During this month of November in the year 1839, the following article
appeared in "The London and Ediburgh Philosophical Magazine and Journal
of Science" (London) Vol. 15 (November 1839) pages 381-385.
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LVIII. On the proper Focus for the Daguerreotype.
By John T. Towson.
To the Editors of the Philosophical Magazine and Journal.
GENTLEMEN,
The universal interest which the discoveries of Daguerreotype and the
photographic art have excited, will, I hope, excuse my soliciting a
space in the pages of your scientific Journal, for the purpose of
explaining an important fact which has hitherto escaped observation, It
appears from a note appended to page 37 of the English translation of
"Daguerre's description," that he does not use an achromatic lens; and
from p. 62, that the focus he uses is obtained by advancing or
withdrawing the frame of the obscured glass until he obtains the
outlines of the subject with the greatest neatness. This method would
be most correct if the chemical rays were identical with the luminous
rays. If such were the case the effect produced on his plate would be
precisely that which had appeared on his obscured glass. But it is a
well-known fact, that the chemical rays are more susceptible of
refraction than the luminous rays; it is therefore necessary, in order
to obtain the neatest effect, that the camera should be adjusted to the
focus of the chemical rays.
M. Frauenhofer, by his investigation of the phaenomena of the
prismatic spectrum, has shown that the index of refraction of each ray
is as follows:
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Red Orange Yellow Green Blue Indigo Violet
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Flint Glass 1*6277 1*6296 1*6350 1*6420 1*6482 1*6602 1*6710
Crown Glass 1*5258 1*5268 1*5259 1*5330 1*5360 1*5416 1*5465
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I also find that the mean index of refraction of the invisible
chemical ray is for flint glass 1.693, and for crown glass 1.536. The
index for plate glass is also about the mean between those of flint and
crown glass.
When we adjust a camera to the point at which the figure appears most
distinct we obtain the mean focus of the luminous power of the united
ray, because each coloured ray possesses a different degree of
illuminating power; therefore the appearance of the figure is mostly
influenced by the yellow ray, because it has the greatest degree of
illuminating power; and least of all by the violet, because it yields
the smallest degree of light.
The proportional light afforded by each ray is as follows: Red *009;
orange *048, yellow 1*000, green *440, blue *84, indigo *010; and
violet *001. On the other hand, each ray also tends to disturb the
disctinctness of the figure in proportion to its distance from the mean
focus of the pencil to which it belongs; thus two rays would but
occasion a similar degree of indistinctness to that which one ray of
equal power would if situate at twice the distance from the mean focus
of the pencil to which it belongs. The elements of our calculation, in
ascertaining the point at which rays of various degrees of
refrangibility produce the most distinct effect, must therefore consist
both of the illuminating power of each portion of the spectrum, and its
distance from the point required. By a calculation founded on these
data, we find that the figure appears most distinct at the focus of the
central yellow ray.
It must however be evident that this focus ought not to be used for
photographic purposes, since the yellow ray, although it yields the
greatest light, produces but a slight degree of chemical action, whilst
the chemical effect of the violet ray is greater than that of any other
luminous ray, but its illuminating power is the least ; the rays that
produce even a greater chemical action than all the luminous rays
combined possess no illuminating power. It has also been shown by Dr.
Herschel that the extreme red ray and the invisible ray beyond the red
portion of the spectrum produce a chemical effect of a contrary nature
to that of the other rays. These considerations are sufficient to
convince us not only that the chemical focus is differently distant
from a lens than its luminous focal length, but also to prove that the
distance between the two foci is sufficiently great to produce
considerable practical results. It therefore becomes an investigation
of considerable importance as connected with the photographic art, to
ascertain the situation of the mean chemical focus of a lens. In
conjunction with the data our previous observations have afforded, the
elements of such a calculation must consist of the chemical power of
those portions of the spectrum as have not already been noticed, which
is as follows. Taking that of the invisible chemical ray as unity,
that of the green will be *01; the blue *1, the indigo *3, and the
violet *45. With these data, and adopting the same formula we used in
calculating the mean luminous focus, we discover the mean chemical
influence to be without the limits of the luminous portion of the
spectrum, very near the extreme violet, ray, and that for all practical
purposes we may find this focus for any lens by multiplying its
distance from the point at which the figure appears most distinct by
the factor *969 if it be of flint glass, *976 of plate glass, or *984
of crown glass. Thus the chemical focus of a lens whose luminous focus
is 16 inches would be if composed of flint glass about 15*504, of plate
glass 15*616, or of crown glass 15*744 inches*. To demonstrate the
importance of obtaining the chemical focus of a lens, I have inclosed
two street views taken on the "improved photo graphic paper" sold by
Mr. Richards of this town. This preparation produces lights which
correspond with lights and shades with shades, consequently the effect
of a correct focus is more perceptible than would be the case on papers
that reverse the tints. The subject of both views is the same; the
paper of each is from the same piece; and the times and the lights
employed in taking them were as similar as possible, the difference of
effect being solely produced by No. 2 having been placed in the mean
luminous focus, but No. 1 in the mean chemical focus, discovered by
the above formula. On observing the very great difference between the
two views, the question immediately occurs, how then does Daguerre
produce such clear pictures if he uses the wrong focus? When however we
observe the imperfect view, No. 2, we are not to conclude that the
luminous focus always produces so little distinctness. During the
summer months I have, together with Mr. Hunt of this town, devoted
considerable attention to the practice of the photographic art, and
have succeeded in obtaining many very tolerably distinct views,
although we used the luminous focus of the lens. This we effected by
reducing the diameter of the lens or stop to a considerable extent, but
by so doing we delayed the process of taking the view. This is also
the mode by which Daguerre in a great measure neutralizes the effect of
the imperfect focus which it appears he is in the habit of using. By
thus reducing the size of the lens of a camera, that aberration of the
glass which arises from the use of a wrong focus is diminished in
direct proportion to the squares of the diameter of the lens or stop,
but the number of rays transmitted is thereby reduced in the same
proportion. The diameter of the stop of the camera employed in drawing
the inclosed views was equal to one-sixth of the focal length of the
lens, whereas it appears from the description of Daguerre's camera that
his lens is of less diameter than 1/18th of its focal length, and the
engraving which represents his camera shows a stop of one half that
diameter. This being the case the aberration arising from the
incorrect focus is reduced to 1/36th of the amount shown in view, No.
2. But by thus reducing the number of rays transmitted, much of the
advantage which would arise from the sensitive character of his
preparation is lost, and the value of less sensitive modes is reduced
in a still greater proportion.
Daguerre informs us, that under very favourable circumstances a
drawing may be obtained by exposing his plates in the camera during
from three to five minutes. If then, by correcting his focus, he were
enabled to use a lens of equal power to the one by which the inclosed
drawings were produced, he would be enabled to make the necessary
impression in from ten to twelve seconds.
During the discussion which took place at the Institute, after M.
Arago had publicly announced the process of Daguerreotype, it was
allowed to be a great desideratum that the art might be applied to
taking portraits from life. The use of large lenses, which the
correction of the focus enables us to adopt, would, I should imagine,
render such an application of the art practicable; and the value of
each use to which this important invention is applied, must also be
increased by a knowledge of the means of obtaining the best possible
effect in the least possible time.
I am, Gentlemen, your obedient servant,
JOHN T. TOWSON.
*From this result we might imagine that crown glass would he the best
material for photographic lenses. This however is not the case. The
least dispersive lenses intercept the greatest number of chemical rays,
and therefore those of crown glass, and consequently achromatic lenses,
cannot be advantageously employed for photographic purposes. This
observation might be exemplified by reference to several interesting
facts, but in so doing we should prematurely anticipate some of the
results of an investigation, which my friend, Mr. R. Hunt, is now
making relative to the power which various transparent media possess of
transmitting chemical rays.
(Viewing this text with a fixed font will enable the table to appear
correctly. --G.E.)
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Posted for your enjoyment. Gary W. Ewer
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11-30-98 |
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