In copying the Solar Spectrum alternately upon a surface of Iodide and Bromide of Silver, we notice a difference in the Photographic properties of these two salts. The latter is affected more extensively, to a point lower in the spectrum, than the former. In the case of the Iodide of Silver, the action ceases in the Blue space; but with the Bromide it reaches to the Green. This is shown in the following diagrams, which are drawn from the observations of Mr. Crookes ('Photographic Journal,' vol. i. p. 100):—
Fig. 1.Fig. 2. Fig. 3.
Fig. 1 represents the chemical spectrum on Bromide of Silver; fig. 2, the same upon Iodide of Silver; and fig. 3, the visible spectrum.
It might perhaps be supposed that the superior sensibility of the Bromide of Silver to green rays of light would render that salt useful to the Photographer in copying landscape scenery; and indeed it is the opinion of many that, in the Calotype paper process, the dark colour of foliage is better rendered by a mixture of Bromide and Iodide of Silver than by the latter salt alone. This however cannot depend upon the greater sensibility of the Bromide to coloured light, as may easily be proved.—
The diagrams given above are shaded to represent nearly, the relative intensity of the chemical action exerted by the rays at different points of the spectrum; and on referring to them it will be seen that the maximum point of blackness is in the indigo and violet space, the action being more feeble in the blue space lower down; there are also highly refrangible rays extending upwards far beyond the visible colours, and these invisible rays are actively concerned in the formation of the image.
It is evident therefore that the amount of effect produced by a pure green, or even a light blue tint, upon a surface of Bromide of Silver is very small as compared with that of an indigo or violet; and hence, as in copying natural objects radiations of all kinds are present at the same time, the green tints have not time to act before the image is impressed by the more refrangible rays.
Sir John Herschel proposed to render coloured light more available in Photography by separating the actinic rays of high refrangibility, and working only with those which correspond to the blue and green spaces in the spectrum. This may be done by placing in front of the Camera a vertical glass trough containing a solution of Sulphate of Quinine. Professor Stokes has shown that this liquid possesses curious properties. In transmitting rays of light it modifies them so that they emerge of lower refrangibility, and incapable of producing the same actinic effect. Sulphate of Quinine is, if we may use the term, opaque to all actinic rays higher than the blue-coloured space. The proposition of Sir John Herschel above referred to was therefore to employ a bath of Sulphate of Quinine, and having eliminated the actinic rays of high refrangibility, to work upon Bromide of Silver with those corresponding to the lower-coloured spaces. In this way he conceived that a more natural effect might be obtained.
If Photographic compounds should be discovered of greater sensibility than any we at present possess, the use of the Quinine bath will perhaps be adopted; but at present we trust to the superior intensity of the invisible rays for the formation of the image, and hence the employment of Bromide of Silver is less strongly indicated.