Jan 30/39.

My dear Sir
I am sure I shall not object to printing your paper as a portion of the P. Trans on the ground of its having been publd 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. but to the best of my recollection prior publicn has always been an objection to printing. Of course however it is competent to ye 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 seems to require some more susceptible body than Mur Silver & some coloured one which light whitens.

In my paper on the aberrations of lenses R.S. March 22/21 p 26 of the Paper are given the radii of curvature of 2 aplanatic combinations which are calcd 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 Esqr
44 Queen Ann Street
Cavendish Square