PHOTOZINCOGRAPHY 573 



A process has recently been introduced (1874) which is highly spoken of. It con- 

 sists essentially in mixing recently-precipitated bromide of silver with very pure gela- 

 tine. This cream}', semi-opaque mixture is poured on glass-plates, and these are dried 

 with application of moderate heat in perfect darkness. The plates are rendered highly 

 sensitive in the usual manner. It is said that in good light a portrait can be taken 

 in five seconds. 



PHOTOMETRY. The measurement of light, or of illuminating power. See 

 ILLUMINATION. 



PHOTO-SCUIiPTURE. The following description of this art is written by the 

 late M. A. Claudet, F. E. S., who most successfully practised it : ' This beautiful ap- 

 plication of photography is called Photo-sculpture, and is the invention of M. Willeme, 

 an eminent .French sculptor. Before explaining how M. Willeme was led to this dis- 

 covery, let me remind you that photography itself was invented by painters of talent 

 by artists who, while using the camera obscura for studying the subject of their 

 intended pictures, were struck with the beauty of those natural representations. In 

 contemplating them they naturally desired that the pictures could be permanently 

 fixed. Considering that these pictures were formed by the light reflected from the 

 objects, they essayed to fix them by availing themselves of the known scientific fact 

 that light had the property of blackening certain chemical compounds. The flash of 

 that idea was enough'; their genius and perseverance solved the problem, and they 

 created that art which they desired no much photography. A similar and no less 

 instructive story may be told of photo-sculptUre. 'M. Willeme was in the habit, 

 whenever he could procure photographs ot his sitters, of endeavouring to communicate 

 to the model the correctness of those unerring types. But how should he raise the 

 outlines of flat pictures into solid form ? Yet these single photographs, such as they 

 were, could serve him to measure exactly profile outlines. He could indeed, by means 

 of one of the points of a pantograph, follow the outline of a photograph, while with 

 the other point directed on the model, he ascertained and corrected any error which 

 had been communicated to his work during the modelling. What he could do with 

 one view or one single photograph of the sitter, he might do also with several other 

 views if he had them. This was sufficient to open the inquiry of an ingenious mind. 

 He saw at once that if he had photographs of many other profiles of the sitter, taken 

 at the same moment, by a number of camera obscuras placed round, he might alter- 

 nately and consecutively correct his model by comparing the profile outline of each 

 photograph with the corresponding outline of the model. Such was the origin of a 

 marvellous and splendid discovery. But it soon naturally occurred to him, that in- 

 stead of correcting his model when nearly completed, he had better work with the 

 pantograph upon the rough block of clay, and cut it out gradually all round in fol- 

 lowing one after the other the outline of the photographs. Now supposing that he 

 had twenty-four photographs, representing the sitter in as many points of view (all 

 taken at once), he had but to turn the block of clay after every operation ^th of the 

 base upon which it is fixed, and to cut out the next profile, until the block had com- 

 pleted its entire revolution, and then the clay was transformed into a perfect solid 

 figure of the twenty-four photographs ; the statue of the bust was made. When this 

 is once explained, everyone must be struck with admiration at the excellence of the 

 process. It is so sure, and so simple, that we are surprised it has not been thought of 

 before.' 



PHOTOZINCOGRAPHY. This is the name given by Major-General Sir Henry 

 James, R.E., Director of the Ordnance Survey, who has thus describfed the process : 

 'For the purpose of producing rapidly and in large numbers, fac-similes of plans, 

 drawings, written and printed documents, &c., of the same size as the originals, or to 

 any required lesser scale, the present Director of the Ordnance Survey successfully in- 

 troduced in 1859, a method combining the accuracy of photography with the facility 

 of printing from zinc-plates, and named the process Photozincography. It is now ex- 

 tensively used at Southampton for supplying the public, at a low cost, with fac- 

 similes of some of the most interesting and valuable State papers that are preserved 

 among the national manuscripts of the United Kingdom. 



' The fac-similes being in ink of which carbon is the basis, are not liable to fade, 

 like photographic prints in silver ; for although the silver is coated with a film of gold 

 in the toning bath, it slowly yields to atmospheric influences, and to the long-continued 

 action of small traces (almost impossible to eradicate) of substances employed in the 

 manipulation. 



' In the process of zincography a tracing of the document is made in a greasy ink, 

 and applied to a zinc-plate, from which any desired number of impressions may be 

 printed ; but in photozincography a photograph is prepared in such a manner that it 

 may be transferred to a zinc-plate, the prints from which are free therefore from any 

 error of the draughtsman's hand. 



