398 Dr. F. P. Kerschbaum : Interference 



and in that of J. J. Thomson by the conception of " quanta" 

 or " units " respectively. 



The Radiation Hypotheses. 



According to the latest form of Planck's hypothesis, only 

 the elementary energy transformation is a discontinuous 

 process. Emission of light — and this emission only — is a 

 quantum process, because the elementary oscillator is only 

 able to radiate energy in amounts e — liv or whole multiples 

 of it. The emitted energy spreads in continuous waves, as 

 on Maxwell's theory, and is absorbed continuously. Planck's 

 hypothesis, therefore, only introduces a discontinuity in time. 

 His view is very successful in its applications to thermo- 

 dynamical problems. For the purely optical phenomena it 

 does not bring anything new. 



J. J. Thomson*, however, assumes a discontinuity in the 

 energy. Radiation energy itself has structure. Not only 

 does the elementary oscillator radiate energy in certain 

 amounts, but this energy is also concentrated in certain 

 parts of space. Radiation energy is built up from "units." 

 J. J. Thomson arrived at his views by imagining that electro- 

 magnetic disturbances can only be propagated along the 

 lines of force. So the light unit represents a disturbance in 

 the lines of force over a cross-section of corpuscular dimen- 

 sions, but of considerable length, carrying a whole train of 

 periodic oscillations. Such units can explain the phenomena 

 of gas ionization by ultra-violet light, the photoelectric effect, 

 and the inter-convertibility of cathode and X-ray energy. 



Einstein f too arrived at the conception of independent 

 light-cells from the consideration that monochromatic radia- 

 tion of not too high intensity may be treated thermodyna- 

 mically like a gas. From this point of view and in ac- 

 cordance with Planck, his light cells are not capable of 

 subdivision. They carry the energy amount € = /iv, where 

 h is Planck's constant. 



In spite of this evident success of energy units for 

 the explanation of the different ionization phenomena, the 

 assumption of a structure of the energy has not been 



* J. J. Thomson, ' Electricity and Matter,' 1903. In this connexion 

 it is perhaps interesting to notice that it is not possible to attribute to a 

 light unit all the same properties as to an equally small volume in a 

 parallel beam of light-waves. As the cross-section of a unit — in order 

 that such a unit may be of any use at all — has to be very much smaller 

 than any wave-length, Huyghens' principle cannot be applied to a beam 

 of this small cross-section. In fact, from Huyghens' principle, it would 

 follow that such a light unit, if coming into existence at all, must 

 disappear at once, the energy being radiated uniformly in space. 



t Einstein, Ann. d. Phys. xvii. p. 132 (1905). 



