Slough
Jan 30/39.

My dear Sir

I am sure I shall not object to printing your paper ^<1> as a portion of the P. Trans ^<2> on the ground of its having been publ^d in the Athenæum under all the circumstances of the case because I think rapidity of Commⁿ of inventions to the Public is of more importance than points of form and the interest of this particular invention supposing it to accomplish all the conditions required to make it practically available is such that it ought to be preserved in some record more durable than the pages of a weekly journal. However I believe such has not been hitherto at all the practice of the R.S. ^<3> but to the best of my recollection prior publicⁿ has always been an objection to printing. Of course however it is competent to y^e Council in a special case to waive their objection – I shall not be the one to object.

I have myself been thinking since I got your note about this enigma.– Here is one fact that forms the groundwork of my view of the process supposing chloride of Silver to be the material used.– That the Image in a Camera obscura may impress itself on a paper imbued with that substance in its pores & rendered more susceptible than usual by moisture & gentle warmth (& perhaps also by the presence of hydrogen gas) I have satisfied myself by experiment. Then we have a picture in which darkness represents light & vice versâ. Now this picture will darken every time it is exposed to light till it is become entirely black & thus our picture is destroyed –

But if the whole of the unreduced Chloride be washed out of the paper which may be done instantaneously and to the last atom by a process I have discovered – leaving the reduced silver still on the paper only in the form of a fine Brown or Sepia tint – our picture becomes permanent.–

The representation of light by light and dark by dark remains difficult but I think not insuperable, even with this material.

But Arago’s Process ^<4> seems to require some more susceptible body than Mur Silver ^<5> & some coloured one which light whitens.

In my paper on the aberrations of lenses R.S. March 22/21 p 26 ^<6> of the Paper are given the radii of curvature of 2 aplanatic combinations which are calc^{d <7>} for Refractive index 1·500.– These I presume if recomputed for the Violet index of the particular glass used instead of the range value 1·500, would I think answer. There is only a cubic to resolve. As to flatness of field I can say nothing about it. The image must be a conoidal surface & its curvature will depend on the focal lengths of the lenses, not on their radii

I hope to see you on Friday. There is a 12 o’clock train, but no one o’clock.– Also there are 8 & 10 o’clock.

Yours very truly
JFW Herschel

H.F. Talbot Esq^r
44 Queen Ann Street
Cavendish Square
London

[draft letter in Herschel’s hand]

draft much altered ^<8>

Slough
Jan 30.

My dear Sir –

I am sure I shall not object to printing your paper as a part of the Phil Trans on the ground of its having been publ^d in the Athenaeum under all the circumstances of the case because I think rapidity of Commⁿ of inventions to the Public is of more importance than points of form and the interest of this particular invention supposing it to accomplish all the conditions required to make it practically available, is such that it ought to be preserved in some more durable form collection than the ephemeral leaves of a Lit^y & Sci Weekly Journal. However I believe that the practice of the R.S. hitherto so far as I recollect it has been very strictly of a contrary character & prior publication in any other form work has to the best of my knowledge always been held a fatal objection. Must you of necessity give to these Journals a verbatim Copy – Is there no matter of detail advantageous in the manipulations, no prospective improvements or hints for future wider applications which may advantageously be for a time withheld? However as before said – I shall not be the one to object.–

It has occurred to me that supposing chloride of Silver to be the substance employed – the chief difficulty in the case is the preserving the imprint intact by further exposure to light.– Now this may perhaps be accomplished thus.– Let paper be prepared with chloride introduced into its pores by moistening 1^st with Nitr. Silver – drying, then moistening again with Mur. Soda, washing & drying in the dark & cold-pressing to give a smooth surface.– Now let the picture be formed. This supposes the reduction to the metallic state of all that part which is black.– Before exposure to light then, let the whole be sponged with or immersed in, a solution of hyposulphite of Soda. This will wash away all the chloride w^ch has escaped the action of the light & leave the reduced silver black as before. Thus there is nothing further left for the light to act on & the picture may afterw^ds be freely exposed like any other drawing.

In my paper on the Aberrations of lenses (R.S. March 22. 1821) p. 26 of the paper are given the radii of aplanatic combinations of 2 lenses of Crown glass 1+μ = 1·500, which I presume will serve your purpose.– As to flatness of field that must be as shall please God, for I do not very well see how that condition is to be combined with the destruction of spherical aberrations which is here very essential. But the Calc^ns ought to be repeated for the real refractive index of the actual glass for violet ray not the rough assumption 1·500.– Au reste ^<9> it is not a long or difficult one – there is only a cubic to resolve with 3 real roots.

Yours very truly
JFW Herschel

P.S. On trial the Hyposulphite appears to answer perfectly the end for which I have above proposed it. It discharges completely all the muriate of silver from the paper except that which has been reduced by the light. Moisture renders the chloride infinitely more susceptible of light than when dry NB P.S. What a beautiful mode of making the sun represent its own spots n times a day or of mapping the moon.

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Notes:

1. ‘Some Account of the Art of Photogenic Drawing’, read before the Royal Society on 31 January 1839.

2. Philosophical Transactions of the Royal Society of London.

3. Royal Society of London.

4. That is, the daguerreotype. All official public announcements concerning the daguerreotype were made by Dominique François Jean Arago (1786–1853), French physicist, astronomer & man of science, it is no wonder that at times the process was considered as his.

5. Muriate Silver.

6. John F. W. Herschel, ‘On the Aberrations of Compound Lenses and Object-glasses’, Philosophical Transactions of the Royal Society of London, 22 March 1821, pp. 222–267.

7. Calculated.

8. Written across the first page by Herschel.

9. Besides