Talbot Photo 3(82)

Letter written by Sir John Herschel to Talbot on the 3rd March 1840, with seal intact. Transcript below.

Slough

March 3. 1840

My dear Sir

Many thanks for your beautiful specimens. I took the liberty (not I hope one you will disapprove) to give them to an Italian friend on his way to Florence via Paris with a request (on my part) that he would shew them to Biot & Arago, who are probably hardly aware of what may be done by paper.

I have got a mode of preparing paper so as to be susceptible of impression (white on a black ground) by the mere calorific rays of the spectrum – in short a thermographic paper. It gives a true & fine picture of the distribution of the thermic rays in, and far beyond the spectrum, and from the nature of the process the action of the Chemical rays has nothing to do with the effect. You will easily see what a field this opens for the analysis of Melloni’s & Forbes’s results

Here is a picture of the spectrum exhibiting the comparative extent & character of the 3 spectra (α Luminous β Chemical, Thermic, θ Thermic Chemical) [illustration] The portion β 2 of the red curve expresses an insulated patch of heat (after transmission by a flint Prism & Achromatic lens) at an enormous distance beyond the extreme perceptible red rays R as you may collect from the following dimensions in which the suns diameter is supposed = 0 YV = + 40.6 YR = −13.2, Ym = −22.7, Yn = −35.0

The nature of the curve δ varies with the paper – but in the generality of cases it has a positive & negative ventre cutting the axis at Y the center of the yellow ray

The greatest extent (Yk) to which I have traced it is [illegible deletion] + 6.9 so that our total spectrum (nk) is now traced to an extent of [illegible deletion] 121.9 parts of which only [illegible deletion] 53.8 is the old [illegible deletion] Newtonian Spectrum,

Yours very truly

J.F.W. Herschel

P.S. I enclose a spectrum exhibiting the singularly definite action of Hydriodate of potash on a spectrum already impressed –under the continued influence of the same rays which originally impressed it. The action commences just at the end of the bright blue ray. – a fortunate joy of the apparatus by throwing the spectra a little aside enables to trace the uncorroded part of the old spectm towards the more refrangible side. – This explains many odd & capricious effects of the Hydriodate

NB. The thermic spectrum extends nearly as far beyond the extreme red rays as the coloured spectrum visible in a cobalt blue glass does in the other direction from the same points! [illustration] I have tried thermic rays as far as θ where Yθ = −47 parts of the scale above referred to and Yθ′ = + 25. or thereabouts!

There is, or I am much mistaken a second insulated patch β″ of thermic spectrum at −45 [illustration]

prepaid

H.F. Talbot Esq

Lacock Abbey

near Chippenham

Wilts

Slough

March 3. 1840

Many thanks for your beautiful specimens. I took the liberty (not I hope one you will disapprove) to give them to an Italian friend on his way to Florence via Paris with a request (on my part) that he would shew them to Biot & Arago, who are probably hardly aware of what may be done by paper.

I have got a mode of preparing paper so as to be susceptible of impression (white on a black ground) by the mere calorific rays of the spectrum – in short a thermographic paper. It gives a true & fine picture of the distribution of the thermic rays in, and far beyond the spectrum, and from the nature of the process the action of the Chemical rays has nothing to do with the effect. You will easily see what a field this opens for the analysis of Melloni's & Forbes's results. Here is a picture of the spectrum exhibiting the comparative extent & character of the 3 spectra (α Luminous β Chemical, θ thermic) [illustration] The portion β 2 of the red curve expresses an insulated patch of heat (after transmission by a flint Prism & achromatic lens) at an enormous distance beyond the extreme perceptible red rays R as you may collect from the following dimensions in which the suns diameter is supposed = 0 yV = +40.6 yR = −22.7, yn = −35.0

The nature of the curve δ varies with the paper – but in the generality of cases it has a positive & negative ventre cutting the axis at Y the center of the yellow ray [missing text] the greatest extent (Yk) to which I have traced it is + 86 [illegible deletion] [missing text] that our total spectrum (nk) is now shewn to an extent of 12 [illegible deletion and missing text] which only [illegible deletion] 53.8 is the old [illegible deletion] Newtonian spectrum,

Yours [missing text] truly

JFW Herschel