[This is copied from an unattributed transcription of a photocopy, found in the Fox Talbot Museum, Lacock, reference files - neither the original letter nor the photocopy have been located. The attribution to Lady Mary Cole is incorrect - she was WHFT's aunt, not cousin.]
39/ Unindexed [Letter] 1839 Aug 9, from Henry Fox Talbot Esq. Sackville Street, London, to Lady Mary Cole, née Talbot [Location unknown, possibly Penrice], in ink
Location of original not known
transcription from poor photocopy
My dear Mary
As you are experimentalizing, I dare say you have already found out that in using nitrate of silver it is best to operate with gloves on as that substance stains most mercilessly & last not eternally indeed, but sometimes for a fortnight or two. In my communication to the Royal Society I did not mention this, because it was "beneath the dignity of Science" but I hope you will be cautious with it.
Your Friend, Sir T. Phillips [sic, per the transcription - should be Phillipps] called to see my specimens & intends himself becoming an artist. [struck out] If you intende making use of the camera you must first acquire the art of making the most sensitive paper by repeated trials, and then I advise not using at first a regular instrument bought at the opticians, but a large lens of shorter focus such as a very strong reading glass or magnifier and fixing it on a board & the paper unpon another exactly in its focus, & excluding all extraneous light-
The object to begin with is a window & its bars placing the instrument in the interior of a room. If you don't succeed tell me & I will see if I can help you further. The lens must be as large as possible compound with its focal length; [if it] makes a good burning glass it will answer.
Your aff[ection]ate Cousin