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, <1> 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 <2> 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 <3> 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 <4> exhibiting the singularly definite action of Hydriodate of potash on a spectrum already impressed –under the continued influence ofthesame 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
[copypress letter:] <5>
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] [missing text] which only [illegible deletion] 53·8 is the old [illegible deletion] Newtonian spectrum,
Yours [missing text] truly
JFW Herschel
Notes:
1. Jean-Baptiste Biot (1774–1862), French scientist, makes no mention that he received these specimens, or that they might have been seen by Dominique François Jean Arago (1786–1853), French physicist, astronomer & man of science.
2. Macedonio Melloni and Prof James David Forbes (1809–1868), Scottish scientist both published numerous articles on the polarisation and refraction of what was to become known as the infrared part of the spectrum. For a complete list see Royal Society Catalogue of Scientific Papers.
3. ‘Ventre’ – a name given to the points where the vibrations present the greatest amplitude, by opposition to the nodes [See Hatchette 1889].
4. The spectrum is attached to the letter.
5. This is a copy made only of the first page of the letter, and apparently before he made the changes noted in the main letter text, and also before he added the results which make up the next pages. As it is incomplete, it differs thoughout and individual differences have not been noted here.