RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 
31 
The breadth of the border, I should observe, is small, not exceeding 0‘5 or 4th part 
of the sun’s radius ; and this, from the circumstances of the experiment, must neces- 
sarily err in excess. 
Existence of a Blackening or deoxidating Power in rays beyond the extreme Red. 
83. The tartrate of silver, whether per se or introduced into the pores of the paper 
by the processes above described, is by no means delicately sensitive. It remains long 
comparatively unaffected, and it is only when that peculiar action, above described, 
comes on, that the darkening process goes on with any degree of rapidity. Ultimately, 
however, it acquires a degree of blackness surpassing almost every other argentine 
compound tried. But there is a mode of accelerating this darkening and rendering 
the tartrated paper very sensitive, which has been already hinted at (Art. 48.), viz. 
by passing over it a wash of hydriodate of potash, which deserves more particular 
mention. 
84. If while a tartrated paper, prepared with two washes of nitrate of silver, and 
one, intermediate between them, of Rochelle salt, is exposed to the spectrum and in 
slow process of darkening, it be washed over with a considerably dilute solution 
of the hydriodic salt, it instantly begins to blacken much faster, not only over the 
whole positive or most refrangible portion of the spectrum, to which the action would 
have been limited without this application (Art. 81.), but over all the least refrangible 
or negative part, and that, not merely up to the limit of the last visible (extreme red) 
rays, but considerably beyond that limit. The spectrum produced, however, has 
nothing of that singular character described in Art. 81, but is of a pretty uniform 
brown over its whole length. Its measured dimensions are as follows : 
Negative or least refrangible limit of darkness — 22‘5 
Positive or most refrangible limit of darkness + 67‘5 
Total length of the dark impression = 90-Q 
There are in this, as in the un-ioduretted paper, two conspicuous maxima of the solar 
action, the most abrupt, which shows like an intense oval spot of about fifteen parts 
(or twice the sun’s apparent diameter) in length, has its centre at + 37, almost ex- 
actly where the more conspicuous maximum occurs in the un-ioduretted paper, but 
the other, which is less definite, takes place at or near the fiducial point itself. If 
from the negative coordinate — 22‘5 of the last perceptible blackening we subtract 
— 3*6, the sun’s semidiameter, we find — 18'9 for the point of extreme action in a 
linear spectrum, which surpasses by — 5 - 6 parts, or by nearly the whole distance 
which separates the mean from the extreme red, the last ray capable of exciting the 
retina. It ought also to be mentioned, that when the red rays are made to act on 
paper thus treated, and which has already undergone the darkening action of the 
blue or violet rays, they do not redden it, as in ordinary cases, which seems to indi- 
cate a difference in the nature of the chemical action exerted in the above experiment. 
