Slough
March 27/39
My dear Sir,
Thank you for the very pretty specimen of the heath, <1> which shall be taken better care of than its predecessor.-
Having had 2 days of occasional (very tantalizing) Sunshine, I have tried some new things. -
1st By placing an etching on a smoked glass (not having a resinous ground) behind an aplanatic lens the smoked side towards the focus - a copy of the etching reduced on any required scale is obtained thus imitating Gonord's celebrated process for taking off engravings in varied dimension. <2>- By exposure to a Solar beam radiating from the focus of a lens - the scale may be enlarged. - The reducing process on trial succeeded perfectly, only a little care is required to follow the Sun. By the use of highly sensitive paper this inconvenience would be much diminished - & by attaching the whole apparatus to an equatorial with clock-motion would be eliminated. -
If a resinous ground is used, the etching must be afterwards varnished or gummed, to destroy the loose light refracted obliquely by the thin edges of the cut up ground which is never quite opake.
2. Sensitive paper.- My attempts to produce sensitive paper at a single washing with one liquid promise to be not wholly unsuccessful I have already made a certain approach to yours but more of this hereafter -
3. Your Sensitive paper is so fully impregnated with Silver that the Hyposul. Soda in the degree of dilution most convenient for use precipitates the Silver as Sulphuret (See my papers) <3> This Evil may be avoided, by substituting the H.S. of Ammonia for that of Soda or by adding ammonia to the H.S. of Soda.
4. Your Paper - or paper impregnd with the chloride of silver is not, or but in a trifling degree more discoloured under a glass pressed on it, than when freely exposed - The contrast in this respect when a slip of your paper, and another merely nitrated are exposed together, half covered by glass, is very striking - It is not therefore to the mere development of heat that this singular phenomenon is owing.
The want of Sunshine however is a terrible drawback in these Expts - If the subject had been started while I was in Africa there would have been no end of the results -
I remain Dear sir Yours very truly
JFW Herschel
H. F. Talbot Esqr
44 Queen Ann Street
Cavendish Square
London
Notes:
1. This is almost certainly the negative inscribed "H.F.T. March 1839 Erica Mutabilis" that is in the Herschel Collection in the Museum for the History of Science, Oxford, Schaaf 2291. It is reproduced, along with a close variant negative now in the J. Paul Getty Museum, in Larry J. Schaaf, The Photographic Art of William Henry Fox Talbot (Princeton: Princeton University Press, 2000), p. 23 and plate 8.
2. François Gonord (b.1756), a French painter of miniatures, developed and patented, 25 July 1818, a method for enlarging and reducing engravings. He died some time after this patent date and before 1825.
3. John Frederick William Herschel, 'On the Hyposulphurous Acid and Its Compounds', Edinburgh Philosophical Journal, v.1, 1819, pp. 8-19 and pp. 396-400. And, 'Some Additional Facts Relating to the Habitudes of the Hyposulphurous Acid and its Union with Metallic Oxides', Edinburgh Philosophical Journal, v.2, 1820, pp. 154-156.