Light and Relativity, a Previously Unknown Eighteenth‐Century Manuscript by Robert Blair (1748–1828)

Abstract

In 1786, Robert Blair, an unknown astronomer from Edinburgh, wrote a paper that would remain unpublished. In his manuscript, Blair gives a systematic treatment of the Newtonian kinematics of light, taking into account in the absolute space of Newton the motion of the light source, that of the observer, and the velocity of the corpuscles of light. Two years before, in the context of Newton's corpuscular theory of light, John Michell had pointed out that the velocity of light could be measured with the help of refraction experiments. Blair went a step further and inferred the existence of what we now call the Doppler effect: a variation of refraction due to a relative motion of the source and the observer. Blair's proposal is at the roots of Arago's well‐known 1806–10 experiments on the velocity of light. In the context of the undulatory theory of light, Blair proposed an experiment to determine the absolute motion of the Earth, laying the bases for the famous experiment performed by Albert Michelson 100 years later. In fact, this manuscript contains the very questions of light relativity, the roots of spectroscopy, and addresses the very problems that would be hotly debated in the nineteenth century, only to be solved by Einstein in 1905.

*At the request of the author, publication has been delayed to coincide with the anniversary of Einstein's paper on special relativity.

Acknowledgements

I would like to thank my good friend, Anne Kox, for excellent advice and discussions, most valuable comments, and linguistic assistance. I would like to thank also Jean‐Claude Houard from Université Pierre et Marie Curie for deep discussions on physical aspects and Michel Combes from Paris Observatory for excellent advice and discussions on astronomical aspects. I would like to thank also Suzanne Débarbat, Ivor Grattan‐Guinness, S. Kichenassamy, François Rigaud, Graham Rule Secretary of The Astronomical Society of Edinburgh for references concerning Robert Blair, John Stachel, and two unknown referees. Part of this paper was written during a stay at the Max‐Planck‐Institut für Wissenschaftsgeschichte, thanks to an invitation from its director, my friend Jürgen Renn.

Notes

*At the request of the author, publication has been delayed to coincide with the anniversary of Einstein's paper on special relativity.

1. By Robert Blair M. D. communicated by Alexander Aubert Esq. J.B.S.U.A.S. Read 6 April 1786. Royal Society, Manuscript L & P, VIII, 182.

2. Alexander Aubert (1730–1805) was fellow of the Royal Society; at the time, he was director of the London Assurance Company. See Christa Jungnickel and Russell McCormmach, Cavendish The Experimental Life (Cranbury, 1999), 586.

3. Hermann Alexander Brück, The Story of Astronomy in Edinburgh from its Beginnings until 1975 (Edinburgh, 1983).

4. Ibid.

5. In 1753, Dollond discovered that by using combinations of lenses, it was possible to correct chromatic aberrations. See Keith Hutchison, ‘Euler, Dollond, Klingenstierna and the Refutation of Newton's Stance on Chromatic Aberration’, in First Australian Conference on the History of Mathematics, edited by John N. Crossley (1981), 135–44.

6. He was born in 1748 and died in 1828. Concerning Blair's scientific life, see H. A. Brück (note 3) and David Myles Gavine, Astronomy in Scotland 1745–1900, Unpublished Ph.D. Thesis, The Open University (1981). There is no entry concerning Robert Blair in Gillispie's dictionary: Charles Coulston Gillispie, editor Dictionary of Scientific Biography (New York, 1970–1980).

7. Actually an achromatic refractor, of course.

8. Robert Blair, ‘A Description of An Accurate and Simple Method of Adjusting Hadley's Quadrant for the Back Observation’, Nautical Almanac (1788). Robert Blair, ‘Experiments and Observations on the Unequal Refrangibility of Light’, Royal Society of Edinburgh Transactions, 3 (1794), 3–76. Robert Blair, ‘The Principles and Application of a New Method of Constructing Achromatic Telescopes’, Journal of Natural Philosophy, 1 (1797), 1–13. Robert Blair, Essays on Scientific Subjects (Edinburgh, 1818). Robert Blair, Scientific Aphorisms, being an Outline of an Attempt to Establish Fixed Principle of Science, and to Explain from them the General Nature of the Constitution and Mechanism of the Material System, and the Dependence of that System on Mind (Edinburgh, 1827).

9. Geoffrey N. Cantor, Optics after Newton. Theories of Light in Britain and Ireland, 1704–1840 (Manchester, 1983), 51–52. Cantor's book shows up a great interest in the emission theory. He analyses Blair's 1791 article on achromatic prisms (67–68), and he discusses Robinson's interest in Blair's manuscript (87–88), but he does not analyse Blair's 1786 manuscript.

10. James Bradley, ‘A letter from the Reverend Mr. James Bradley … to Dr. Edmund Halley … giving an account of a new discovered motion of the fix'd stars’, Royal Society of London. Philosophical Transactions, 35 (1728), 637–61.

11. Precisely because the aberration is an effect in ‘v/c’ where v the (relative) velocity of the Earth is known. The aberration angle being constant, c, the velocity of light, was generally thought to be constant at the precision of the measurements. Concerning this question, see Section 5.

12. Jean Eisenstaedt, ‘L'optique balistique newtonienne à l'épreuve des satellites de Jupiter’, Archive for History of Exact Sciences, 50 (1996), 117–56.

13. Bechler calls it the ‘velocity‐model’; see: Zeev Bechler, ‘Newton's Search for a Mechanistic Model of Colour Dispersion a Suggested Interpretation’, Archive for History of Exact Sciences, 11 (1973), 1–37.

14. See: J. Eisenstaedt (note 12); Zeev Bechler, ‘Newton's Law of Forces Which Are Inversely As the Mass a Suggested Interpretation of His Later Efforts to Normalize a Mechanistic Model of Optical Dispersion’, Centaurus, 18 (1974), 184–222; and Alan E. Shapiro, ‘Newton's ‘Achromatic’ Dispersion Law Theoretical Background and Experimental Evidence’, Archive for History of Exact Sciences, 21 (1979), 91–128.

15. J. Eisenstaedt (note 12) and Alan E. Shapiro, Fits, Passions, and Paroxysms Physics, Method, and Chemistry and Newton's Theories of Colored Bodies and Fits of Easy Reflection (Cambridge, 1993), Ch. 4.

16. See Z. Bechler (note 13) and J. Eisenstaedt (note 12). Thomas Melvill was also to deal with colour and aberration.

17. Actually, it is not that clear, as we will see in Section 5.

18. See Kurt Möller Pedersen, ‘Water‐filled Telescopes and the Pre‐history of Fresnel's Ether Dragging’, Archive for History of Exact Sciences, 54 (2000), 499–564.

19. John Michell, ‘On the Means of Discovering the Distance, Magnitude, &C. of the Fixed Stars, in Consequence of the Diminution of the Velocity of Their Light, in Case Such a Diminution Should Be Found to Take Place in Any of Them, and Such Other Data Should Be Procured from Observations, As Would Be Farther Necessary for That Purpose’, By the Rev. John Michell, B. D. F. R. S. In a letter to Henry Cavendish, Esq. F. R. S. and A. S. Royal Society of London Philosophical Transactions, 74 (1784), 35–57. Concerning Michell, see: Archibald Geikie, Memoir of John Michell, Cambridge (1918); Russell McCormmach, ‘John Michell and Henry Cavendish Weighing the Stars’, The British Journal for the History of Science, 4 (1968), 126–55; Russell McCormmach, ‘Henry Cavendish: A Study of Rational Empiricism in Eighteenth Century Natural Philosophy’. Isis, 60 (1969), 293–306; C. Jungnickel and R. McCormmach (note 2); Jean Eisenstaedt, ‘De l'influence de la gravitation sur la propagation de la lumière en théorie newtonienne. L'archéologie des trous noirs’, Archive for History of Exact Sciences, 42 (1991), 315–86; Hélène Vignolles, ‘La distance des étoiles au dix‐huitième siècle: l'échelle des magnitudes de John Michell’. Archive for History of Exact Sciences, 55 (2000), 77–101.

20. Arago made his well‐known experiments on the velocity of light in 1806–10, but his article on the subject was only published in 1853; cf. Dominique François Jean Arago, ‘Mémoire sur la vitesse de la lumière, lu à la première Classe de l'Institut le 10 décembre 1810’ Académie des Sciences (Paris). Comptes Rendus 36 (1853), 38–49. In his article, Arago hinted at Blair's ideas, but he did not make any reference to Blair's papers; most probably, he never read Blair's manuscript but heard of it through John Robison's article. See: John Robison, ‘On the Motion of Light, As Affected by Refractiong and Reflecting Substances, Which Are Also in Motion’, Royal Society of Edinburgh. Transactions, 2 (1790), 83–111, 98. An article on Arago's experiment is to be published elsewhere.

21. Essentially the ‘ordinary’ addition of velocities.

22. See J. Eisenstaedt (note 19).

23. J. Michell (note 19).

24. Concerning Michell's experiment, see J. Eisenstaedt (note 19) and: Jean Eisenstaedt, ‘The Prehistory of Relativity’, in Revisiting the Foundations of Relativistic Physics, edited by A. Ashtekar et al. (Dordrecht, 2003), 3–12.

25. We will not discuss here the details of Michell's ideas on the subject: see R. McCormmach (note 19, 139–46), and J. Eisenstaedt (note 19, 343–49).

26. J. Michell to H. Cavendish, 20 April 1784 2, 1783 in C. Jungnickel and R. McCormmach (note 2, 587).

27. In Section XIV of Book I of the Principia: Isaac Newton, Sir Isaac Newton's Mathematical Principles of Natural Philosophy & His System of the World, translated by Andrew Motte and Florian Cajori (Berkeley 1687) (US 1962). See also Z. Bechler (note 13); J. Eisenstaedt (note 12).

28. Equations and detailed calculations will be given below.

29. J. Michell to H. Cavendish, 26 May 1783 in C. Jungnickel and R. McCormmach (note 2, 564).

30. Ibid. Concerning this question of secrecy see R. McCormmach (note 19, 147–48).

31. H. Cavendish to J. Michell, 27 May 1783 in C. Jungnickel and R. McCormmach (note 2, 567).

32. J. Michell to H. Cavendish, 2 July 1783 in C. Jungnickel and R. McCormmach (note 2, 570–76).

33. J. Michell to H. Cavendish, 2 July 1783 in C. Jungnickel and R. McCormmach (note 2, 570).

34. J. Michell to H. Cavendish, 2 July 1783 in C. Jungnickel and R. McCormmach (note 2, 573. Michell used to come to London ‘to see all [his] friends there once a year & to learn what was going forward in the literary world’, J. Michell to W. Herschel 22 January 1781 quoted from C. Jungnickel and R. McCormmach (note 2, 565, note 7). Because of ill health, he did not come to town very often; he was in London during or before the 1782–83 winter, but he did not come until May 1784: see C. Jungnickel and R. McCormmach (note 2, 565).

35. J. Michell to H. Cavendish, 2 July 1783 in C. Jungnickel and R. McCormmach (note 2, 574). Concerning Michell's experiment, see J. Eisenstaedt (note 19, 343–50).

36. As Michell put it in his letter to Cavendish introducing his paper: J. Michell (note 19), 35; see also Joseph Priestley, The History and Present State of Discoveries Relating to Vision, Light and Colour (London, 1772) (Millwood, 1978), 1, 786–91; R. McCormmach (note 19, 143), and J. Eisenstaedt (note 19, 322–28).

37. H. Cavendish to J. Michell, 27 May 1783: in C. Jungnickel and R. McCormmach (note 2, 567).

38. R. McCormmach (note 19, 146).

39. H. Cavendish to J. Michell, 12 August 1783: in C. Jungnickel and R. McCormmach (note 2, 579).

40. R. Blair (note 1, 11.)

41. R. Blair (note 1, 13). Actually, the use of a refraction telescope was also Cavendish's afterthought: H. Cavendish to J. Michell, 12 August 1783: in C. Jungnickel and R. McCormmach (note 2, 579). See also McCormmach (note 19, 148).

42. R. Blair (note 1, 14).

43. ‘within these few days’. R. Blair (note 1, 12). The note clearly concerns Michell's 1784 article.

44. The argument works this way: the angle of aberration is α = v/c, where v is the (relative) velocity of the Earth around the Sun and c the velocity of light at the telescope. In the context of the emission theory of light, the Newtonian kinematics has to be applied, and we have: c = v_s +c ₀−v ₀, where v _s is the (radial) velocity of the source, v ₀ the (radial) velocity of the observer, and c ₀ the emission velocity); thus, the aberration writes as: . But in the context of the undulatory theory, the light velocity is independent of the source, and thus: .

45. What I wrote concerning aberration at the beginning of this article (note 11) was the ‘common view’. It is much more complicated than that, as I have shown here, but it is not the aim of this paper to produce an history of aberration. Aberration will not be really understood before Einstein's 1905 paper: Albert Einstein, ‘Zur Elektrodynamik bewegter Körper’ Annalen der Physik (1905) 17: 891–921, translated in Arthur I. Miller, Albert Einstein's Special Theory of Relativity: Emergence (1905) and Early Interpretation (1905–1911) (Reading, MA, 1981), 392–415. See also: Edward Eisner, ‘Aberration of Light from Binary Stars—a Paradox?’, American Journal of Physics, 35 (1967), 817–19.

46. ‘L'observation directe de l'aberration était peu propre à résoudre cette question d'une manière décisive, puisqu'une différence dans la vitesse de la lumière égale à 1/20 de la vitesse totale, ne doit produire dans l'aberration qu'une différence de 1″, précision que l'on ne peut se flatter de surpasser, même à l'aide des meilleurs instruments’; F. Arago 1853 (note 20, 40).

47. ‘Quelques étoiles doivent d'ailleurs se mouvoir dans l'espace avec des vitesses très‐considérables, puisque, malgré la petitesse de leurs parallaxes, elles sont annuellement assujetties à des déplacements très‐sensibles; la vitesse des rayons qu'elles nous envoient doit donc être la résultante de leur vitesse primitive d'émission combinée avec celle de l'étoile elle‐même, et varier, par conséquent, avec la grandeur et la direction du mouvement des astres’; F. Arago 1853 (note 20, 47).

48. This difficulty of the wave theory was linked to the fact that it was not coherent with the principle of relativity. The ether was a kind of reification of the absolute space. See: John Stachel, ‘“What Song the Syren Sang”: How Did Einstein Discover Special Relativity’ in Einstein from B to Z (Boston, 2002), 157–69, 161. See also: John Stachel, ‘History of Relativity’, in Twentieth Century Physics, edited by Laurie M. Brown, Abraham Pais and Sir Brian Pippard (New York, 1995), I, 249–356 (257–58).

49. Euler's views, even if they are not quoted in Blair's manuscript, are of great interest; for example, Robison refers to ‘Euler's explanations of refraction’ just before quoting Blair: J. Robison (note 20, 98).

50. Leonhard Euler, ‘Explicatio phaenomenorum quae a motu successivo lucis oriuntur’, Commentarii Academia Scientiarum Petropolitanae, 11 (1739), 150–93. Also in Operia Omnia, Series III, 5, 46–80.

51. David Speiser followed by Giulio Maltese studied Euler's work on the relativity of motion in detail: David Speiser, ‘L. Euler, the Principle of Relativity and the Fundamentals of Classical Mechanics’, Nature, 190 (1961), 757–59; Giulio Maltese, ‘On the Relativity of Motion in Leonhard Euler's Science’, Archive for History of Exact Sciences, 54 (2000), 319–48, 328–36. On Euler's position, see also Paul Acloque, ‘L'aberration stellaire: Un mirage qui a destitué l'éther’ Cahiers d'histoire et de Philosophie des Sciences (1991) n° 36, 39.

52. G. Maltese (note 51, 329); L. Euler (note 50, 48).

53. See G. Maltese (note 51, 331).

54. D. Speiser (note 51, 758).

55. See G. Maltese (note 51, 331).

56. G. Maltese (note 51, 330).

57. For example, in her ‘Histoire du principe de relativité’, Marie‐Antoinette Tonnelat jumped over one century of research: from Bradley to Fresnel: see Marie‐Antoinette Tonnelat, Histoire du principe de relativité (Paris, 1971), 84–91. Pedersen (note 18), in his lengthy article on water‐filled telescopes, does not make such a point.

58. This is importent from a pedagogical point of view, as well. As a student, I remember this as a dogma very well: ‘the velocity of light is independent of the source’, not of the observer. But why this asymmetry? It was not a question to be raised. Only special relativity was consistent.

59. R. Blair (note 1, 3–4).

60. But published in 1790: J. Robison (note 20).

61. In this article, Robison gives the algebraic calculations concerning refraction in precisely the same way as Clairaut fifty years before: Alexis‐Claude Clairaut, ‘Sur les Explications Cartésiennes et Newtoniennes de la Réfraction de la Lumière’, Académie Royale des Sciences (Paris), Mémoires pour 1739 (1741), 259–75; concerning Clairaut's calculations, see J. Eisenstaedt (note 12, 120–24); see also G. Cantor (note 9, 212–16).

62. Remarkably, in this article, Robison does not allude to the velocity–refraction effect in the sense that he does not point out that a differential measure of refraction is a differential measure of light‐velocity.

63. J. Robison (note 20, 106). It is worth noting that the relativity is restricted to that of the light and the observer; it is not extended to the source.

64. J. Robison (note 20, 98). The question has to be raised: Could Robison have borrowed the idea from Blair's 1786 manuscript, especially since he is judged as ‘extremely competent, scrupulous and well‐read’? G. Cantor (note 9, 87). Concerning Robison's reticence to refer to Blair, see also the discussion concerning Wilson's work further on.

65. R. Blair (note 1, 9).

66. Better: any possible cases are considered and even ‘particles of light at rest’.

67. R. Blair (note 1, 13–14).

68. The emission velocity of light c ₀, the velocity of light relative to its source, was supposed to be constant anyway.

69. Let us recall once more that the accepted view in these matters was that the velocity of light was not constant and that we deal with a ballistic theory of light: actually, light is just treated as an ordinary particle.

70. Such a calculation is not in Blair's manuscript. But some calculation of the same sort must have been made by him in order to obtain his results. We will give all these calculations to verify Blair's alleged data.

71. Concerning Newton's corpuscular theory of light and Clairaut's algebraic elaboration of it, see: J. Eisenstaedt, (note 12 and also note 20). These equations are discussed in detail therein.

72. This calculation is also consistent with Arago's supposed calculations: F. Arago (note 20, 45–46).

73. Blair assumed a complete relativity of light. But it was very generally thought that, in an undulatory context (and in an emission context, as well), the velocity of light was independent of the velocity of the source. This implies that v _s = 0. In order to get a formula as general as possible, we will not make this supposition immediately.

74. Christian Doppler, ‘Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels’. Abhandlung der mathematische‐naturwissenschaftlischen Classe der Königlich Böhmischen. Gesellschaft der Wissenschaften (1842) 2, 465–83.

75. R. Blair (note 1, 13–14).

76. Rosmorduc is, as far as I know, the only historian of science who understood that Arago's experiment was a Doppler–Fizeau experiment. See Jean Rosmorduc, ‘L'expérience de Fizeau’, Bulletin de l'Union des Physiciens, 632 (1981), 841–57 (843, note 7).

77. J. B. Hearnshaw, The Analysis of Starlight: One Hundred and Fifty Years of Astronomical Spectroscopy (Cambrige, 1986), 4.

78. J. B. Hearnshaw, ‘Doppler and Vogel. Two Notable Anniversaries in Stellar Astronomy’, Vistas in Astronomy, (1992) 35, 157–77 (162). It is ‘Lockyer who in 1869 made the first observations of Doppler shifts of spectral lines’ (Hearnshaw, 163).

79. James Clerk Maxwell, ‘On the Influence of the Motions of the Heavenly Bodies on the Index of Refraction of Light’, in Maxwell, James Clark to William Huggins, 10 June 1867, in: William Huggins, ‘Further Observations on the Spectra of Some of the Stars and Nebulae, with an Attempt to Determine Therefrom Whether These Bodies Are Moving Towards or from the Earth, Also Observations on the Spectra of the Sun and of Comets II’, Royal Society of London Philosophical Transactions 158 (1868), 529–64, 532. Maxwell implicitly deals with the (Blair)–Doppler–Fizeau effect but in the context of the theory or undulations.

80. See: William Hyde Wollaston, ‘A Method of Examining Refractive and Dispersive Powers by Prismatic Reflexion’ Royal Society of London Philosophical Transactions 92 (1802), 365–80.

81. R. Blair (note 1, 4).

82. Pierre Bouguer, Traité d'optique sur la gradation de la lumière: Ouvrage posthume publié par M. l'Abbé de la Caille (Paris: Guérin, 1760). The English edition is recent: Pierre Bouguer, Optical Treatise on the Gradation of Light, Translated, with introduction and notes, by W. E. Knowle (Toronto, 1961).

83. R. Blair (note 1, 16).

84. R. Blair (note 1, 17).

85. Ibid.

86. R. Blair (note 1, 18).

87. R. Blair (note 1, 21). The rosin is a kind of glue near to isinglass that he had also tried.

88. R. Blair (note 1, 20).

89. R. Blair (note 1, 5, and 22).

90. R. Blair (note 1, 21).

91. R. Blair (note 1, 8).

92. Another consequence is that the ray will be less and less parallel to the prism's bases. But as the effect is slight, it does not affect the calculations. Blair does not address this point at all.

93. R. Blair (note 1, 7). 2′ 9″ 12″′ is 24×5″ 23″′.

94. R. Blair (note 1, 4). It is Bradley's value.

95. R. Blair (note 1, 6).

96. Ibid.

97. Ibid.

98. R. Blair (note 1, 41).

99. R. Blair (note 1, 42).

100. R. Blair 1794, 1797 (note 9).

101. H.A. Brück (note 3, 13).

102. R. Blair (note 1, 43).

103. R. Blair (note 1, 43).

104. See note 20.

105. R. Blair (note 1, 23).

106. Ibid.

107. R. Blair (note 1, 8).

108. See J. Eisenstaedt (note 12 and 20) see also Jean Eisenstaedt, ‘Laplace: l'ambition unitaire ou les lumières de l'astronomie’, Académie des Sciences Paris Comptes Rendus, Série IIb 324 (1997), 565–74.

109. J. Michell (note 19).

110. R. Blair (note 1, 30). Blair's proposal is much simpler than Michell's. Michell did not aim to ascertain his effect; he used it to determine the distances of stars. His experiment was a nice but an incredibly sophisticated one: he proposed comparing light‐rays' refraction coming from each component of a double‐star located in the Pleiades. However, Michell's instrument was simpler than Blair's: just an achromatic prism on a telescope. See J. Eisenstaedt (note 19, 343).

111. That is to say, the vectorial value of the velocity calculated from a classical point of view: c±v in the case where the motions are collinear. Of course, throughout this paper, we use the classical kinematics which allows the velocity of light to be possibly ‘augmented or diminished’.

112. R. Blair (note 1, 34). In such a case, the prism acts as a convergent lens.

113. Ibid.

114. Effects of this kind appear in special relativity.

115. R. Blair (note 1, 37).

116. We are to follow all of these ideas in their proper context where Blair's speculations are very consistent indeed.

117. R. Blair (note 1, 29).

118. R. Blair (note 1, 10).

119. R. Blair (note 1, 38).

120. R. Blair (note 1, 33).

121. John Robison, ‘Mr. Wilson on the Solar System (Part 1 History of the Society)’, Royal Society of Edinburgh Transactions, 1 (1788), 28–30.

122. Patrick Wilson, ‘An Experiment Proposed for Determining, by the Aberration of the Fixed Stars, Whether the Rays of Light, in Pervading Different Media, Change Their Velocity According to the Law Which Results from Sir Isaac Newton's Ideas Concerning the Cause of Refraction; and for Ascertaining Their Velocity in Every Medium Whose Refractive Density Is Known’, Royal Society of London Philosophical Transactions, 72 (1782), 58–70. Concerning this line of research, see K. Pedersen (note 18). Pedersen refers and analyses Wilson's paper of 1782 but not Robison's paper of 1788.

123. We have to bear in mind that from 1783 Blair had let his proposal be known with ‘copies granted … both in Edinburgh and in London’ R. Blair (note 1, 11).

124. J. Robison (note 121, 29).

125. J. Robison (note 121, 30).

126. In the same way, Pedersen points to a question of priority involving Wilson: K. Pedersen (note 18, 513–22).

127. J. Robison (note 20, 98).

128. John Robison, ‘Optics’, Encyclopædia Britannica, 3rd ed., 13 (1797), 231–364 (284–85). In his contribution, aside from the velocity–refraction effect, Robison refers also to the question of the colour of Jupiter's satellites, an explanation which ‘must be given up’ (283–84). Concerning this question, see J. Eisenstaedt (note 12).

129. J. Robison (note 128, 284 in the margin). Arago will take up this question in his 1806–10 experiments: F. Arago (note 20).

130. J. Robison (note 128, 284).

131. Ibid.

132. Ibid.

133. J. Robison (note 128, 284). Robison also proposes observing the tail of a comet in order to discover ‘its greater aberration and refrangibility’.

134. J. Robison (note 128, 284, in the margin).

135. J. Robison (note 128, 284).

136. F. Arago (note 20).

137. J. Robison (note 128, 284).

138. G. Cantor (note 9, 87–88).

139. J. Robison to W. Herschel, 14 April 1804: in G. Cantor (note 9, 88). As Cantor suggested, ‘he was in part responding to Young, a fearsome scientific opponent, whose revisions of the vibration theory had recently been published’. Ibid.

140. J. Robison to W. Herschel, 14 April 1804: in G. Cantor (note 9, 88).

141. It is difficult to give a precise date from the manuscript itself: it it clear from his handwriting that Herschel was writing his queries in his notebook from time to time. Most probably, notes 70–71–72 were written after Herschel had heard of Michell's method in 1783.

142. William Herschel, ‘Hints. Desiderata. Experiments to be made' Memorandum of Experiments to be made relating to Powers of Att[ractions]. & Rep[ulsions]’, Royal Astronomical Society, Ms. Real 23 6/9 (n.d.).

143. ‘Queries’ as it is reminiscent of Newton's Optiks.

144. W. Herschel (note 142, §71). Note that, in the same way as Blair, he considers the possibility of a dependence of the light‐velocity on that of the source and of course on that of the observer as well.

145. W. Herschel (note 142, §72). Note that Herschel was not to observe a special ray but observed ‘prismatic intervals’, and he correctly waited for a widening of the spectral lines, much in the same way as for a Doppler–Fizeau effect.

146. R. McCormmach (note 19, 148–49).

147. R. Blair (note 1, 25–26).

148. Clearly, in such a case, there is no relativity of motion. The candle and the Earth are at rest … In the unulatory theory of light, it is possible that a light‐ray would not move with the same velocity relative to the Earth and relative to the candle: it is the logic of the ether; now, such a doctrine is very difficult to follow!

149. At the beginning of the 1870s, Eleuthère Mascart actually realized this very experiment. In 1874, he quoted Arago, who conducted the experiment first (note 11), but not Blair's proposal: ‘One may conclude from these observations that the motion of translation of the Earth is without influence on the apparent refraction of light coming from a terrestrian source’. (‘On peut conclure de ces observations que le mouvement de translation de la Terre est sans influence sur la réfraction apparente de la lumière qui provient d'une source terrestre’.) Eleuthère Mascart, ‘Sur les modifications qu'éprouve la lumière par suite du mouvement de la source lumineuse et du mouvement de l'observateur II’, Ecole Normale Annales, 3 (1874), 363–420 (388).

150. Loyd S. Swenson, ‘The Michelson–Morley–Miller Experiment before and after 1905’, Journal for the History of Astronomy, i (1970), 56–78.

151. R. Blair (note 2, 27).

152. Clearly, in this case, there is relativity of motion. The candle and the Earth are at rest … Any corpuscule and a light‐ray as well will move with the same velocity relative to the Earth and relative to the candle.

153. Ibid.

154. Ibid.

155. R. Blair (note 1, 27–28).

156. ‘Mes expériences étaient à peu près achevées, lorsque la lecture d'un des beaux Mémoires que le Dr Young a inséré dans les Transactions philosophiques, m'aprit que M. Robisson (sic), professeur de physique à Édinburgh, avait considéré théoriquement cette question de la vitesse de la lumière; j'ai, depuis, trouvé, dans divers ouvrages, qu'elle avait été examinée sous différents points de vue par Boscowich, Michell, Wilson et Blair’ F. Arago (note 20, 41).

157. It was my aim as an historian to discuss these developments in their own right. But it is interesting to analyse the reasons for the negative results that Arago obtained, arising mainly from the achromatism of his prisms. The precision of such measures was also questionable.

158. Tetu Hirosige, ‘The Ether Problem, the Mecanistic Worldview, and the Origins of the Theory of Relativity’, Historical Studies in the Physical Sciences, 7 (1976), 3–82 (12).

159. I. Newton (note 27, I, Section XIV).

160. Isaac Newton, Opticks (London, 1704), (I, I, Prop. VI. Theor V).

161. A. Clairaut (note 61). See also: J. Eisenstaedt (note 12).

162. For example, the idea of the velocity–refraction effect is not present in Priestley's history of opticks: Joseph Priestley, The History and Present State of Discoveries Relating to Vision, Light and Colour (London, 1772).

163. See chapter 6.

164. As an aside, in the middle of the eighteenth century, the mechanistic model of colour dispersion, the velocity‐model as Bechler called it, was really close to the velocity–refraction effect and was widely discussed. It could have been the occasion to point at the second one.

165. G. Cantor (note 9).

166. R. McCormmach (note 19).

167. In addition, Arthur Miller did not understand Arago's experiment, which was certainly not ‘a measure of the “aberration angle”’, Miller (note 45, 15).

168. By Father Secchi and William Huggins: see J.B. Hearnshaw, The Analysis of Starlight: One Hundred and Fifty Years of Astronomical Spectroscopy (Cambridge, 1986), 143–207. Fraunhofer first observed stellar specta in 1814: see J. B. Hearnshaw (note 77, 51).