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annotated by R. D. Wood
Robert Hunt, Researches on Light (2nd edition 1854), Appendix 2, pp. 371-5 
MY DEAR SIR,
|[The second half of Reade's letter draws attention to Thomas Young's ideas on light and Young's experiment half a century earlier on the spectral rays including his use of silver nitrate paper to record the violet and invisible UV end of the spectrum]|
Sir David Brewster, in his History of Photography, passes immediately from the experiments of Wedgwood to those of Talbot; but the Transactions of the Royal Society, to which my friend Mr. Gravatt has directed my attention, will enable us to insert, if not a chapter, at least a very pregnant parenthesis. The Bakerian Lecture, in 1803 , by Dr. Young, who never touched a subject without leaving his mark upon it, contains a highly interesting and original experiment on the photographic representation of the invisible chemical rays beyond the blue end of the spectrum. This experiment does not happen to be recorded in the first edition of your Researches on Light; but no one will refer to it with greater pleasure than yourself, not only because it is the first photographic analysis of the spectrum, but also because it has the higher merit, even as it stands alone, of being the one sufficient fact which establishes the consummation so devoutly looked for, at the conclusion of your work, from the persevering accumulation of facts only; for it is in itself a simple and demonstrative proof, to use the words of Dr. Young, of the general law of interference, and, in your own words, reconciles the chemical action of the photographic force, energia, with the undulatory theory of light. Dr. Young's experiment forms the conclusion of his lecture, and is given in the following terms: The existence of solar rays accompanying light more refrangible than the violet rays, and cognisable by their chemical effects, was first ascertained by Mr. Ritter; but Dr. Wollaston made the same experiments a very short time afterwards, without having been informed what had been done on the Continent. These rays appear to extend beyond the violet rays of the prismatic spectrum, through a space nearly equal to that which is occupied by the violet. In order to complete the comparison of their properties with those of visible light, I was desirous of examining the effect of their reflexion from a thin plate of air capable of producing the well-known rings of colours. For this purpose I formed an image of the rings, by means of the solar microscope, with the apparatus which I have described in the Journals of the Royal Institution, and I threw this image on paper dipped in a solution of nitrate of silver, placed at a distance of about nine inches from the microscope. In the course of an hour, portions of three dark rings were very distinctly visible, much smaller than the brightest rings of the coloured image, and coinciding very nearly in their dimensions with the rings of violet light that appeared upon the interposition of violet glass. I thought the dark rings were a little smaller than the violet rings, but the difference was not sufficiently great to be accurately ascertained; it might be as much as 1/30th or 1/40th of the diameters, but not greater. It is the less surprising that the difference should be so small, as the dimensions of the coloured rings do not by any means vary at the violet end of the spectrum so rapidly as at the red end. For performing this experiment with very great accuracy a heliostate would be necessary, since the motion of the sun causes a slight change in the place of the image, and leather impregnated with the muriate of silver would indicate the effect with greater delicacy. The experiment, however, in its present state is sufficient to complete the analogy of the invisible with the visible rays, and to show that they are equally liable to the general law (of interference), which is tbe principal subject of this paper. 
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Footnotes. By R. Derek Wood © - Midley History of early Photography
There are authoritative short biographies in the Oxford Dictionary of National Biography, Oxford University Press 2004, of persons mentioned on this page: Brayley, Edward William (1801/2-1870), writer and lecturer on science by J. N. Hays; Brewster, Sir David (17811868), natural philosopher and academic administrator, by A. D. Morrison-Low; Hunt, Robert (1807-1887) chemist and photographer, by Alan Pearson; Peacock, George (17911858), mathematician and university reformer by Harvey W. Becher; Reade, Joseph Bancroft (18011870), microscopist and experimenter in photography by R. D. Wood; Wedgwood, Thomas (1771-1805), chemist by Trevor H. Levere; Wollaston, William Hyde (17661828), chemist, physicist, and physiologist by Trevor I. Williams; Young, Thomas (17731829), physician and natural philosopher, by Geoffrey Cantor.
1. Robert Hunt's Researches on Light, 2nd edition London 1854, appendix 2 on pp. 371-5.
2. This mistake of March (it had earlier appeared in North British Review in 1847) misled historians of the beginnings of photography. The situation created by this mistaken date was not fully resolved until 1972 - see J. B. Reade's Early Photographic Experiments: further evidence, by R. Derek Wood, British Journal of Photography, 28 July 1972, Volume 119, No. 5845, pp. 644646, 643, on-line here on another page of this Midley History of early Photography.
Rev. J. B. Reade. Communicated by J. G. Children, Esq., Sec. R. S. Observations and Experiments on the Solar Rays that occasion Heat; with the application of a remarkable property of these rays to the construction of the Solar and Oxyhydrogen Gas Microscopes, read at Royal Society meeting 22 December 1836, Proceedings of the Royal Society, Vol. 3 (18301837), No. 28 (8 December 1836 16 March 1837), p. 457. Also published in Philosophical Magazine (3rd Series), March 1837, Vol. 10 (No. 60), pp. 21920.
4. Thomas Young's third Bakerian lecture, The Bakerian lecture. Experiments and Calculations relating to physical Optics, was read at Royal Society on 24 November 1803 and published in Philosophical Transactions of the Royal Society of London for the year MDCCCIV, (1804), pp. 1-16
5. Reade correctly quotes this long passage from Young's third Bakerian lecture, although the italics of leather impregnated with the muriate of silver were emphasis by Reade not Young.
Also Reade has placed at a distance although in the Phil. Trans. original it is placed at the distance. Young's final sentence - a remark about the present impractical use of thermometers to reveal the infrared heat end of the spectrum - was omitted.
6. George Peacock, Life of Thomas Young, London: John Murray 1855.
7. Thomas Wedgwood's article (with Faraday's notes) appeared in 1802:
8. Wedgwood's account of producing images by the effect of light on silver nitrate was published one and a half years before Young read his third Bakerian lecture at the Royal Society. Young was clearly in a position to be well acquainted with Wedgwood's experiments, for in July 1801 Young had been appointed Professor of Natural Philosophy at the Royal Institution and edited that first volume of the Institution's Journal in which Wedgwood's work was published. For brief information of Young at Royal Institution see Memoir of the life of Thomas Young, anon [Howard Gurney]. pp. 20, 53 (London 1831), and Life of Thomas Young, by George Peacock, pp. 134-5, 188 (London 1855)
9. Reade's enthusiasm for Young might seem at first reading to be suggesting that Young's spectrum experiments have an equal rating to the actual copying of images by Wedgwood. However, it is worth noting that Reade is pointing out that Young's name and place in the history of the use of silver salts should be duly marked. And, as we have seen in the above three footnotes No.s 6-8, Wedgwood's use of silver salts was published before Young's third Bakerian lecture. Reade's letter to Hunt did indeed have the effect of placing Young into the history. The historiographic line can be traced from Hunt's book to other influential publications such as the long essay on Researches on Light in North British Review, August - November 1858 (p. 202); Photography and the Stereoscope in Stories of Inventors and Discoveries London 1860, by John Timbs (pp. 313-4).
10. Reade is not referring here to the same lecture in which Young reports his use of silver salts, but to Young's previous second Bakerian lecture, 'On the theory of light and colours', read at Royal Society on 12 November 1801, published in Phil. Trans., 1802, pp. 12-48.
11. Thomas Young's second Bakerian lecture of 1801, was his most influential work 'On the [wave] theory of light and colours', see above note 10.
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Robert Hunt, Researches on Light, 2nd edition 1854, pp. 84-86
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