31 Sackville St <1>
Dear Sir
There is one circumstance presented by the glass films which we examined yesterday, <2> which still needs explanation nor do I see how it is deducible from the Newtonian rings <3>– It is this: if the daylight reflection from one if viewed thro’ another held at any distance (tho’ the nearer the better) is separated into coloured bars. The same is more distinctly seen if the light be transmitted thro’ the first film and reflected from the second. Neither of them separately shew any colour whatever in the daylight. I have thoughts of drawing up for Brewster’s journal a short account of this and several other phenomena presented by the Monochromatic Lamp; as I am not acquainted with Dr Brewster perhaps you will have the kindness to transmit it. <4>
Will you favor me with Capt Kater’s <5> direction –
You were of opinion that the variations in the thickness of the film were appreciable by the spherometer – and could be found equal to the length of one undulation of yellow light between each of the obscure bands – This yellow light is identical with the rays named D by Fraunhofer and he has given the length of their undulations with the utmost accuracy in his Kurzer Bericht 21 ¾ millionths of a Paris Inch.– p. 359. <6>
The spectrum afforded by the Red Fire of the Theatre <7> is one of the most beautiful of phenomena, from the number of maxima of light which it presents.
In the Red – First, a streak beyond the limits of the ordinary spectrum due to the nitre, followed by a considerable space of darkness: then very numerous crowded bright streaks with dark space between.
One orange streak
One yellow (due to the sulphur)
Three green, one of them very broad
One very light blue, & several fainter
I remain Dear Sir Yours most truly
H. F. Talbot
Notes:
1. 31 Sackville Street, London residence of the Feildings, often used as a London base by WHFT.
2. Herschel recorded in his diary on 3 January, “Talbot called. Also Kater. Talbot stayed from 2 till 5 much talk about Light. he mentioned a curious obs. with 2 homogenous yell. lights. also a property of v. thin glass plates (a broken bubble)”. See HRHRC, Herschel Collection, reference number W0008.
3. These bands of colour, seen under a variety of conditions, were observed before Sir Isaac Newton (1642–1727), but investigated thoroughly by him in Newton, ‘Discourse on Light and Colours’, Opticks, book ii, 1675. WHFT, like Newton, examined very thin, slightly curved glass under some pressure.
4. Sir David Brewster (1781–1868), Scottish scientist & journalist, editor of the Edinburgh Journal of Science, published WHFT’s article in December as, ‘On Monochromatic Light’, Quarterly Journal of Science, Literature and the Arts, v. 22, December 1826, p. 374.
5. Captain Henry Kater (1777–1835), instrument maker & man of science.
6. Joseph von Fraunhofer (1787–1826), optician, Munich, Kurzer Bericht von den Resultaten neuerer Versuche, über die Gesetze des Lichtes, und die Theorie derselben (Leipzig, 1823); see also Annalen der Physik, 1823.
7. “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 no.1, June 1826, pp. 77–82.