1929 L DE BROGLIE irradiated body: one is thus led to the theory of light quanta proposed Einstein in 1905 and which is, after all, a reversion to Newtons corpuscular theory, completed by the relation for the proportionality between the en- ergy of the corpuscles and the frequency. A number of arguments were put forward by Einstein in support of his viewpoint and in 1922 the discovery by A H. Compton of the X-ray scattering phenomenon which bears his name confirmed it. Nevertheless, it was still necessary to adopt the wave theory to account for interference and diffraction phenomena and no way whatsoever of reconciling the wave theory with the existence of light cor- puscles could be visualized As stated, Planck's investigations cast doubts on the validity of very small scale mechanics. Let us consider a material point which describes a small tra- jectory which is closed or else turning back on itself. According to classical dynamics there are numberless motions of this type which are possible com- plying with the initial conditions, and the possible values for the energy of the moving body form a continuous sequence On the other hand Planck was led to assume that only certain preferred motions, quantized motions, are possible or at least stable, since energy can only assume values forming a discontinuous sequence. This concept seemed rather strange at first but its value had to be recognized because it was this concept which brought Planck to the correct law of black-body radiation and because it then proved its uitfulness in many other fields. Lastly, it was on the concept of atomic mo- tion quantization that Bohr based his famous theory of the atom; it is so familiar to scientists that i shall not summarize it here The necessity of assuming for light two contradictory theories-that of waves and that of corpuscles-and the inability to understand why, among the infinity of motions which an electron ought to be able to have in the atom according to classical concepts, only certain ones were possible: such were the enigmas confronting physicists at the time I resumed my studies of When I started to ponder these difficulties two things struck me in the main. Firstly the light-quantum theory cannot be regarded as satisfactory since it defines the energy of a light corpuscle by the relation W= hu which con- ains a frequency D. Now a purely corpuscular theory does not contain any element permitting the definition of a frequency. This reason alone renders it necessary in the case of light to introduce simultaneously the corpuscle concept and the concept of periodicity246 1929 L.DE BROGLIE irradiated body: one is thus led to the theory of light quanta proposed by Einstein in 1905 and which is, after all, a reversion to Newton’s corpuscular theory, completed by the relation for the proportionality between the en￾ergy of the corpuscles and the frequency. A number of arguments were put forward by Einstein in support of his viewpoint and in 1922 the discovery by A. H. Compton of the X-ray scattering phenomenon which bears his name confirmed it. Nevertheless, it was still necessary to adopt the wave theory to account for interference and diffraction phenomena and no way whatsoever of reconciling the wave theory with the existence of light cor￾puscles could be visualized. As stated, Planck’s investigations cast doubts on the validity of very small scale mechanics. Let us consider a material point which describes a small tra￾jectory which is closed or else turning back on itself. According to classical dynamics there are numberless motions of this type which are possible com￾plying with the initial conditions, and the possible values for the energy of the moving body form a continuous sequence. On the other hand Planck was led to assume that only certain preferred motions, quantized motions, are possible or at least stable, since energy can only assume values forming a discontinuous sequence. This concept seemed rather strange at first but its value had to be recognized because it was this concept which brought Planck to the correct law of black-body radiation and because it then proved its fruitfulness in many other fields. Lastly, it was on the concept of atomic mo￾tion quantization that Bohr based his famous theory of the atom; it is SO familiar to scientists that I shall not summarize it here. The necessity of assuming for light two contradictory theories-that of waves and that of corpuscles - and the inability to understand why, among the infinity of motions which an electron ought to be able to have in the atom according to classical concepts, only certain ones were possible: such were the enigmas confronting physicists at the time I resumed my studies of theoretical physics. When I started to ponder these difficulties two things struck me in the main. Firstly the light-quantum theory cannot be regarded as satisfactory since it defines the energy of a light corpuscle by the relation W = hv which con￾tains a frequency v. Now a purely corpuscular theory does not contain any element permitting the definition of a frequency. This reason alone renders it necessary in the case of light to introduce simultaneously the corpuscle concept and the concept of periodicity