[The wrapper for this letter is in a private collection:]
H.F. Talbot Esqr
31 Sackville Street
I am very sure that an account of your interesting expt on the glass films will be acceptable to Dr Brewster <1> and I shall be happy to transmit it to him according to your wish. With regard to the phænomena seen by common daylight, I have tried to produce them by dispersed candle light (which cannot materially differ from daylight) unsuccessfully, but as the films I used were only such fragments as I picked up on the tables & floor after we broke up & were mostly dirty, I may not have given them a fair trial. However you shall judge.
Exp. 1. C a Candle. a a screen of Silver paper. <2> b a lens set in an eye tube dd ff a film covering the orifice gg the reflecting film. This gave as an image to the eye placed laterally, a round disc of uniform light, and whatever inclinations I gave to the films f and g, I could not see any bars crossing this image.
Exp. 2. C the Candle
a the silver paper
b the lens as before
ff a film set obliquely, so as to reflect the dispersed light and gg another to transmit it to the Eye e.-
As before, a circular disc of equable illumination was seen & no bars however f & g were inclined.
Exp. 3.- When gg was so inclined as to reflect the light reflected at ff to the eye - no bars.
In all these Expts. the focus of the eye was adjusted 1st to the circular disc - 2 to the film f 3dly to the film g - but the effect was the same.
I have therefore probably misconceived your experiment, or it must depend on hitting some particular angle or other condition not mentioned by you.
If I had my optical
apparatus &c papers by me (they are in the country) I could shew you an NS Exp. where, in the deflagration of Sulphur & Nitre, a red was seen which struck me forcibly as being less refrangible than the extreme solar red (a ray I was familiar with & which is equally definite with the yellow of the alcohol lamp) but as I could not measure its refrangibility, I suppressed the mention of this circumstance, for fear it might be only an illusion. If you are sure of the existence of such a red it is a very curious thing and in that case perhaps you will mention in your account of it, this soupcon of mine. Where is the orange streak in the red fire? <3>
Capt. Kater, No 12. York Gate
Regents Park <4>
The 50,000th of an inch is certainly a very small quantity, but I think I have perceived smaller with the spherometer. <5>
It would be well worth while [illustration] in a very regular plate of air included between two perfectly plane sextant glasses to note exactly the space ab taken up by a certain number of bands of homogeneous yellow light by the Alcohol lamp & then the space taken up by the same number of bands of homogeneous extreme red (by the compound glass described in my paper) The ratio of these will give very exactly the ratio of their undulations, which I have no doubt will agree precisely with measures otherwise obtained. A perfect coincidence must be searched for, or the depassement <6> of one band over the other estimated, or a micrometer screw & high magnifier applied, at once. †
The bands seen from the films prove two things
1st the very great degree of homogeneity of the lamp light
2d That this quality though possessed in a very high degree by it, is not absolute. The intermediate spaces are only obscure, not blank.
J. F.W. Herschel
†NB. If a coincidence be hit, if it be at a distance of 500 bands & the thickness of the film of air (or glass) be measured at the extremitus, it is no matter how irregular the film be, and would be a very beautiful measure. The best way would be by object glasses laid together. Newton's process, <7> pushed to the extreme of refinementPray excuse this straggling note. You see at least it is an unpremeditated one.
1. WHFT expressed a wish to publish the results of these experiments on monochromatic light in the Edinburgh Journal of Science, edited by Sir David Brewster (1781-1868), Scottish scientist & journalist. [See Doc. No: 01349]. In fact, it was not published until December and then as 'On Monochromatic Light', Quarterly Journal of Science, Literature and the Arts, v.22, December 1826, p. 374.
2. Silver paper (or silvered paper) - usually an article of commerce, prepared in various proprietary ways. Silver may or may not have been present, with the colour sometimes coming from metals such as tin.
3. "Fires", flammable mixtures that burn with coloured flames. They contain fuels, oxidizers and salts of various metals to produce the desired colour. "Red fire" refers to a mixture containing a strontium salt, which would produce the red color by the molecular band emission of gaseous strontium monohydroxide (SrOH), and, if potassium chlorate were present in the mixture, of gaseous strontium monochloride (SrCl). WHFT mentions his examination of the spectra emitted by the red fires used in theatres in 'Some Experiments on Coloured Flames', Edinburgh Journal of Science, v.5 n.1, June 1826, pp. 77-82.
5. An instrument used to measure the radius of the curvature of a lens.
7. Sir Isaac Newton (1642-1727), natural philosopher, made his famous experiments on the spectrum by sending a beam of light through two prisms, one set after the other.