A simple and excellent account of the effects produced by the motion of a charged ion and also of the electronic theory of matter was given by Sir Oliver Lodge in 1903 in a paper entitled “Electrons” (_Proceedings of the Institution of Electrical Engineers_, Part 159, Vol. 32, 1903). See also J. J. Thomson’s _Electricity and Matter_ (Scribner, New York, 1904). Footnote 84: J. J. Thomson, _Phil. Mag._ April, 1887. Footnote 85: Heaviside, _Collected Papers_, Vol. II. p. 514. Footnote 86: Searle, _Phil. Mag._ Oct. 1897. Footnote 87: Abraham, _Phys. Zeit._ 4, No. 1 b, p. 57, 1902. Footnote 88: A full account of the path described by a moving ion under various conditions is given by J. J. Thomson, _Conduction of Electricity in Gases_ (Camb. Univ. Press, 1903), pp. 79–90. Footnote 89: J. J. Thomson, _Phil. Mag._ p. 293, 1897. Footnote 90: Lenard, _Annal. d. Phys._ 64, p. 279, 1898. Footnote 91: Kaufmann, _Annal. d. Phys._ 61, p. 544; 62, p. 596, 1897; 65, p. 431, 1898. Footnote 92: Simon, _Annal. d. Phys._ 69, p. 589, 1899. Footnote 93: A complete discussion of the various methods employed to measure the velocity and mass of electrons and also of the theory on which they are based will be found in J. J. Thomson’s _Conduction of Electricity through Gases_. Footnote 94: Goldstein, _Berlin Sitzber._ 39, p. 691, 1896; _Annal. d. Phys._ 64, p. 45, 1898. Footnote 95: Wien, _Annal. d. Phys._ 65, p. 440, 1898. Footnote 96: Larmor, _Phil. Mag._ 44, p. 593, 1897. Footnote 97: J. J. Thomson, _Phil. Mag._ Feb. 1897. Footnote 98: Barkla, _Phil. Mag._ June, 1903. CHAPTER III. METHODS OF MEASUREMENT. =53. Methods of Measurement.= Three general methods have been employed for examination of the radiations from radio-active bodies, depending on (1) The action of the rays on a photographic plate. (2) The ionizing action of the rays on the surrounding gas. (3) The fluorescence produced by the rays on a screen of platinocyanide of barium, zinc sulphide, or similar substance. The third method is very restricted in its application, and can only be employed for intensely active substances like radium or polonium. The photographic method has been used very widely, especially in the earlier development of the subject, but has gradually been displaced by the electrical method, as a quantitative determination of the radiations became more and more necessary. In certain directions, however, it possesses distinct advantages over the electrical method. For example, it has proved a very valuable means of investigating the curvature of the path of the rays, when deflected by a magnetic or electric field, and has allowed us to determine the constants of these rays with considerable accuracy.