COLLODIONIZED-PAPER PROCESS. In this process a negative is taken upon a sheet of paper coated with collodion. The process has not yet been extensively practised, but it appears to offer some im portant advantages, in one or two respects, over every other process now employed by the tourist. It will be better to describe the manipulation first, and point out these advantages afterwards.
The operations in the collodionized-paper process consist in attaching a sheet of paper to a glass plate, coating the paper with collodion, and proceeding in the ordinary way to the completion of the picture ; then removing the paper from the glass by soaking it in water, and treating it as an ordinary paper negative.
The only operations we have to describe are those of attaching the paper to the plate, and removing it when the picttut is finished; the others being identical with those on collodionize,d glass.
To attach the Paper to the Glass Plate. Immerse it in a warm solution of gelatine, made by dissolving six grains of gelatine to the ounce of water. Let it remain for a minute or two in the solution, then remove it, and immediately apply the back of it to a clean glass plate, pressing it into close contact with the glass by means of a bent glass rod. Let it dry thoroughly. When dry it will stick firmly to the glass.
Coat the paper with collodion, excite it by immersion in the nitrate bath, and develope and fix the picture precisely as in the collodion process. Then soak the plate in water, and remove the paper negative as soon as it will leave the glass without risk of being torn. It should then be well washed in several changes of water, dried, and waxed in the usual way.
The advantages of the process are, that the excited paper is highly sensitive,—more so than an excited collodionized plate,—so that skies and instantaneous pictures may be taken upon it ; and, also, that it gives far better definition and half-tone than the ordinary methods upon paper. The photographic tourist may, therefore, employ paper for the same class of subjects as glass, without subjecting himself to the expense, trouble, and risk of travelling with boxes of glass plates, and valuable glass negatives. On the other hand, when all has been done to render a paper negative evenly transparent, it can hardly be rendered quite equal to glass for supporting a negative picture.
Such are the merits and drawbacks of an excellent process, which deserves to be more extensively practised.
COLOUR.-If the coloured spectrum produced by passing a ray of sunshine through a prism, and thereby decomposing it into coloured rays of different refrangibilities, be thrown upon a sheet of •sensitive photographic paper, the paper is most darkened by the violet rays, and least by the yellow and red. This shews that the chemical power of light resides chiefly in the violet rays. Now the colour of an object when illuminated by white light is supposed to depend upon its absorbing all the coloured rays which fall upon it, and, when combined, produce white light, except the rays- of the particular colour which it emits : so that, on this hypothesis, it might be supposed that the images of yellow and orange coloured objects would scarcely produce an impression upon a photographic tablet, while those of objects of a blue or deep violet colour would produce a comparatively marked effect. This is found by experi ments to be true to some extent in the case of coloured objects copied in the ordinary way, and to a greater extent in the case of light transmitted through coloured media. 'For instance, if a paint ing in fresco were exposed to full sunshine, and copied in the ordi nary way, the photograph would not be so perfectly truthful in its mode of rendering shade for colour, as an engraving from the same picture, because the yellows and reds would be darker, and the blues and violets lighter in the photograph than in the engraving, in which light colours would be properly represented by lights, and dark colours by shades. But, if the same fresco were faithfully copied by a trans parent painting upon glass, and this copy were photographed by superposition upon a sensitive tablet, and a positive printed from a negative so obtained, the untruthfulness of this positive, as regards light and shade, would be much more marked than in the former case. It appears, therefore, that a coloured object emits from its surface certain actinic rays besides the rays of its peculiar local colour, and that the photographic copy of a coloured object obtained in the ordinary way is not quite so inaccurate in its mode of ren dering colours by shades as might at first be supposed.