September i, 192 i] 



NATURE 



The Present Position of the Wave Theory of Light. 

 By Dr. R. A. Houstoun. 



I. 



THE emission theory of light prevailed for a 

 century after Newton's death. During this 

 time his " Opticks " was regarded as of equal 

 importance as his " Principia," and his emission 

 theory as of equal value as his law of gravitation. 

 Then, principally owing to the work of Fresnel, 

 the emission theory was overthrown, and the wave 

 theory established in its place. The latter in its 

 turn has prevailed foi a centur}-, but now in cer- 

 tain quarters doubts are being expressed as to 

 whether it is competent to explain the results of 

 recent experimental work, and whether, after all, it 

 may not be advisable to hark back to some form of 

 emission theory, at least for certain fields of work. 



There are two great diflerences between the 

 situation now and as it existed a hundred years 

 ago. Then the wave theor}^ under Fresnel pre- 

 sented a clear and definite alternative to the emis- 

 sion theorv of Newton, explaining certain decisive 

 experiments in a simple and natural manner. The 

 critics of the wave theory at present are not so 

 much hostile as neutral towards it. They present 

 no alternative to it ; they admit its strong position 

 and also admit the impossibility of Newton's emis- 

 sion theor\- in the light of the experimental work 

 of to-day. But they direct attention to certain 

 results which they have difficulty in reconciling 

 with the wave theory, and hint at somehow com- 

 bining the advantages of both theories. 



Another difference between now and a hundred 

 years ago is the manner in which we regard our 

 theories. Then a theory was true or false ; we 

 were engaged in interpreting the processes of 

 Nature which existed independently of us and 

 outside of us, and it was necessary- that the true 

 solution should be true for all time. Nowadays 

 we do not so much speak of the truth of a theory 

 as of its utility, or rather the truth of a theory 

 lies in its utility. Truth is what works. Conse- 

 quently we require of a theory only that it should 

 be true for our day and our generation. .\ theory 

 works if it connects the facts together and enables 

 us to predict new facts. We can never penetrate 

 to the essential nature of things ; we can only com- 

 pare them W'ith other things. Physical theories are 

 metaphors. When we say that light is propagated 

 in wave motion, we mean that it is propagated like 

 wave motion. This change in the attitude of the 

 physicist towards his theories had been pretty 

 widelv adopted before the results of the principle 

 of relativity became known ; the latter made the 

 change of attitude known to the public. 



The criticisms directed against the wave theory 

 at present arise from two quarters, namely, the 

 principle of relativity and the quantum 

 phenomena. 



The special theory of relativity requires that 

 the mass of a system should vary with its internal 

 energy, and that consequently radiation, including 

 light waves, should have mass. In connection 



NO. 2705, VOL. 108] 



with this result a paper by Sir Joseph Thomson, 

 entitled " Mass, Energy, and Radiation," which 

 appeared in the Philosophical Magazine for June, 

 1920, is of the greatest interest and importance, 

 not so much for its actual results as for the devel- 

 opment it foreshadows. It is well known that tne 

 relativists operate with symbols and not with 

 physical ideas; this paper is "an attempt to help 

 those who like to supplement a purely analytical 

 treatment of physical problems by one which 

 enables them to visualise physical processes a% 

 the working of a model ; who like, in short, to 

 reason by means of images as well as by 

 symbols. " 



The paper assumes that all mass, that of atoms 

 as well as that of electrons, is distributed through 

 space with a density determined by the electric 

 field at the place where the mass is supposed to 

 exist, and that energy of every kind, kinetic, 

 potential, thermal, chemical, or radiant, is of one 

 and the same type, being the kinetic energy pos- 

 sessed by the particles which are supposed to 

 constitute mass. Transformation of energy is 

 merely the flow of the mass particles from one 

 place to another. Thus, for example, on this view, 

 when a body gains kinetic energy, it is not because 

 any of its mass particles are moving faster ; it is 

 because the mass of the body has been increased, 

 and the increase in the mass implies a proportional 

 increase in the energy. 



We are not yet in a position to calculate the 

 mass of any one of these mass particles, but at 

 least lo^i are required to supply the mass of 

 one electron. If energy is indivisible beyond a 

 certain limit, the inverse square law of electrical 

 attraction cannot hold over more than a certain 

 finite distance. 



The distribution of these particles and their 

 movement from one place to another are determined 

 by the distribution of the lines of electric force. 

 In addition to mass particles it is assumed that 

 there are in the universe lines of force spreading 

 through space, the electron being at one end of 

 a line of force and a unit of positive electricity at 

 the other. The mass particles are concentrated in 

 the places where the electric field is strongest. 

 Thus, for example, if the radius of an electron is 

 io~^^ cm., onlv one-thousandth part of its mass 

 will be at a distance from the electron greater 

 than lo"^" cm. The mass particles perform the 

 functions of both aether and matter. Comparing 

 the physical universe with a living organism, we 

 may regard the mass particles as the flesh and 

 the lines of force as the nervous system. 



A light ray is consequently a jet of particles and 

 lines of force moving sideways, the density of 

 both varviner periodically along the jet. Refrac- 

 tion is exolained bv the action of the secondary 

 waves emitted bv the electrons of the refracting 

 med'"m under the stimulus of the incident wave. 



The paper is noteworthy, because it points the 



