Sunlight transmitted through red glass produces the same effect as the red rays of the spectrum in the foregoing experiment. Sir John Herschel placed an engraving 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 photographic copy was obtained in which the shades were black, as in the original engraving; but the lights, instead of being white, were of the red colour of venous blood, and no other colour could be obtained by exposure to light, however long. Sir John ascertained that every part of the spectrum impressed by the more refrangible rays is equally reddened, or nearly so, by the subsequent action of the less refrangible; thus the red rays have the very remarkable property of assimilating to their own colour the blackness already impressed on photographic paper.
That there is a deoxydating property in the more refrangible rays, and an oxydating action in the less refrangible part of the spectrum, is manifest from the blackening of one and the bleaching effect of the other; but the peculiar action of the red rays in the experiments mentioned shows that some other principle exists different from contrariety of action. These opposite qualities are balanced or neutralized in the region of the mean yellow ray. But, although this is the general character of the photographic spectrum, under certain circumstances even the red rays have a deoxydating power, while the blue and violet exert a contrary influence; but these are rare exceptions.
The photographic action of the two portions of the solar spectrum being so different, Sir John Herschel tried the effect of their united action by superposing the less refrangible part of the spectrum over the more refrangible portion by means of two prisms; and he thus discovered that two rays of different refrangibility, and therefore of different lengths of undulation, acting simultaneously, produce an effect which neither, acting separately, can do.