PHOTO-LITHOGRAPHY, the name of a new art which we owe to Mr Macpherson of Rome. A photograph, impressed on the surface of a lithographic stone, is treated in the ordinary way, and copies afterwards taken of it. Two different processes have been patented in England, one by M. Poitevin, and the other by Mr Newton as the invention of Messrs Culling and Bradford of Boston.
In M. Poitevin's patent the stone is covered with one or more
like a telescope, and thus obtain the most perfect adjustment of Photo-Li-
the luminous focus of the lens, the usual means being taken to ob-
tain a coincidence of the luminous and actinic focus.
The portable form of the camera, as constructed by Captain Fowkes, is shown in the annexed figures—when it is open (fig. 2), and when it is shut (fig. 3).3
A section of the tube containing the achromatic lenses of almost every camera is shown in figs. 4 and 5, where AB, CD, are
the two lenses used in taking portraits, with a suitable stop, seen between them. In order to take landscapes, the lens CD is removed, and the lens AB placed as in fig. 5, with the stop seen in front of it. Mr Skelton uses what he calls a pisto-camera, with a single lens three-fourths of an inch in diameter, to produce photographs on small concave lenses.4
In all these cameras the picture is inverted on the glass, so that the artist cannot possibly form a correct judgment of the pose of individual figures, or of the grouping of several figures whose portraits are required. We would recommend, therefore, the use of an erecting camera,5 for the single purpose of enabling the artist to judge of the pose or grouping of the picture. When the artist is satisfied with this, the erecting camera should be replaced by the inverting one. (See arts. xvii. and xx.)
Among the various lenses used by photographers or proposed by optical writers, that of M. Petzval has been ascertained, not only by theory, but by direct experiment, to be the best. It has been the practice to test a lens by the pictures which it produces; but the picture thus used as a test is influenced by various causes, so that it is impossible to separate the effect produced by these causes from that which is due to the lens. The lens of every camera should be made the object-glass of a telescope, and tried with different eye-pieces and different apertures; and from the data thus procured a photographer of any optical knowledge will have no difficulty in ascertaining the value of his lens. With large apertures, it will doubtless be found, especially from the projected pictures of rectilinear objects, that straight lines are much bent into curves; and the photographer will learn, what so few of them know, that if he wants a true picture he must attend to the aperture of his lens. M. Petzval has proposed the excellent test of copying a map on the scale of a fifth part of the original, and observing the difference between the copy and the original.
In almost all the cameras used in taking portraits, the focal length of the lens exceeds, and is sometimes double, the focal length of the eye; so that all the irregularities in the human skin are magnified, thus adding imperfections not generally visible to those of large and imperfect lenses.
In an article of limited extent like the present, we cannot find room for drawings and descriptions of copying and enlarging cameras; or of the ingenious pieces of apparatus invented by Mr Claudet, such as the photogrammeter, dynactinometer, and foelometer; and of the various pieces of apparatus which are used in photographic processes. For an account of these, we refer the reader to the Abbé Moigno's Repertoire d'Optique Moderne, parts II., III., IV.; Hunt's Manual of Photography, 5th edition, 1857; Hunt's Practice of Photography, 1857; Thornthwaite's Guide to Photography, 14th edition, 1857; and Ross and Thomson's Plain Answers to Common Questions regarding Photography, 1853. (D. M.)
films of a mixture of albumen or gelatine, and a concentrated solution of bichromate of potash. This film is then dried if a negative picture is to be copied upon it; but it may be used in a moist state if it is to receive the picture in the camera. When the surface is dry it is moistened with a sponge, and while moist the lithographic ink is applied to the surface by a tail or dabber, or other means, when it will be found to adhere only to those parts which have been affected by light.
In Messrs Culling and Bradford's process, a film of prepared gum-arabic is used which has been deprived of its power of intimate union
1 See the Photographic Journal, vol. iv., p. 83, where this subject is treated at considerable length.
2 On the subject of photographic portraiture, we would refer the reader to a clever article in the Art-Journal, No. xlv., p. 273.
3 An improved camera on this principle, by Mr Kinnear, is described in the Phot. Jour., vol. iv., pp. 116, 165.
4 Phot. Jour., vol. v., p. 98, Dec. 11, 1858.
5 This camera may be a very common one, with a small single lens, and may be constructed at a small expense.
with the stone, while at the same time it is rendered capable of becoming fixed or insoluble by the action of light. When this film is submitted to the action of a solution of soap, the parts acted upon by light are not injuriously affected by it, and an insoluble soap is formed on the stone to produce the printing surface. The exciting solution is
Water ..... 1 qt. | Sugar or molasses ..... 100 gr.
Gum-arabic ..... 4 oz. | Bichromate of potash ..... 100 gr.
The sugar retards the immediate fixing of the gum upon the stone, and the chromic salt causes it to be more firmly fixed, or to be less soluble when exposed to light. When the film is dried, it may be exposed in the camera, or placed under a negative.
In the first of these processes the blacks are produced by the parts on which the light has acted; and on the second by those upon which it has not acted. (D. B.)