The process just described yields perfect results, and has super seded all others for the purpose of making • Photogeuio Drawings, as they wore formerly termed. Leaves, grasses, prints, feathers, lace, and indeed any well-defined object, translucent or not, may be depicted by simple contact with the prepared paper and subsequent exposure of the whole to light, under a sheet of glass pressed tightly on by springs or screw's, as in the ordivary copying-frame.' In the absence of the sun, a quarter of an hour'e exposure to the electric light will produce a good positive impression.
There is auother branch of photography which is worthy of the attention of the student, but which has not yet come into practical operation in a perfectly satisfactory manner. It is that of photographic engraving. The labours of Niapee, Grove, Fizeau, Talbot, Pretsch, Poitevin, and others have done much to forward this art, and are well worthy of attention. Although the first Niepce laid the foundation of the art of Photographic Engraving, by his experiments on asphaltum, or Jew's pitch, it is to Mr. Talbot that we are indebted for opening up new processes in which organio substances such as starch, gum, albumen, and gelatine mixed with a salt of chromic acid furnish a photographic deposit which is capable of being worked by the ordinary processes of the engraver and lithographer. Mr. Talbot's plan is to pour upon a steel plate a mixture of bi-chromate of potash and gelatine, so as to obtain by drying a sensitive film : upon this film a positive photographic drawing is placed ; now, by exposure to light, the gelatine becomes or nearly insoluble wherever the light has fallen through the positive picture. An engraving acid poured upon the plate will now etch only the shaded parts of the plate, and thus an engraved surface is obtained to be printed front with printer's ink. Mr. l'retsch, instead of etching the plate obtained by the action of the light on the gelatine compound, acts- upon it by liquids ; and what is most remarkable, gets a grained image in relief from which a mould is taken for the purpose of being electrotyped to form the copper-plate to print from. By proper manipulation, Mr. Pretsch can produce plates fit to print by the method called surface-printing, as with an ordinary wood block.
Of course, impressions taken from these plates by proper means can be conveyed to porcelain ur glass, and burnt in by the enameller in the usual manner, or as at first devised by Mr. Malone, and described in a joint patent taken out with Mr. Talbot. To Messrs. Ponton. and Becquerel we owe the first application of chromium to general photo graphic purposes. Mr. Pauncey and others have also devised ingenious processes by which pigments or inks may be made to adhere only to the altered gelatine and chromium compound surface, whether on paper, stone, glass, porcelain, or metals. These processes are as yet under trial. They lack the delicacy of tint of the albumen process,
but they will doubtless yield ultimately valuable results.
Stimulated by the experiments of Sir John Herschel, M. Ed. Bec querel and others, M. Niepce de St. Victor commenced a series of beautiful experiments upon coloured flames and their photographic images. He laid before the Academy of Sciences, Paris, a detailed memoir upon the subject on the 4th of March, 1851. This was followed by others on June 2, 1851, Feb. 9, 1852, and November 6, 1852. By the method described in these papers, M. Niepce succeeded, he says, in obtaining upon silver plates which had been rendered sensitive by a chloride of copper, images which faithfully reproduced the colours in coloured engravings, flowers both artificial and natural, lay-figurea dressed in stadia and gold and silver lace, precious stones, &c. These were obtained both by the process of photographic printing and in the camera ; the light and brilliant colours being obtained with comparative ease, but the darker and more sombre colours more slowly. The colours he rendered more vivid and at the same time more lasting by the action of ammonia. But beautiful as were the results, and much more nearly than in any other previous experiments as they seemed to approach the solution of the problem of photographing the colours of nature, they proved to be only comparatively permanent. The colours soon began to fade and eventually disappeared altogether.
This method (due to M. Ed. Becquerel) M. Niepce named Heliochrome.
M. Ed. Becquerel, by the use of silver plates, coated with a dark compound of chlorine and silver, obtained by the voltaic decom position of hydrochlorio acid, has succeeded in obtaiuing coloured images of the solar spectrum, but no method of fixing them permanently has been discovered.
In scientific photography much remain, to be done. We know but little of the properties of light In its influence on vegetation and animal life. 141r. Robert Hunt and others have, however, established some interesting facts in the former direction, and lately some experi ments made upon the eggs of insects seem to show that light of various colour and intensities acts diflerently accordin? to its colour and other peculiar qualities. There is no branch of science which will better repay the philosophical experimentalist for his investigations than that of photography. The most marvellous and unexpected results have been constantly obtained.
Those who would pursue photography further should consult Hunt's ' Researches on bight, the Abb6 Ifoigno'a • Repertoire d'Op tique Moderns,' and 11r. Hardwich's 'Treatise on Photographic Chemistry.' There are many papers also of interest to be found In the ' Comptes Rendus ' of the Paris Academy of Sciences, in our own Royal Society. Transactions,' and above all in the journals of the various Photographic Societies.