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 –

3^dly – 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. –

1^st) The tint from a to b is a dull brick-col^d sort of red. – gradually shading into

2^d) from b to c a green-black formed by the mixture of a yellow black & a blue black

3^d) from c to d or a little farther as I have drawn the figure – Blue-black

4^th) 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 D^o – 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 Esq^r
31 Sackville Street
Piccadilly

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Notes:

1. See Doc. No: 03905.

2. This enclosure not located.

3. Joseph von Fraunhofer (1787–1826), optician, Munich.