RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 23 
paper so covered was exposed for a day to desultory and feeble gleams of sunshine, 
and when withdrawn and compared with a portion of the same piece of paper pre- 
served in the dark, to have become materially less dark than before, and to have ac- 
quired a high red tint instead of the greenish blackness it had before. Papers 
darkened by exposure under cupro-sulphate of ammonia and by free exposure, were 
reddened in exactly the same way. 
63. It was natural, perhaps, in this stage of the inquiry to regard, as I did, this 
change of tint, accompanied as it was with a diminution of total intensity, as a first 
step in the process of ultimate restoration to perfect whiteness, and that I should, in 
consequence, in the way of practical application, have forthwith proceeded to attempt 
the production of a positive photographic copy of an engraving by placing it on a 
frame, as usual, over a paper blackened by exposure to sunshine, covering the whole 
with a dark red-brown glass previously ascertained to absorb every ray beyond the 
orange. In this way a copy was obtained in effect, so far of a positive nature that 
the shades of the original engraving were expressed in black. But the lights, instead 
of white , were found to be eaten out in red, the colour of venous blood. Beyond this 
I could not succeed in pushing it, nor could I obtain any other result on exposing 
blackened paper under the standard red glass, however long continued. 
Combined action of rays of di fferent degrees of ref rangibility. 
64. It is evident from this experiment, that the conservative whitening, or oxidating 
action of the red rays depends on some other principle than a mere contrariety of 
action to that of any one species of ray in particular. It is proved, 1st, that a very 
powerful red ray is capable of neutralizing, in the moment of its action, the photogra- 
phic effect of a feeble white one, in which, besides the red, are contained in due pro- 
portions all the other rays. But on the other hand, it is equally shown, that a long- 
continued subsequent action of the red rays is incapable of destroying entirely the effect 
of other rays once fully produced, though it superinduces on that effect the very 
remarkable modification of assimilating the blackness arising from their action to its 
own proper colour. A question of high interest therefore arose, as to what would be 
the effect of the combined action of a red with any other single ray in the spectrum, 
if acting at the same instant ; or more generally, whether any, and what differences 
subsist between the joint and the successive actions of any two given rays of different 
and definite refrangibilities, and whether the joint action be capable, or not, of pro- 
ducing effects which neither of them acting alone would be competent to produce. 
65. As regards the successive action of differently coloured rays, I have not been 
able to perceive, hitherto, any influence which the prior action of the less refrangible 
rays of the spectrum exercises in modifying the subsequent effects produced by the 
more refrangible. That portion of a slip of paper, for example, which has been pre- 
served in a state of whiteness under the action of dispersed light by the long-con- 
tinued action of red light, is darkened quite as rapidly, and assumes the same shade 
