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A few notes on today's text:
  In correspondence I had with R. Derek Wood some time ago, Derek
mentioned this about "The Athenaeum" text:
  "The first and last paragraphs are the only part of the text written
by 'The Athenaeum' contributor, the rest is a translation into English
from the first half of A. Donne's original report in 'Journal des Debats
(Paris), 20 August 1839, pp. 1-3. It is the most significant report of
Arago's lecture that appeared during August 1839."
  I'll add to this that the figure of "A. Donne" can be seen in the 8
December 1839 lithograph, "La Daguerreotypomanie." He is depicted on the
right hand side of the illustration operating an instrument of
portrait-torture under the banners "EPREUVE DAGUERRIENNE SUR PAPIER" AND
"SYSTEM DU DOCTEUR DONNE." Donne formulated a method of taking paper
proofs directly from daguerreotypes. Etching the plates by
electrotyping, Donne could print up to 40 proofs from one plate.
  It was over a month later that this text appeared in the US press. 
"The New-Yorker" carried the text in their 28 September 1839 edition.
(see http://www.daguerre.org/resource/dagnews/09-28-95.html)
                                             --Gary W. Ewer

* * * * * * * *
On the day (August 21) in the year 1839, the following news item was
written from Paris and appeared a few days later in "The Athenaeum -
Journal of Literature, Science, and the Fine Arts" (London, Saturday,
August 24, 1839, No. 617, pp. 636-7):
-----------------------------------------------------------------

               PRINCIPLE OF THE DAGUERREOTYPE.
                                    Paris, August 21st
    
   I write to you to report--though of necessity hastily--the
proceedings of the Academie des Sciences of Monday last, when M. Arago,
in the presence of a crowded audience, which had besieged the doors of
the Institute three hours before the commencement of the sitting,
divulged the secret of M. Daguerre's invention, which has now, as you
are aware, become public property.  Three drawings having been
exhibited, by way of specimens, M. Arago began be recapitulating the
discoveries--or rather hints toward discoveries--of former chemists.  He
afterward dwelt upon the progressive experiments of M. Niepce, since
carried out by M. Daguerre.  As, however, your columns already contain
notices of these, I will come at once to the publication of the secret
of the perfect invention; and in order to give you this as fully and
clearly as possible, I send you an abstract from the report published in
yesterday's Journal des Debats.
   M. Arago stated that, according to M. Daguerre's process, copper
plated with silver is washed with a solution of nitric acid, for the
purpose of cleansing its surface, and especially to remove the minute
traces of copper which the layer of silver may contain.  This washing
must be done with the greatest care, attention, and regularity.  M.
Daguerre has observed, that better results are obtained from copper
plated with silver, than from pure silver; whence it may be surmised,
that electricity may be concerned in the action.
   After this preliminary preparation, the metallic plate is exposed, in
a well-closed box, to the action of the vapor of iodine, with certain
precautions.  A small quantity of iodine is placed at the bottom of the
box, with a thin gauze between it and the plate, as it were to sift the
vapor, and diffuse it equally.  It is also necessary to surround the
plate with a small metallic frame, to prevent the vapor of iodine from
condensing in larger quantities round the margin than in the centre; the
whole success of the operation depending on the perfect uniformity of
the layer of ioduret of silver thus formed.  The exact time to withdraw
the sheet of plated copper from the vapor, is indicated by the plate
assuming a yellow color.  M. Dumas, who has endeavored to ascertain the
thickness of this deposit, states that it cannot be more than the
millionth part of a millimetre.  The plate thus prepared is preserved
with great care from the faintest action of light.  It is, in fact, so
sensitive, that exposure for a tenth part of a second is more than
sufficient to make an impression on it.
   At the bottom of the dark chamber, which M. Daguerre has reduced to
small dimensions, is a plate of ground glass, which advances or recedes
until the image of the object to be represented is perfectly clear and
distinct.  When this is gained, the prepared plate is substituted for
the ground glass, and receives the impression of the object.  The effect
is produced in a very short time.  When the metallic plate is withdrawn,
the impression is hardly to be seen, the action of a second vapor being
necessary to bring it out distinctly.  The vapor of mercury is employed
for this purpose.  It is remarkable, that the metallic plate, to be
properly acted upon by the mercurial vapor, must be placed at a certain
angle.  To this end, it is enclosed in a third box, at the bottom of
which is placed a small dish filled with mercury.  If the picture is to
be viewed in a vertical position, as is usually the case with
engravings, it must receive the vapor of mercury at an angle of about
45o[degrees].  If, on the contrary, it is to be viewed at that angle,
the plate must be arranged in the box in a horizontal position.  The
volatilization of the mercury must be assisted by a temperature of
60o[degrees] (of Reaumur.)
   After these three operations, for the completion of the process, the
plate must be plunged into a solution of hypo-sulphite of soda.  This
solution acts most strongly on the parts which have been uninfluenced by
light; the reverse of the mercurial vapor, which attacks exclusively
that portion which has been acted on by the rays of light.  From this it
might perhaps be imagined, that the lights are formed by the
amalgamation of the silver with mercury, and the shadows by the
sulphuret of silver formed by the hypo-sulphite.  M. Arago, however,
formally declared the positive inability of the combined wisdom of
physical, chemical and optical science, to offer any theory of these
delicate and complicated operations, which might be even tolerably
rational and satisfactory.
   The picture now produced is washed in distilled water, to give it
that stability which is necessary to its bearing exposure to light
without undergoing any farther change.
   After this statement of the details of M. Daguerre's discovery, M.
Arago proceeded to speculate upon the improvements of which this
beautiful application of optics was capable.  He adverted to M.
Daguerre's hopes of discovering some farther method of fixing not merely
the images of things, but also of their colors; a hope based upon the
fact that, in the experiments which have been made with the solar
spectrum, blue color has been seen to result from blue rays, orange
color from orange, and so on with the others.  Sir John Herschel is sure
that the red ray alone is without action.  The question arose, too,
whether it will be possible to take portraits by this method?  M. Arago
was disposed to answer in the affirmative.  A serious difficulty,
however, presented itself:  Entire absence of motion on the part of the
object is essential to the success of the operation; and this is
impossible to be obtained from any face exposed to the influence of so
intense a light.  M. Daguerre, however, believes that the interposition
of a blue glass would in no way interfere with the action of the light
on the prepared plate, while it would protect the sitter sufficiently
from the action of the light.  The head could be easily fixed by means
of supporting apparatus.  Another more important desideratum is, the
means of rendering the picture unalterable by friction.  The substance
of the pictures executed by the Daguerreotype is, in fact, so little
solid--is so slightly deposited on the surface of the metallic plate,
that the least friction destroys it, like a drawing in chalk; at
present, it is necessary to cover it with glass.
   From his numerous experiments on the action of light on different
substances, M. Daguerre has drawn the conclusion that the sun is not
equally powerful at all times of the day, even at those instants when
his light is the same above the horizon.  Thus, more satisfactory
results are obtained at six in the morning than at six in the afternoon.
From this, too, it is evident that the Daguerreotype is an instrument of
exquisite sensibility for measuring the different intensities of
light--a subject which has hitherto been one of the most difficult
problems in Natural Philosophy.  It is easy enough to measure the
difference in intensity between two light viewed simultaneously: but
when it is desired to compare daylight with a light produced in the
night--that of the sun with that of the moon, for example--the results
obtained have had no precision.  The preparation of M. Daguerre is
influenced even by the light of the moon, to which all the preparations
hitherto tried were insensible, even when the rays were concentrated by
a powerful lens.
  In physics, M. Arago indicated some of the more immediate applications
of the Daguerreotype independently of those which he had already
mentioned in Photometry.  He instanced some of the most complex
phenomena exhibited by the solar spectrum.  We know, for example, that
the different colored rays are separated by black transversal lines,
indicating the absence of these rays at certain parts; and the question
arises whether there are also similar interruptions in the continuity of
the chemical rays?  M. Arago proposes, as a simple solution of this
question, to expose one of M. Daguerre's prepared plates to the action
of a spectrum--an experiment which would prove whether the action of
these rays is continuous or interrupted by blank spaces.
   I shall only add, that M. Daguerre has entered into a contract with
Giroux, the celebrated toyman, for the practical application of his
discovery; and that it is said he has already in petto some new results
of importance, which he sill submit to the Academie at an early
opportunity.


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Posted for your enjoyment.     Gary W. Ewer     
--------------------------------------------------------------
08-21-97


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