Oxford.
May 18. 1846
Dear Sir
I am engaged in some researches on a subject in which I believe you take an interest – viz. the apparent enlargement of a bright object seen on a dark ground, often indistinctly referred (as other kindred phenomena have been) partly to diffraction, partly to irradiation – the latter being assumed to be a physiological effect on the retina.
Now it has occurred to me that this question (so far as the oculae effect is concerned) might be decided, if it were possible to form sufficiently exact pictures of the appearances in question by photography. everything of course turns on the degree of delicacy to which the process can be carried<,> and I thought that possibly in your hands such a trial might be made as would be at least decisive on the practicability of the [illegible] idea. I would suggest as two remarkable instances for such a trial,
(1) The well known appearance of the moon when a few days old – then the light part appears bounded by a larger circle than the visible dark portion – thus: – [diagram at left]
(2) The appearance of two halves of a card – the one left white the other blackened, & placed respectively on a black & a white ground when the white part appears broader, under strong illumination [diagram below]
Should you be able to try the experiment in these cases, or in any other of the same kind which may occur to you I should feel extremely glad to see the results. I trouble you on the subject because in less experienced hands I should not feel confidence in results when everything depends on the extreme accuracy & delicacy with which the process is conducted.
Perhaps indeed it may have occured [sic] to you before to take such photographs – if so I should be greatly obliged by being informed whether upon accurate measurement the apparent enlargement of the bright parts is preserved in the picture which would be decisive against the effect being purely ocular.
Believe me to remain Very sincerely yours
Baden Powell
H. Fox Talbot Esqre &c &c
[envelope:]
W H Fox Talbot Esqre
31 Sackville St
London
Laycock Abbey
Chippenham
Wilts