29 Albemarle St <1>
May 31st 1833
Dear Sir
I have always used lamplight in repeating Brewster’s expt. on the Nitrous Gas, <2> but I imagine there cannot be the least doubt that when Solar light is used, the lines of the Solar Spectrum are simply superadded to those of the Nitrous Spectrum. I have added a line of homogenous yellow light to the Nitrous Spectrum, & I find that it falls close to one of the dark bands of the latter, but not upon it. Accordingly a spirit lamp is very visible thro’ an atmosphere of nitrous gas, altho’ it would be wholly invisible thro’ a gas having its absorption power distributed in a slightly different ratio.
I consider Brewster’s discovery to possess great novelty & importance. For it shows us a gaseous body exerting a peculiar influence upon light, which it wholly loses when it assumes the liquid form –
I think it affords an argument almost amounting to demonstration that the lines in the Solar Spectrum are caused by the absorptive power of the Sun’s atmosphere: certainly not by the earth’s atmosphere, for then the stars would exhibit the same lines in their spectra, which are however different for each star. But there is not much likelihood of ones being able to determine what gas constitutes the sun’s atmosphere, since it has probably a very slight absorptive power & only produces so much effect owing to the immense thickness of it which the light traverses – Thus, it may be oxygen or some other gas which produces no perceptible effect in the small thicknesses which we can experiment with –
However I must say that I think the vapour of Iodine produces an effect still more extraordinary which I remarked 14 or 15 months ago – It has been since described by Mr Miller, <3> but incorrectly. This gas produces a multitude of narrow bands, not equidistant, as Mr M says, but gradually becoming more & more crowded towards the blue end, until they finally vanish somewhere in the blue rays – The extreme Red likewise does not contain them, which causes the purple colour of the gas when heated highly, the 2 extremities of the spectrum being then alone transmitted.– But if you confine your attention to any given part of the spectrum, the bands are beautifully equidistant, as their convergence is very gradual towards the blue end – Now it appears to me that a very important inference follows from this experiment, viz. that the vibrations of the vapour of Iodine have a degree of rapidity which is comparable to those of the luminous ether which constitutes light, and that the Iodine being caused to vibrate by the light passing thro’ it, its vibrations are successively in complete accordance & in complete discordance with those of the light, according as the tint of the light varies, in passing from one end of the spectrum to the other. As each particle of the Iodine is probably surrounded with an atmosphere of heat, which is perhaps a fluid of the same nature as light, if not the same, I conceive that there is nothing incredible in supposing that the vibrations of the heat may interfere with those of the light, which hypothesis I would prefer to that of supposing the particles of the gas themselves to act. I have read the papers of Mr Knox & Sir W. Herschel <4> which you were so good as to point out. I find they have anticipated me in the most essential particulars, but I wonder that neither in your treatise on Light, nor in Brewster’s, <5> is any mention of it. As certainly these are the most beautiful effects of interference which I have seen, those excepted which require a darkened room, Solar light, and light diverging from a single point. But I am at a loss to account for many of the phenomena, for instance in one case I get a set of coloured rings, and a second set which possesses no colour of their own, but have the property of destroying the colour of the first set.
In consequence of which the first become discontinuous & are formed of successive coloured fragments: and a second set are seen in white. But by throwing a shadow on them these invisible rings suddenly appear of an intense black. I wish you would read in Knox’s paper page 178 the expert 21, he there anticipates an expt of mine which I once described to you, where I looked thro’ a film of glass at another film lying on a table some feet distance, & saw coloured bands appear in the air between them. Of which I cannot yet understand the reason.
The expts you & I made formerly on Cyanogen Flame <6> &c. I fully agree are to be referred to the same order of facts as Brewster’s nitrous gas – But the best coloured flame is Copper: Barytes also if I mistake not absorbs the light in many hundred distinct places in the Spectrum, but certainly copper does –
Believe me to remain Yours very truly
H. F. Talbot
Notes:
1. WHFT lived at this London address, just down the street from the Royal Institution, for half of April and the whole of May, 1833.
2. Sir David Brewster (1781–1868), Scottish scientist & journalist, described a series of experiments in ‘Observations on the lines of the Solar Spectrum, and on those produced by the Earth’s Atmosphere, and by the Action of Nitrous Acid Gas’, Transactions of the Royal Society of Edinburgh, v. 12, 1834, pp. 519–530.
3. William Allen Miller (1817–1870), made many discoveries in optical experiments.
4. 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. 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. JFW Herschel, ‘Light’, and probably Sir David Brewster (1781–1868), Scottish scientist & journalist ‘On a new analysis of Solar Light’, Edinburgh Journal of Science V, 1831, pp. 197–206.
6. These were experiments made at the Royal Institution, London with Prof Michael Faraday (1791–1867), scientist, during the month of May 1827. [See Doc. No: 01560, and related letters].