On this day (September 25) in the year 1839, the following article appeared in 
the "United States Gazette" (Philadelphia). My text today is from the reprint of 
the article in the "Journal of the Franklin Institute" Vol. 24, No. 3 (September 
1839) pp. 209-210.
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            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.



("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].)
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Posted for your enjoyment.     Gary W. Ewer     
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09-25-98