Sunday July 7/39
My
My dear Sir,
As the same Happy chance which led you to the Committee Last Monday may not do so tomorrow, I write this just to say that in an hour or 2 of good sun this morning I procured a distinct verification of the view I took in my letter of yesterday 1 – viz: that the black developed by the blue-green rays of the Spectrum is in fact a compound tint – of red, blue, & yellow – at least so I interpret the following beautiful phenomena
I formed a very concentrated & intense spectrum of small dimensions by throwing a spectrum on a large lens and recieving the focus on sensitive paper –
Luminous – spectrum [illustration]
Effect and of Chemical Spectrum [illustration]
The effects were as follows.
1. At the extreme Red R1 – no effect
2. Effect begins about the end of R2 the least refrangible rays and the intensity of darkness is represented by the ordinate all along the spectrum –
3dly – The tint produced on the sensitive paper is actually a coloured picture of the spectrum. But this is to be understood in a very moderate & humbly imitative manner. – It is as if the tints were coloured in darkness, not in light. As we speak of a red-black – a brown black, a green black, a blue black &c. –
1st) The tint from a to b is a dull brick-cold sort of red. – gradually shading into
2d) from b to c a green-black formed by the mixture of a yellow black & a blue black
3d) from c to d or a little farther as I have drawn the figure – Blue-black
4th) from d to f black
from f beyond Violet black
My present interpretation of these facts is that in Red light (I do not mean red refrangibility) kills destroys to a certain feeble extent the red, blue, and yellow reflected by the paper – but less of red.
Blue refrangible rays destroy very energetically all the rays but most less of blue –
Yellow Do – less energetically than blue but more than red, all, but least of yellow.
Violet must be regarded (as I always have regarded it as a mixture of red & yellow blue.
Excuse this from Yours very truly
JFW Herschel
I enclose a specimen. 2 it is not fixed. The curvature of the Spectrum is intentional
P.S– One of Fraunhofer’s 3 Crown Prisms was used in the above Expts –
H.F. Talbot Esqr
31 Sackville Street
Piccadilly
Please cite as “TALBOT3906,” in Ɛpsilon: accessed on