CHAP. V., 7.] OPTICS (PHOTOGRAPHY). DAGUERRE MR TALBOT. 



125 



keeping his secret until brought to perfection, that 

 he did not even show his results until early in 1839, 

 when the numerous specimens he had to exhibit ri- 

 valled in delicacy anything that the art has since 

 produced. 1 



The daguerreotype is depicted in the camera on a 

 plate of silver, coated with an evanescent film of 

 iodide, by exposure for a short time to the vapour of 

 iodine. After the light has acted, and the plate has 

 been withdrawn from the camera, no trace of a pic- 

 ture is visible until it has been exposed to the vapour 

 of mercury, which, by its peculiar action on the places 

 where light has acted, produces a correct picture of 

 the object (or positive image, as it is called, light an- 

 swering to light). It is then fixed, as it is called, by a 

 bath of dissolved hypo-sulphite of soda, which has 

 the property of removing the iodide of silver wherever 

 the light has not acted. This singular and elaborate 

 process has since been but slightly modified by vari- 

 ous plans for rendering the iodide more exquisitely 

 sensitive. To M. Niepce belongs the credit (1) of 

 having fixed an impression of light, (2) of using 

 metal plates, (3) of forming a picture by means of a 

 camera obscura ; to Daguerre, on the other hand, 

 the novel and ingenious use of vapours instead of 

 washes, and the whole succession of operations in 

 the daguerreotype. When the French government 

 acquired for the public (the French public, however, 

 only) a right of property in the invention, they 

 marked their sense of the share of merit of the in- 

 ventors, by awarding to Daguerre 6000, to M. I. 

 Niepce 4000 francs per annum. 



(571.) Mr W. H. Fox TALBOT, a Wiltshire gentleman 

 Mr Fox o f great ingenuity and perseverance, and well ac- 

 the calo^ quainted with mathematics and physics, applied him- 

 type. self in 1834 to the problem of fixing shadows, in 

 entire ignorance of what Wedgewood and Davy had 

 attempted. He used paper washed with the nitrate 

 of silver, and soon succeeded in obtaining impressions 

 of lace and leaves of plants, but without fixing the 

 shadows. This great step he, however, made a 

 year or two later : a wash of iodide of potassium, 

 or of common brine, was found to effect it. The 

 announcement of his success and of his methods 

 was called forth early in 1839 2 by the first reports of 

 Daguerre's discovery. His method then consisted in 

 dipping writing paper alternately in nitrate of silver 

 and common salt, drying between the operations, 

 and afterwards fixing the image. An unnatural or 

 negative picture was thus obtained, the lights of na- 



ture being darkened on the paper, and vice versa; 

 but the truth was restored by pressing the drawing 

 thus obtained against a second prepared sheet of 

 paper, and exposing it to light, when the natural 

 lights and shades were of course obtained. This 

 derived impression is called a positive. This process 

 has evidently several advantages over Daguerre's ; 

 such as, that paper is used instead of metal plates ; 

 and that from a single impression (negative from 

 nature) copies may be at leisure indefinitely multi- 

 plied. Pictures were thus obtained by Mr Talbot 

 with the camera obscura. 



Mr Talbot's chief improvement on his first me- (572.) 

 thod he called the Calotype (1841), and consisted in Progress, 

 washing the paper successively with nitrate of silver, 

 iodide of potassium, 3 and gallo-nitrate of silver. It 

 is then exposed in the camera, but no impression 

 appears until again washed with the gallo-nitrate of 

 silver. It is then fixed with bromide of potassium, or 

 with hypo-sulphite of soda, as in Daguerre's process. 



By a subsequent invention, Mr Talbot has obtained (573.) 

 what he justly calls an instantaneous process. 1 An Instanta- 

 image was formed in a camera of a revolving wheel, "g g u8 pn> 

 to which was affixed a printed bill ; the room being 

 darkened, and the wheel made to revolve with the 

 speed of 200 revolutions in a second, and being then 

 illuminated by an electric spark, a legible impression 

 of the printing was obtained. We doubt if, in the 

 whole history of physics, a more astonishing result 

 is recorded. Thus Mr Fox Talbot, by his rare energy, 

 brought his inventions almost to perfection. Nume- 

 rous competitors, of course, appeared on the field, and 

 obtained many interesting results. The only one of 

 much importance to the art of photography is the 

 substitution of a film of iodized collodion on a glass 

 plate, for the prepared paper in the first or negative 

 process. 



The daguerreotype and calotype processes, though (574.) 



seemingly so different, have much in common : (1) Ana lgy 



f i L -L. j / j-j f -i of the two 



a sensitive surface has to be prepared (iodide oi silver processeSi 



is the basis in both) ; (2) it is exposed in the camera ; 

 (3) the picture (still invisible) is developed; (4) it is 

 fixed. In the calotype the printing process for ob- 

 taining positives must be added to these. 



The chemical theory is very far behind the art of (575.) 



photography. The most important steps of these Chemical 



n T , T i -L x theory im- 



cunous and complicated processes have been at- fe * t> 



tained by a kind of divination after a multitude of 

 failures. The salts of silver are of a highly de- 

 composable nature, the iodides and bromides pecu- 



1 The present writer had the benefit of seeing Daguerre's marvellous productions, and making his acquaintance at Paris, 

 through the courtesy of M. Arago, while the secret was still preserved, and the public interest was excited to the highest pitch. 

 About the same time he saw M. Isidore Niepce, son of the first photographer, and the specimens in his bands, as well as those in 

 the possession of Mr Bauer of Kew, with whom they had been left by M. Nicephore Niepce, when he visited England in 1827 

 and exhibited them at the Royal Society. One of the latter was engraved on a plate resembling pewter. 



a Communicated to the Royal Society of London, 31st January, and printed in the Philosophical Magazine for March. 



3 Mr Talbot says (Phil. Mag., March 1839, p. 203) that Sir H. Davy had recommended iodide of silver as a sensitive substance ; 

 but in his ea.-lier experiments he had found it the contrary. But now, like Daguerre, he requires no immediate visible action 

 of light, but developes it by a subsequent reaction. 



4 The process is described in Hunt's Researches on Light, 2d edit., p. 140. 



