I originally recommended chlorophyl extracted from dried leaves, because I had not yet learned how to preserve the solution for more than a few weeks; and at some seasons it would be difficult, if not impossible, to obtain fresh leaves. The tea organifier which I recommended is also a color sensitizer, and when it is used in connection with the chlorophyl from dried leaves the plates are as sensitive to red as can be safely prepared and developed in the light of an ordinary photographic "dark-room." Plates prepared with chlorophyl from fresh leaves do not require treatment with the tea organifier to secure this degree of sensitiveness. Recently I have used the tea organifier and some other sensitizers, in connection with the solution from fresh myrtle-leaves, and in this way have produced plates having such an exalted color sensitiveness as to be unmanageable in ordinary "dark-room" light. Possibly, such plates might be prepared and developed in total darkness, by the aid of suitable mechanical contrivances, but I am not sure that they would work clear even then, because they appear to be sensitive to heat as well as to light.
DISTORTION FROM EXPANSION OF THE PAPER IN PHOTOGRAPHY.
The effect of the unequal expansion of paper, when wetted, in causing distortion of the photographic image impressed upon it, has, in the case of ordinary photographs upon albumenized paper, been well recognized; but the extent to which such distortion may exist under different treatment is worthy of some special consideration, particularly with reference to the method of printing upon gelatinized paper, which has been thought by some likely to supersede the method now usually employed with albumenized paper.
When a print upon the ordinary photographic (albumen) paper is wetted, the fiber expands more in one direction than in the other, so that the print becomes unequally enlarged, very slightly in one and much more so in the other way of the paper. When the paper is dried without any strain being put upon it, the fibers regain very nearly their original dimensions and position, so that the distortion which has existed in the wet condition nearly disappears.
If the photograph is cemented, while in the expanded condition, upon a rigid surface, the distortion then existing is fixed, and rendered permanent. Such a cementation or method of mounting is that which has been generally adopted, and the consequence has been that every now and then complaints have justly been made of the untruthfulness—owing to this particular distortion—of photographs; productions whose chief merit has often been asserted to consist in their absolute truthfulness. This distortion is very manifest when, in a set of portraits, some of the prints happen to have been made in one direction of the paper, and others with the long grain the other way. I have known a case where a proof happened to increase the face in width, and all the other prints increased it in length. Of course, neither was correct, but the proof had been accepted and liked, and the remainder of the set had to be reprinted with the grain of the paper running in the same direction as that in the first one which had been supplied.
Another evil arising from mounting prints while expanded with moisture is, that in drying the contraction of the paper pulls round the card into a curved form and although by rolling this curvature may be temporarily got rid of, the fiber of the paper is in a strained condition, and the bent state of the mount is, sooner or later, renewed thereby.
To remedy these evils it has been proposed to mount the print when dry, by forcible pressure against a slightly damped card, the back of the print having been previously coated with a cement and dried. This plan is, to a great extent, successful; but that it does not give absolute immunity from distortion is, I think, evident from the following consideration. The prints, after being mounted a few days, will show a certain tendency to curl inward. This curling, I take it, is a measure of the strain upon the print, produced by the more complete return to its original dimensions of the paper photograph. Probably it would be well to keep the prints a few days after drying, or to subject them to alternations of damp and dryness, in order to facilitate this complete return before being placed upon the card. The evil of distortion is, however, very slight—perhaps imperceptible—compared with that existing when the prints are mounted wet. I may mention, en passant, that I have found gum much more satisfactory as a mountant than starch paste in what is known as the "dry mounting" system.