(570.) 
The da- 
guerreo- 
type. 
(571.) 
Mr Fox 
Talbot— 
the calo- 
type. 
Cuar. V., § 7.] 
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.! 
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been withdrawn from the camera, no trace of a pic- 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 mereury, 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). Itis 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. 
Mr W. H. Fox Taxzor, a Wiltshire gentleman 
of great ingenuity and perseverance, and well ac- 
quainted with mathematies and physics, applied him- 
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 izing 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? 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- 
OPTICS (PHOTOGRAPHY ).—DAGUERRE—MR TALBOT. 
923 
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- 
thod he called the Calotype (1841), and consisted in Progress. 
washing the paper successively with nitrate of silver, 
iodide of potassium,* 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.) 
An Instanta- 
what he justly calls an instantaneous process.‘ 
image was formed in a camera of a revolving wheel 
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 
seemingly so different, have much in common :—(1) Anslogy 
a sensitive surface has to be prepared (iodide of silver 
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 
curious and complicated processes have been at- 
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 hands, 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 atthe Royal Society. One of the latter was engraved on a plate resembling pewter. 
2 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 earlier experiments he had found it the contrary. But now, like Daguerre, he requires no immediate visible action 
of right, but developes it by a subsequent reaction. 
4 
he process is described in Hunt’s Researches on Light, 2d edit., p. 140. 
? cess, 
theory im- 
perfect. 
