when the action was 
arrested soon after the first impression made, such was the character of the whole 
photographic spectrum. But if the light was allowed to continue its action, there 
was observed to come on, suddenly, a new and much more intense impression of 
darkness, confined, in length, to the blue and violet rays, and, what is most remark- 
able, confined also in breadth to the middle of the sun’s image, so far, at least, as to 
leave a border of the lead-coloured spectrum traceable not only round the clear and 
well-defined convexity of the dark interior spectrum, at the less refrangible end, but 
also laterally along both its edges. And this border was the more easily traced, 
and the less liable to be mistaken, by reason of its singular contrast of colour with 
the interior spectrum. That of the former, as observed above, was lead-grey : of the 
latter an extremely rich, deep, velvety brown. The less refrangible end of this inte- 
rior brown spectrum presented a sharply terminated and regular elliptic contour, 
the more refrangible, a less decided one. The coordinates of its extremities may be 
stated at -f- 10-0 and + 48\5. Its length is therefore 38'5. Within this length occur 
two very distinct and evident maxima of intensity, viz. at -j- 21*7 and 36'9, the latter 
being by far the most conspicuous and abrupt. Another maximum also occurs be- 
yond the limits of the dark-brown spectrum, viz. at + 56'7, in the grey or rather 
grey-brown colour, which the ground assumes at this end of the spectrum. The en- 
tire spectrum, including the lead-coloured portion at one end, and the grey-brown 
portion at the other, extends from — 3'9 to -(- 71*5, giving a total range of action 
= 75*4. 
82. It may seem too hazardous to look for the cause of this very singular pheno- 
menon in 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 I am disposed to think decisive evidence (which will find its place elsewhere,) of 
the existence of an absorptive solar atmosphere, extending beyond the luminous one. 
