The modern corpuscular theory of radiation was born in 1905 when Einstein advanced his light quantum hypothesis; and the steps by which Einstein's hypothesis, after years of profound scepticism, was finally and fully vindicated by Arthur Compton's 1922 scattering experiments constitutes one of the most stimulating chapters in the history of recent physics. To begin to appreciate the complexity of this chapter, however, it is only necessary to emphasize an elementary but very significant point, namely, that while Einstein based his arguments for quanta largely on the behaviour of high-frequency black body radiation or ultra-violet light, Compton experimented with X-rays. A modern physicist accustomed to picturing ultra-violet light and X-radiation as simply two adjacent regions in the electromagnetic spectrum might regard this distinction as hair-splitting. But who in 1905 was sure that X-rays and γ-rays are far more closely related to ultra-violet light than to α-particles, for example ? This only became evident after years of painstaking research, so that moving without elaboration from Einstein's hypothesis to Compton's experiments automatically eliminates from consideration an important segment of history—a segment in which a major role was played by William Henry Bragg.
1 “Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt”, Annalen der Physik, xvii (1905), 132–148; translated in Boorse, H. A. and Motz, L., ed., The World of the Atom (New York, 1966), i, 544–557.
2 “A Quantum Theory of the Scattering of X Rays by Light Elements”, Physical Review, xxi (1923), 483–502.
3 “Photo-electricity”, Proc. Royal Institution, xxv (1928), 341.
4 Ibid., 343. See also Bragg, 's Universe of Light (London, 1933); reprint by Dover (New York, 1959). 269–270.
5 “A Comparison of Some Forms of Electric Radiation”, Trans. R. Soc. S. Aust., xxxi (1907), 90.
6 For Stokes's 1898 paper, see “On the Nature of the Röntgen Rays”, Mathematical and Physical Papers (Cambridge, 1905), v, 255. A. Schuster and E. Wiechert had also suggested the pulse theory. See Rutherford, E., Radioactive Substances and their Radiations (Cambridge, 1913), 83; W. H. and Bragg, W. L., X Rays and Crystal Structure (4th edn., London, 1924), 2.
7 The result Thomson published in the first edition of his Conduction of Electricity through Gases (London, 1903) was out by a factor of 2, which he corrected in time for the second edition (London, 1906), 321–325.
8 “Polarized Röntgen Radiation”, Phil. Trans., cciv (1905), 467–479; Proc. R. Soc., lxxiv (1905), 474–475; preliminary announcement in Nature, lix (1904), 463.
9 “Polarization in Secondary Röntgen Radiation”, Proc. R. Soc., lxxvii (1906), 247–255; “Secondary Röntgen radiation”, Proc. Phys. Soc., xx (1906), 200; Phil. Mag., xi (1906), 812–828.
10 For brief accounts of Barkla's and Bragg's life and work, see Allen, H. S., “Charles Glover Barkla”, Obituary Notices of Fellows of the Royal Society, v (1945–1948), 341–366; Andrade, E. N. da C., “William Henry Bragg”, ibid., iv (1942–44), 277–300. See also articles by Forman, Paul in Dictionary of Scientific Biography, ed. by Gillispie, C. C., New York, 1970.
11 Bragg, , op. cit. (1), 79–93; “The Nature of Röntgen Rays”, Trans. R. Soc. S. Aust., xxxi (1907), 94–98; reprinted as “On the Properties and Natures of Various Electric Radiations”, Phil. Mag., xiv (1907), 429–449; Annual Reports of the Smithsonian Institution (1907), 195–214.
12 Andrade, , op. cit. (10), 280, suggests that Bragg's neutral pair was a modified version of P. Lenard's “dynamid”. Not until 1908 did Rutherford and Geiger show that the α-particle is doubly charged. See Rutherford, , op. cit. (6), 136.
13 Bragg, , op. cit. (11), Phil. Mag., 442.
14 Ibid., 445. For Haga and Wind's work see “Die Beugung der Röntgenstrashlen”, Annalen der Physik, x (1903), 305–312; “Über die Polarisation der Röntgenstrahlen und der Sekundärstrahlen”, Annalen der Physik, xxiii (1907), 439–444.
15 “Ueber die lichtelektrische Wirkung”, Annalen der Physik, viii (1902), 149–198, especially 166; Innes, P. D., “On the Velocity of the Cathode Particles emitted by Various Metals under the Influence of Röntgen Rays, and its Bearing on the Theory of Atomic Disintegration”, Proc. R. Soc., cxxix (1907), 442–462.
16 Marx, E., “Die Geschwindigkeit der Röntgenstrahlen”, Phys. Z., vi (1905), 768–778; 834–835.
17 Bragg, , op. cit. (11), Phil. Mag., 448.
18 “Note on X-Rays and Scattered X-Rays”, Phil. Mag., xv (1908), 288–296; “The Nature of X-rays”, Nature, lxxvi (1907), 661–662.
19 Ibid., 288, 294, 296.
20 “The Nature of γ and X-rays”, Nature, lxxvii (1908), 270–271.
21 Ibid., 270.
22 “The Nature of Röntgen Rays”, Nature, lxxvii (1908), 320.
23 “The Nature of the γ and X-Rays”, Nature, lxxvii (1908), 560.
25 “The Nature of the γ and X-Rays”, Nature, lxxvii (1908), 271; “An Experimental Investigation of the Nature of γ Rays—No. 2”, Phil. Mag., xvi (1908), 918–939 (with J. P. V. Madsen).
26 Lenard, , op. cit. (15).
27 “Secondary Radiation from a Plate exposed to Rays from Radium”, Phil. Mag., xiv (1907), 176–187, especially 187. Mackenzie also independently observed the forward-backward asymmetry in secondary β-rays excited by γ-rays.
28 Bumstead, H. A., “The Heating Effects produced by Röntgen Rays in Different Metals, and their Relation to the Question of Charge in the Atom”, Phil. Mag., xi (1906), 292–317; “On the Heating Effects Produced by Röntgen Rays in Lead and Zinc”, Phil. Mag., xv (1908), 432–437; Angerer, E., “Bolometrische Untersuchungen über die Energie der X-Strahlen”, Annalen der Physik, xxi (1906), 87–117; “Ursprung der Wärmeentwickelung bei Absorption von Röntgenstrahlen”, Annalen der Physik, xxiv (1907), 370–380.
29 Bragg, , op. cit. (25), 935.
30 Ibid., 934; Thomson, J. J., “On the Ionization of Gases by Ultra-Violet Light and on the evidence on the Structure of Light afforded by its Electrical Effects”, Proc. Camb. Phil. Soc., xiv (1906–1908), 421. Also see McCormmach, Russell, “J.J. Thomson and the Structure of Light”, Brit. J. Hist. Sci., iii (1966–1967), 362–387.
31 Bragg, , op. cit. (25), 936.
32 “The Nature of γ and X-Rays”, Nature, lxxvii (1908), 509–510. Bragg did not mention that Cooksey disagreed with his interpretation, believing rather that the electromagnetic momentum of a pulse was responsible for ejecting the β-rays asymmetrically.
33 Bragg, , op. cit. (25), 938.
34 “The Nature of X-Rays”, Nature, lxxviii (1908), 7.
35 “Homogeneous Secondary Röntgen Radiations”, Phil. Mag., xvi (1908), 550–584.
36 Barkla, , op. cit. (34).
37 “The Nature of the y and X-Rays”, Nature, lxxviii (1908), 294.
38 “The Nature of X-Rays”, Nature, lxxviii (1908), 665.
39 Ibid., note added by Bragg.
40 “The Absorption of Röntgen Rays”, Phil. Mag., xvii (1909), 758 (with C. A. Sadler).
41 Barkla, and Sadler, , op. cit. (35), 579.
42 “The Nature of the γ Rays”, Proc. Camb. phil. Soc., xiv (1906–1908), 540.
43 “On the Secondary Radiation caused by the β and γ Rays of Radium”, Phil. Mag., viii (1904), 669–685. More recently Kleeman, R. D., “On the Different Kinds of γ Rays of Radium, and the Secondary γ Rays which they Produce”, Phil. Mag., xv (1908), 638–663, had found the same thing. Kleeman, a former student of Bragg then studying under Thomson, interpreted this observation in terms of the pulse theory.
44 “On a Want of Symmetry shown by Secondary X-Rays”, Phil. Mag., xvii (1909), 855–864 (with J. L. Glasson).
45 Ibid., 863.
46 Andrade, , op. cit. (10), 282.
47 “Primary and Secondary Gamma Rays”, Phil. Mag., xviii (1909), 275.
48 “Secondary γ Radiation”, Phil. Mag., xvii (1909), 423–448. Madsen, thoroughly convinced of Bragg's corpuscular ideas, described the softening as due to billiard-ball type collisions of the γ-ray particles.
49 “Primary and Secondary γ Rays”, Phil. Mag., xx (1910), 921–938.
50 For specific papers, see Allen, , op. cit. (10). Barkla found evidence for “excess scattering”, an increase in the forward-backward asymmetry with an increase in the primary wavelength. Both Crowther, J. A., “On the Energy and Distribution of Scattered Röntgen Radiation”, Proc. R. Soc., lxxxv (1911), 29–43, and Owen, E. A., “On the Scattering of Röntgen Radiation”, Proc. Camb. phil. Soc., xvi (1911), 161–166, observed the same phenomenon.
51 “The Röntgen Radiation from Substances of Low Atomic Weight”, Phil. Mag., xxiv (1912), 138–149.
52 For more details on Thomson's work, see McCormmach, , op. cit. (30), as well as Thomson's papers “On a Theory of the Structure of the Electric Field and its Application to Röntgen Radiation and to Light”, Phil. Mag., xix (1910), 301–313, and “On the Theory of Radiation”, Phil. Mag., xx (1910), 238–247.
53 “Zur experimentellen Entscheidung zwischen Ätherwellen- und Lichtquantenhypothese. I. Röntgenstrahlung”, Phys. Z., x (1909), 902–913; “Zur experimentellen Entscheidung zwischen der Lichtquantenhypothese und der Ätherimpulsetheorie der Röntgenstrahlen”, Phys. Z, xi (1910), 24–31; “Zur experimentellen Entscheidung zwischen Lichtquantenhypothese und Ätherwellentheorie. II. Sichtbares, und Ultraviolettes Spektrum”, Phys. Z., xi (1910), 179–187. The last paper appeared in the 1 March issue, a few weeks after Bragg had written his first letter to Sommerfeld.
54 Bragg, , op. cit. (54), 386.
55 For a discussion of Einstein's Salzburg paper in the context of the development of his thought, see Klein, Martin J., “Einstein and the Wave-Particle Duality”, The Natural Philosopher, iii (1964), 3–49. Also see Hermann, Armin, ed., Die Hypothese der Lichtquanten (Stuttgart, 1965).
56 “Über die Verteilung der Intensität bei der Emission von Röntgenstrahlen”, Phys. Z., x (1909), 969–976; xi (1910), 99–101.
57 The three letters from W. H. Bragg to A. Sommerfeld are deposited in the Archive for History of Quantum Physics (AHQP) at the American Philosophical Society Library (Philadelphia), University of California Library (Berkeley), and the Universitets Institut for Teoretisk Fysik (Copenhagen); the letter from Sommerfeld to Bragg was found by Sir Lawrence Bragg in his father's papers, and a copy of it was kindly sent to me by Sir Lawrence. I should like to express my gratitude to Sir Lawrence and to Dr.-Ing. Ernst Sommerfeld for permission to reprint them all here. The translation of Sommerfeld's letter is my own. For W. Friedrich's Munich Dissertation see “Räumliche Intensitätsverteilung der X-Strahlen, die von einer Platinaantikathode ausgehen”, Annalen der Physik, xxxix (1912), 377–430; E. Bassler's 1909 work on the polarization of X-rays is discussed and referenced on p. 378.
57a Walter and Pohl had cast serious doubt on the validity of Haga and Wind's earlier experiments.
58 “Über die Struktur der γ-Strahlen”, Sbr. bayer. Akad. Wiss. Mathematisch-physikalische Klasse, xxxxi (1911), 1–60. For a modern discussion of the “forward peaking”, see for example Jackson, J. D., Classical Electrodynamics (New York, 1962), 473.
59 For C. T. R. Wilson's original photographs and paper, see “On a Method of Making Visible the Paths of Ionizing Particles through a Gas”, Proc. R. Soc., lxxxv (1911), 285–288.
60 “The Consequences of the Corpuscular Hypothesis of the γ and X Rays, and the Range of β Rays”, Phil. Mag., xx (1910), 415.
61 Ibid., 416.
62 “Energy Transformations of X-Rays”, Proc. R. Soc., lxxxv (1911), 349–365. See also Bragg, W. H., “The Mode of lonization by X-Rays”, Phil. Mag., xxii (1911), 222–223; “On the Direct or Indirect Nature of the lonization by X-rays”, Phil. Mag., xxiii (1912), 647–650.
63 Bragg, , op. cit. (25), Phil. Mag., 938.
64 Bragg, , op. cit. (59), 386.
65 Studies in Radioactivity (London, 1912), 191–192. This is a somewhat different explanation than he gave in “The Secondary Radiation produced by the Beta Rays of Radium”, Physical Review, xxx (1910), 638–640.
66 Bragg, , op. cit. (59), 389.
67 “Radio-activity as a kinetic theory of a fourth state of matter”, Nature, lxxxv (1911), 494.
69 “A Difference in the Photoelectric Effect caused by Incident and Divergent Light”, Nature, lxxxiii (1910), 311; Phil. Mag., xx (1910), 331–339.
70 “A Difference in the Photoelectric Effect caused by Incident and Divergent Light”, Nature, lxxxiii (1910), 339; “On the Direction of Motion of an Electron ejected from an Atom by Ultra-Violet Light”, Proc. R. Soc., lxxxiv (1910), 92–99.
71 Bragg, , op. cit. (67).
72 Reports of the British Association for the Advancement of Science (1911), 340–341.
73 Ibid., 341. Whiddington, R., “The Production of Characteristic Röntgen Radiation”, Proc. R. Soc., lxxxv (1911), 323–332; “Characteristic Röntgen Radiation”, Nature, lxxxviii (1911), 143.
74 Ibid., 341.
75 Nature, lxxxvii (1911), 501.
76 Ibid. This is a curious statement, in view of the radiation pressure experiments of Lebedev and Nichols and Hull at the turn of the century.
78 Bragg, , op. cit. (65), vii.
79 Ibid., 191.
80 Ibid., vii.
81 Ibid., 192.
82 Ibid., 192–193.
83 Ibid., 193.
84 Ibid., 188.
85 Ibid., 193.
86 “Radiations Old and New”, Nature, xc (1913), 560; see also 529–532.
87 Ibid., 558.
88 Bragg, , op. cit. (67).
89 See reference (55).
90 Ewald, P. P., “Max von Laue”, Obituary Notices of Fellows of the Royal Society, vi (1960), 139.
91 W. H. and Bragg, W. L., op. cit. (6), 3.
92 “Interferenzerscheinungen bei Röntgenstrahlen”, Sbr. bayer. Akad. Wiss. Mathematische-physikalische Klasse, xlii (1912), 303–322; “Eine quantitative Prüfung der Theorie für die Interferenzerscheinungen bei Röntgenstrahlen”, ibid., xlii (1912), 363–373.
93 Quoted in Compton, Arthur H., X-Rays and Electrons (New York, 1926), 90.
94 “Atomic Theories of Radiation”, Science, xxxvii (1913), 119–133, especially 127, 132–133; also see Allen, , op. cit. (10), 349.
95 “Are Electrons Waves?”, Bell Laboratories Record, iv (1927), 258.
96 “X-rays and Crystals”, Nature, xc (1912), 219.
97 On deposit in the AHQP, reference (69).
98 See identical statement by Bragg in “X-rays and Crystals”, Nature, xc (1912), 360.
100 For Nobel Lecture see “The Diffraction of X-Rays by Crystals”, Nobel Lectures: Physics (Amsterdam, 1967), i, 370–382.
101 “The Reflection of X-rays by Crystals”, Proc. R. Soc., lxxxviii (1913), 436.
102 “Aether Waves and Electrons”, Nature, cvii (1921), 374.
103 On deposit in the O. W. Richardson Collection in the Miriam Lutcher Stark Library at the University of Texas. I am indebted to Sir Lawrence Bragg and to the Stark Library for permission to quote from this letter.
104 The exact relationships between Thomson and Compton scattering may be seen from the plots presented by Ann T. Nelms in “Graphs of the Compton Energy-Angle Relationship and the Klein-Nishina Formula”, National Bureau of Standards Circular Number 542 (1953).
105 There is of course a longer wavelength Compton component in scattered X-rays as well as in scattered γ-rays. Since, however, the change in wavelength for X-rays is only a few per cent, while for γ-rays it is of the order of 100 per cent, the longer wavelength γ-ray component is much easier to observe than the longer wavelength X-ray component.
106 For a discussion of Kossel's work see Heilbron, John L., “The Kossel-Sommerfeld Theory and the Ring Atom”, Isis, lviii (1967), 451–485, especially 462–466.
107 Curiously, as I shall show in a future publication, Arthur Compton, while in no way building on Einstein's work, may have received this crucial insight (that a single quantum must interact with a single electron) from Bragg's work. I should also mention that my reason for terming Einstein's hypothesis “long-neglected” in spite of Millikan's 1915 photoelectric effect experiments is that Millikan (in common with virtually every other contemporary physicist except Einstein) did not accept these experiments as proof of Einstein's hypothesis. See my “Non-Einsteinian Interpretations of the Photoelectric Effect” in Stuewer, Roger H., ed., Historical and Philosophical Perspectives of Science (Minneapolis: University of Minnesota Press, 1970).
108 Eddington, Arthur, The Nature of the Physical World; paperback reprint (Ann Arbor, 1958), 194.
* Acknowledgement: I should like to express my sincere gratitude to Sir Lawrence Bragg, C.H., O.B.E., M.C., F.R.S., and to Professor Paul Forman for reading the manuscript and pointing out several errors.
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