Contemporary Advances in Physics — XV 



The Classical Theory of Light, First Part 



By KARL K. DARROW 



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 ap- 

 propriate 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 deter- 

 mine 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 successful — 

 the great theory of light dimly foreshadowed by Huygens, endowed 

 with its essential 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 with- 

 out 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. 



For purposes of drill it might seem better to study the example of 

 water-waves, which are visible ; or sound-waves, of which no one denies 



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