Contemporary advances in physics — XV: The classical theory of light, first part

FOR twenty years and more we have been hearing continually about the conflict between the corpuscular and the undulatory theories of light, and it is possible that for years to come we may be hearing about a similar contest between the wave-theory and the particle-theory of matter. Furthermore, there are intimations that if an adequate theory either of light or of matter ever is attained, it will involve conceptions of waves which in certain limiting cases approach to conceptions of particles. Already it is established that the appropriate way to attack the typical problems of the atom consists in setting up a wave-equation, and dealing with it in the same manner as one adopts to solve the typical problem of acoustics: how to determine the resonance-frequencies of a piece of elastic matter, such as a taut wire or a drumhead or a column of air in a tube. Therefore it seems opportune to restudy, with care and in detail, the great classical example of a wave-theory highly developed and widely successfulthe great theory of light dimly foreshadowed by Huygens, endowed with its essen tial attributes by Young and Fresnel and Kirchhoff and a host of their coevals, utilized in the design of a multitude of ingenious instruments, perfected by Maxwell and connected with the theory of electricity and magnetism, and serving to this day as the basis for the theory of quanta. So doing, we shall be reminded of many triumphs of the past century of physical research, discoveries which in their time were as exciting as new quantum phenomena in ours; we shall notice certain achievements themselves as recent as those of quanta, and perhaps not less impressive; we shall retrace the reasonings which led to certain conclusions which the quantum-theories, unable to do without them and yet incompetent to derive them, have taken bodily over from their forerunner; we shall reconsider the evidence which in the litigation of a century ago caused the verdict to be rendered in favour of the wave-theory over the particle-theory; and perhaps incidentally we shall be drilling ourselves to test the evidence lately submitted and still to be submitted in the appeal of that case, and in the hearing of that other which impends.