THE GROWTH OF PHYSICAL IDEAS 113 



Planck's atomic theory of the structure of energy led to a 

 revival of the old argument as to whether radiation was in 

 the form of waves or of streams of particles. According to 

 Planck, radiation was in quanta, each of which had an energy 

 content of Jjv, where h is a universal constant and v is the 

 frequency, that is, the inverse of the wave length, of the radi- 

 ation. This involved a discussion of the physical structure of 

 these quanta— whether they consisted, for instance, of short 

 trains of ^vaves, since the wave structure was implicit in the 

 definition of frequency. Another form of radiation is that of 

 the cathode ray; it is known to consist of streams of electrons. 



Louis V. de Broglie, a gifted French amateur who has de- 

 voted his life to research in physics, suggested that if the 

 structure of radiant energy, which is associated with ^vave 

 length, had an atomic and discontinuous nature, then matter, 

 which obviously is atomic and discontinuous, inight also have 

 properties associated wdth waves. This was confirmed by the 

 experiments of C. J. Davisson and L. H. Germer in the Bell 

 Telephone Research Laboratory. They succeeded in dif- 

 fracting electron beams, w^ork which has been followed by the 

 development of the electron microscope, in which a beam of 

 electrons forms images parallel to those formed by light in 

 a microscope. The t^vo aspects of radiation were finally 

 reconciled by the work of W. Heisenberg and of Erwin 

 Schroedinger, who took up the old ideas of Hamilton with 

 regard to the dualistic aspect of rays and waves and initiated 

 the physical theories classed as quantum mechanics^ into 

 which they introduced the theory of probability. 



The theory of radiation and of the structure of matter has 

 been greatly affected by the development of the relativity 

 theory of Einstein. The adoption of the ether by Augustin 

 Fresnel and Clark Maxwell as the medium in which radia- 

 tion is transmitted led to the suggestion that, since the earth 

 was moving through the ether, the velocity of light as meas- 

 ured by an observer on the earth should be different if it were 

 measured in the direction of the earth's travel or across that 

 direction. This was tested by Albert Michelson in a series 



