The 
Daguerreian Society


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.
- - - - - - - - -

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
-----------------------------------------------------------------------

  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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