86 Mr. J. H. Jeans [March 30, 



sponding size — shall we not be justified in supposing that we are not 

 altogether on the wrong track in believing that the fishes really 

 exist ? 



Allowing for the necessary imperfections of an analogy, evidence 

 of the kind just described is provided by the photo-electric effect. 

 High-frequency light falHng on a clean metallic surface breaks up the 

 atoms of the metal, the breakage being shown by the liberation of 

 electrons. The energy required to liberate an electron from an atom 

 of any given metal is known from other sources — this corresponds to 

 the mesh of the net ; in addition to this, the electron brings away with 

 it a certain amount of kinetic energy. On adding this to the energy 

 required to liberate the electron we invariably obtain one quantum of 

 energy of the frequency of the incident light. If the light is of 

 frequency such that one quantum is less than the energy required to 

 liberate an electron we may allow the light to fall on the metallic 

 surface for years, and no atoms are In-oken up, no matter how intense 

 the light, while even the feeblest light, if of sufficiently high 

 frequency, will at once start to liberate electrons. 



Such phenomena as the photo-electric effect and the line-spectra 

 of the elements are quite inexplicable in terms of the old dynamics, 

 while they receive such complete and convincing explanations in 

 terms of the quantum-theory that we might be inclined to jump to 

 the conclusion that the quantum-theory contained the whole truth 

 and nothing but the truth. On the other hand, all interference- 

 phenomena and phenomena of reflection, polarisation, etc., receive a 

 simple explanation in terms of the old dynamics, and seem at present 

 inexplicable in terms of the new. It seems impossible to reconcile 

 the new quantum-theory with the old undulatory theory of light. 

 There is a very real difficulty here ; indeed, it constitutes the big out- 

 standing puzzle of present-day physics. The evidence of interference 

 suggests that light must be continuous and almost infinitely divisible, 

 while the evidence of the photo-electric effect is that light consists 

 of discrete " quanta " which are discontinuous and completely in- 

 divisible. 



No solution has yet been found ; one has hardly been suggested. 

 It may perhaps be worthy of notice that the theory of relativity is 

 also to some extent antagonistic to the undulatory theory of light, for 

 when the medium through which the waves were supposed to be 

 propagated is aljolished, the waves themselves reduce to little more 

 than a mathematical fiction, and the undulatory theory of light 

 reduces to the solution of a differential equation. In the past the 

 waves in the ether have been regarded as the ultimate reality, while 

 the differential equation has been regarded merely as a means of 

 calculating the motion of the wave. Perhaps the scientist of the 

 future will regard the differential equation as the ultimate reality, 

 while the whole mechanism of the undulatory theory — ether, forces, 

 waves, interference, etc. — will be regarded as an extraordinarily 



