Slough.
May 30/1833.
My dear Sir/
On the subject of the intersectional rings read a very interesting paper by Mr Knox of Belfast in the Phil Trans Vol 105 (1815) page 161. <1> – A very odd fact relative to these fringes is that they are visible as fine lines. Where the primaries to wh they owe their origin are not even to be suspected. The fringes described by Amici <2> on the wings of the Papilia Idas about which I blundered in Art. 695 of my Essay on Light, <3> are in this respect in perfect analogy and many other Phænomena.
I have been in the habit of regarding the phænomena of the intersectionaries (which are remarkably beautiful) as a corollary so easily deduced from the undulatory theory that I have never been at the pains to follow out the steps of the deduction – and I believe it was owing to this impression on my mind that I said little or nothing about them in my “Essay”. Perhaps I am wrong in this – Perhaps too your intersectionaries are not identical with Knox’s but of that you will judge on reading his paper.
The effects of shading the rings in different ways have been pointed out by my Father in his Expts. on the concentric rings Phil. Tr. – 1807 (§.15.) <4> – Those papers of his contain many very elegant experiments though the reasoning thereon I confess appears to me unsound. But whether the particular effect you allude to is among those enumerated by him you must also decide “on a view of the body” of evidence. I write in a hurry as I am now unfortunately compelled to do everything – my days & hours being numbered and more pressing on me than I can accomplish.
Do you not remember the expts we made with Faraday on the flames of Cyanogen & the red fire? <5> – the numerous lines in the former flame seem to me very like those (in a general way & less definite) in nitrous gas. I confess I am at a loss to see what novelty of Principle there is in Brewster’s expt of the nitrous acid gas. <6> A most elegant Expert. it is – or rather a most singular absorptive colour is that of the gas – totally unexpected certainly to me but where is the novelty? Where is the new class of Phænomena? If B. or any one else could re-produce in Solar light those rays it has not but which I believe to have had originally either before it entered our air, or before it left the Sun’s Surface – or before it left the vibrating atom which give it its existence as light – there would indeed be a new fact of a most important kind At present it seems to reenter (rentrer) into the [illegible deletion] complicated but by no means inexplicable subject of absorption. I think I have a key to it in the and if I can possibly find time to throw off a few pages on the subject before starting I cannot help thinking I could relieve the undulatory theory of any objection which at present seems to press against it from these Phænomena.– Whatever Brewster & Potter <7> may say, that theory is not to be upset on any grounds having no better points d’appui <8> than our ignorance generally, or the want of perception of the essential points in question, of particular reasoners.–
You will find in a note on page 212 of my Astronomy <9> what I mean by what I have said above of solar light.– It must be remembered there. In the expt of the nitrous gas as done we only subtracted from solar light what it has (am I right? Does Brewster restore anything – if so pray inform me – I have seen no acct of the expt – only hastily repeated it.) The Nitr. Gas spectrum therefore has its lines compounded with those in-herent in the Sun’s light. I wish you would read with reference to this what is said in Art 505 of my “Light” correcting an error line 5 p. 434 where the printers have perversely put organ for origin.
Yours very try
JFW Herschel
H. F. Talbot Esqr MP.
29 Albemarle Street <10>
Piccadilly
London
Notes:
1. John Knox ‘On some phenomena of colours, exhibited by thin plates’, Philosophical Transactions of the Royal Society of London, v.105, 1815, pp. 161–181.
2. Prof Giovanni Battista Amici (1786–1868), Italian optician & man of science.
3. John Frederick William Herschel wrote his article ‘Light’, in 1827. By 1828 some form of printed copy was available, including the Plates 1–14. The article appeared in the 1830 volume of Encyclopaedia Metropolitana, 2nd Division: Mixed Sciences, v. 2, 1830, pp. 341–586.
4. Sir William Herschel (1738–1822), ‘Experiments for investigating the Cause of the Coloured Concentric Rings, discovered by Sir I. Newton, between two Object-glasses, laid one upon another’, Philosophical Transactions of the Royal Society of London, 1807, pp. 180–233.
5. In May of 1827, WHFT met with Prof Michael Faraday (1791–1867), scientist, and Herschel to examine the spectra emitted by Cyanogen and “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.
6. Sir David Brewster (1781–1868), Scottish scientist & journalist.
7. Probably Richard Potter (1799–1886), scientific writer.
8. Supporting.
9. John Frederick William Herschel, ‘Physical Astronomy’, Encyclopaedia Metropolitana, 2nd Division, Mixed Sciences, v.1, 1829, pp. 647–729.
10. WHFT lived at this London address, just down the street from the Royal Institution, for half of April and the whole of May, 1833.