||From the United States Gazette (Philadelphia) 25 September 1839 as reprinted in the Journal of the Franklin Institute Vol. 24, No. 3 (September 1839) pp. 209-210.
To the Editor of the United States Gazette.
Dear Sir:A scientific friend having kindly sent to me, from Paris, a paper containing the authorized explanation of the process of M. Daguerre, which has attracted so much attention, I have made an abstract of it, which is at your service. The condition under which M. Daguerre received a pension from the French Government was, that he should make all the steps of his process public. His communication was first made confidentially to a Committee of the Institute, and M. Arago, acting as their organ, states that some of the steps of the process are not explicable by known laws.
Yours,A. D. B.
The Daguerreotype Explained.
The Academies of Sciences and of Fine Arts of the Institute, met on the 19th of August last, to hear the explanation of the process of M. Daguerre, from the perpetual Secretary, M. Arago.
It appears that in the earlier attempts to form a permanent picture of external objects, by the aid of the camera obscura, chloride of silver was used as in the method of Photogenic Drawing of Mr. Talbot. In this case the light blackens the paper, the depth of the shade being proportionate to the intensity of the light; the lights are thus represented by dark parts of the picture, and vice versa. M. Niepce first ascertained that an effect the reverse of that just mentioned might be produced by using a particular kind of bitumen, dissolved in oil of lavender, and that this picture might be preserved. The picture formed upon the bitumen required, to render it fully visible, the use of petrolium, which acted upon the parts of the picture not affected by the light. This process was a very imperfect one, requiring a long time for its execution, and yielding but imperfect results. The invention had been brought to this stage, when M. Daguerre undertook to improve it; by many curious experiments, and much labor, he was gradually led to his present process, the details of which present some very strange, and so far unexplained phenomena.
A sheet of copper, plated with silver, is carefully cleaned on the silver side by the aid of nitric acid. The cleansing requires great care and especial precaution, the plate requiring to be rubbed backwards and forwards in a fixed direction. Plated copper is found to answer better than silver. The plate thus prepared is exposed to the action of the vapour of iodine. For this purpose, it is placed in a box upon the bottom of which a small quantity of iodine is strewed, separated from the plate by a gauze screen, so as to diffuse the vapour uniformly The plate must be enclosed in a metallic frame, to prevent the vapour from acting more upon the edges than near the centre, the success of the whole operation depending essentially upon the uniformity of the coating of iodide of silver, which is formed upon the surface of the plate. A yellow tint indicates that the plate has been sufficiently long exposed to the action of the vapour. It is then transferred to the camera obscura carefully excluding it meanwhile from the light. M. Daguerre is understood to have made some improvements to this instrument; to understand the progress of the present operation, however, it is only necessary to observe, that a plate of ground glass having been previously placed so as to receive a distinct image of the object to be delineated, the prepared silver plate is substituted for it. The effect is immediate, but is only very slightly perceptible. The plate is next exposed to the action of the vapour of mercury, and a condition stated to be essential is, that it shall be placed under a particular angle. It is therefore placed in a second box, at the bottom of which is a small trough of mercury, heated to between 160¡ and 170¡ Fahrenheit, and if the picture is to be viewed when hanging vertically, the inclination of the plate to the surface of the mercury must be 45¡. The vapour of mercury appears to affect only the parts which have been already acted upon by the light, forming, probably, an amalgam of mercury and silver. After this operation the plate is dipped into a weak solution of hyposulphite of soda, and then washed with distilled water. The process is now complete, and the plate presents a drawing in which the light and shade is truly represented, and which may be exposed, without change, to the action of the light.
(Transcriber's note: "A.D.B." is "Alexander Dallas Bache" as noted in Stapp, William, Marion Carson, and M. Susan Barger, Robert Cornelius: Portraits from the Dawn of Photography [Washington: Smithsonian Institution Press, 1983] which reprints the text as "Appendix 4" [pg. 139].)
(End of text. Please refer to our textnote regarding this text.)