¹ Young considered matter to be composed of particles of finite extension, impenetrable to one another, but possibly penetrable to certain immaterial influences such as gravitation. Although substances appear to be continuous, he believed there to be pores between their component particles which permit the free passage of subtler substances.

² Young, Thomas, “Outlines of Experiments and Inquiries Respecting Sound and Light”, Phil. Trans., xc (1800), 106–150.CrossRefGoogle Scholar This paper is in the form of a letter dated 8 July 1799. Its tenth section is entitled “On the Analogy between Light and Sound”.

³ Young, Thomas, “On the Theory of Light and Colours”, Phil. Trans., xcii (1802), 12–48.CrossRefGoogle Scholar Read as a Bakerian Lecture on 12 November 1801.

⁴ Young, Thomas, A Syllabus of a Course of Lectures on Natural and Experimental Philosophy, London, 1802.Google Scholar

⁵ MS. Add. 13, University College London. References to the microfilm edition of these notebooks are given in terms of the notebook number followed by a solidus and then the frame number. Where only a single page has been photographed, only the above information is given. Where a double page occurs on the same frame, the left (v) or right (r) side is stated. Thus Notebook, 16/12r refers to the right side of the 12th frame of notebook 16.

⁶ Young, Thomas, “An Account of some Cases of the Production of Colours not hitherto Described”, Phil. Trans., xcii (1802), 387–397.CrossRefGoogle ScholarYoung, Thomas, “Experiments and Calculations relative to Physical Optics”, Phil. Trans., xciv (1804), 1–16.CrossRefGoogle Scholar

⁷ Young, Thomas, A Course of Lectures on Natural Philosophy and the Mechanical Arts, London, 1807.Google Scholar This work will hereafter be termed Lectures.

⁸ These appear in several places—“Mr. Isaac Newton's Answer to some Considerations [of Hooke] upon his Doctrine of Light and Colours”, Phil. Trans., No. 88, 18 11 1672, 5084–5103Google Scholar; “Newton's second paper on Colour and Light” (1675/6), Birch, , History of the Royal Society of London (London, 1757), iii, 247–305Google Scholar; “Newton's Letter to Boyle”, dated 28 Feb. 1678/9, Birch, , The Life of the Honourable Robert Boyle (London, 1744), i, 70–73.Google Scholar There are many similarities between Young's application of the ether distribution hypothesis and the contents of this letter. The work in which it was published is cited in the second volume of the Lectures. Newton's later speculations on the ether appear in the “General Scholium” to the Philosophiae Naturalis Principia Mathematica (London, 1713)Google Scholar and in Queries 17–24 of the Opticks (London, 1730).Google Scholar

⁹ Op. cit. (2), 126.Google Scholar In Euler, 's Letters to a German Princess (London, 1795Google Scholar, ii, letter XXV), he stated that “all the phenomena of electricity are a natural consequence of want of equilibrium in the ether … consequently, electricity is nothing else but a derangement of the equilibrium of the ether”.

¹⁰ Op. cit. (2), 127.Google Scholar

¹¹ Ibid., 128.

¹² Op. cit. (3).

¹³ Young's full statement of these hypotheses is as follows—I. “A luminiferous ether pervades the universe, rare and elastic in a high degree”; II. “Undulations are excited in this ether whenever a body becomes luminous” III. “The Sensation of different Colours depends on the different frequency of Vibrations excited by Light in the Retina”, ibid. 14–21.

¹⁴ Of the ten passages quoted, two were abstracted from Newton's answer to Hooke, (1672)Google Scholar, four from his second paper on colour and light (1675/6). Four quotations were taken from the Opticks, three being drawn from the Queries which first appeared in the Latin edition of 1706. By quoting these passages out of context Young misrepresented Newton and it was this consideration which excited most comment when the paper was first published.

¹⁵ Op. cit. (3), 21.Google Scholar

¹⁶ Newton dealt with this problem in Opticks, Bk. iii, Part 1, Observation 1.

¹⁷ Op. cit. (3), 27.Google Scholar

¹⁸ Newton, , Opticks (fourth edition, London, 1730), Query 28.Google Scholar

¹⁹ Op. cit. (3), 29.Google Scholar Here he made a mistake which he corrected in the Lectures of 1807 in which he identified the concept of elasticity with incompressibility and inextensibility—i.e. Young's modulus. Hence the ether is more elastic than water and water more elastic than air. This mistake arose because he referred to water waves rather than to sound waves propagated in water.

²⁰ Ibid., 32–33.

²¹ Ibid., 31. The fourth hypothesis does not state whether the density of the ethereal atmosphere changes abruptly or continuously.

²² Ibid., 42.

²³ Ibid., 43.

²⁴ Ibid., 43. For Hooke's views on optics, see Micrographia (London, 1665, reprinted New York, 1961), 55 et seq.

²⁵ “… for if it [inflection] were conceived to originate from the same cause that produces refraction, it might be expected to differ when produced by different inflecting substances: this however is not the case …” Notebook, 16/16r.

²⁶ Ibid., 16/16r.

²⁷ Ibid., 16/20r.

²⁸ Birch, T., History of the Royal Society of London (London, 1757), iii, 268.Google Scholar

²⁹ Notebook, 19/6r.

³⁰ The Gentleman/s Magazine, 04 1792, 303.Google Scholar

³¹ Notebook, 19/23v; Lectures, i, 653.Google Scholar

³² Phil. Trans., xc (1800), 255–326 and 437–538.Google Scholar See also Lovell, D. J., “Herschel's Dilemma in the Interpretation of Thermal Radiation”, Isis, lix (1968), 55.Google Scholar Young in his first optical memoir quoted Rumford's “vibrational” theory of heat in this context. But Young's writings on heat bear little resemblance to Rumford's theory. Instead Herschel's experiments provide much more evidence to support the analogy between light and heat. These experiments were repeated at the Royal Institution by SirEnglefield, Henry at Young's request, Journals of the Royal Institution, i (1802), 202–8.Google Scholar One of Young's major objections to caloric arose from his inability to conceive of continuously moving streams of a medium crossing one another. Likewise this was one of the objections cited against the corpuscular theory of light by both Young and Euler, op. cit. (9), iGoogle Scholar, letter XVII. A similar consideration was involved in his dispute with John Gough in Nicholson's Journal (several notes, late 1802 and early 1803).

³³ “… if the undulating and vibrating system of light should ever be generally received, there will be little ground for believing in the existence of a caloric fluid unless as identical with the ethereal medium which may be supposed to be the common vehicle of light.” Notebook, 19/23v. Cf. Query 18 of Newton's Opticks.

³⁴ Birch, T., The Life of the Honourable Robert Boyle (London, 1744), i, 71.Google Scholar

³⁵ “Professor Robison has ascertained by experiment the force necessary to produce the greatest possible degree of contact, and finds it equivalent to a pressure of about a thousand pounds for every square inch of glass.” Lectures, i, 611Google Scholar; Notebook, 19/9. Young was considerably influenced by Robison's writings—the index to the Lectures contains 45 references to him—but it does not appear that they met while Young was studying in Edinburgh.

³⁶ Notebook, 19/13r.

³⁷ Ibid., 19/14v.

³⁸ Ibid., 19/14v. In this calculation he made the gratuitous assumption that in any given area of cross-section of the metal, only one-tenth is “actually occupied by particles perfectly united”.

³⁹ Birch, , op. cit. (34), 73.Google Scholar

⁴⁰ Query 21 in the 1730 edition.

⁴¹ Notebook, 19/12r.

⁴² Young's magnetic fluid bears some similarities to his electric fluid, but it is endowed with the remarkable property of being attracted only to metallic iron.

⁴³ Franklin, , Experiments and Observations on Electricity (London, 1769), 54.Google Scholar

⁴⁴ Notebook, 20/2r.

⁴⁵ Ibid., 20/2r.

⁴⁶ In the 1820's Young contributed several papers on atmospheric refraction to the Quarterly Journal of Science.

⁴⁷ Notebook, 19/12r. Cf. Newton, “Thus perhaps may all things be originated from aether.” Birch, , op. cit. (28), 250.Google Scholar

⁴⁸ Notebook, 19/12r.

⁴⁹ “… it is not improbable that they [electrical phenomena] may depend on some modification of the actions of the medium [the ether] which appears to be concerned in the effects of light, heat, cohesion and repulsion…”, op. cit. (4), Art. 493.Google Scholar “There seems to have been some reason to suspect, from the phenomena of cohesions and repulsions and from the apparent displacement of the elastic medium concerned in cohesion by means of friction or of heat, that such a medium, if it exists, is… connected with the electric fluid.” Notebook, 20/9. It should be noted that in this passage as in some others, Young treats the atmosphere of ethereal fluid as a hypothesis.

⁵⁰ “Perhaps, however time will show that electricity is very materially concerned in the mechanical actions and affectations which probably are the foundation … of all chemical properties …”, Notebook, 20/9.Google Scholar “That heat and light should both be capable of producing chemical effects and that these effects should frequently be different, can form no kind of difficulty in this system.” Notebook, 19/22r.

⁵¹ Fresnel mathematized many of the optical problems Young tackled qualitatively.

⁵² Notebook, 19/12r and 19/14v.

⁵³ Edinburgh Review, i (01 1803), 456.Google Scholar For further statements on Brougham/s attitude to the ether see ibid., 162 and 454.

⁵⁴ “Dufay/s opinion of two distinct fluids, a resinous & a vitreous electricity, has now few or no advocates …”, Notebook, 2O/2r.

⁵⁵ Phil. Trans., xcii (1802), 387–397.Google Scholar

⁵⁶ Ibid., 388.

⁵⁷ Prévost, , “Quelques Remarques sur la Chaleur, et sur l'Action des Corps qui l'interceptent”, Phil. Trans., xcii (1802), 403–447.CrossRefGoogle Scholar

⁵⁸ Notebook, 16/20v.

⁵⁹ Journal of the Royal Institution, i (1802), 194.Google Scholar This report was written by Young.

⁶⁰ These figures depend on the following formula used by both Newton and Young, where V stands for the velocity of propagation, D for density and E for elasticity. Newton took the ratio V_ether/V_air = 700,000, which is rather low. In making D_air/D_ether = Eether/Eair, Newton proposed a numerical example devoid of physical basis.

⁶¹ “And so small a resistance would scarce make any sensible alteration in the Motions of the Planets in ten thousand Years” (Query 22). Young was probably referring to this purely metaphorical statement when he concluded that Newton had “calculated erroneously”. It is not clear on what evidence Young reached this conclusion. He may have had in mind Johann Bernouilli's objection to Prop. 3 in Book II of the Philosophiae Naturalis Principia Mathematica (first edition, 1686—for an early nineteenth-century discussion of this problem, see Brinkley, J., Trans. of the Royal Irish Academy, ii (1807), 45)Google Scholar or a later recalculation of the data which showed the resistance to be greater than Newton supposed.

⁶² Notebook, 19/12r.

⁶³ Lectures, i, 616.Google Scholar

⁶⁴ “Experiments and Calculations relative to Physical Optics”, Phil. Trans., xciv (1804), 1–16.Google Scholar

⁶⁵ Ibid., 12.

⁶⁶ Lectures, i, 630.Google Scholar

⁶⁷ Ibid., 616.

⁶⁸ Ibid., 662.

⁶⁹ Aepinus was one of many philosophers who postulated that the excess electric fluid is accumulated in the surface of conductors. Young quoted Coulomb (Histoires et Mémoires de l'Académie Royale des Sciences, 1786)Google Scholar in this context. Young also made frequent reference to Robison, 's article “Electricity” in the Encyclopaedia Britannica (3rd edn., Edinburgh, 1797), vi, 418–545.Google Scholar

⁷⁰ “The miscellaneous papers are reprinted with some corrections and additions, but with no other alterations, of any kind, than might have been made at the time when they were first published …”, Lectures, ii, v.Google Scholar Young made this enigmatic claim in order to conceal his earlier speculations.

⁷¹ Lectures, ii, 618.Google Scholar

⁷² Ibid., 621.

⁷³ Ibid., 629.

⁷⁴ Lectures, i, 610.Google Scholar

⁷⁵ The catalogue included in the second volume of the Lectures (pp. 105–520)Google Scholar contains “about twenty thousand articles”.

⁷⁶ Euler, , op. cit. (9), iGoogle Scholar, letters XIXff, LXVIIIff; ii, letters XXIVff. Like Young, Euler frequently used analogies; in particular he also expounded the analogy between light and sound. Young often praised the “profound” Euler, although on several occasions he blamed him for lack of rigour in his investigations.