190 THE QUANTUM THEORY 



Different views may be held as to how the prize- 

 drawing is conducted on the sweepstake theory. Some 

 hold that the lucky part of the wave-front is already 

 marked before the atom is reached. In addition to the 

 propagation of uniform waves the propagation of a 

 photon or "ray of luck" is involved. This seems to me 

 out of keeping with the general trend of the modern 

 quantum theory; and although most authorities now take 

 this view, which is said to be indicated definitely by 

 certain experiments, I do not place much reliance on the 

 stability of this opinion. 



Theory of the Atom. We return now to further experi- 

 mental knowledge of quanta. The mysterious quantity 

 h crops up inside the atom as well as outside it. Let us 

 take the simplest of all atoms, namely, the hydrogen 

 atom. This consists of a proton and an electron, that is 

 to say a unit charge of positive electricity and a unit 

 charge of negative electricity. The proton carries nearly 

 all the mass of the atom and remains rock-like at the 

 centre, whilst the nimble electron moves round in a 

 circular or elliptic orbit under the inverse square-law 

 of attraction between them. The system is thus very 

 like a sun and a planet. But whereas in the solar system 

 the planet's orbit may be of any size and any eccentricity, 

 the electron's orbit is restricted to a definite series of 

 sizes and shapes. There is nothing in the classical 

 theory of electromagnetism to impose such a restriction; 

 but the restriction exists, and the law imposing it has 

 been discovered. It arises because the atom is arranging 

 to make something in its interior equal to h. The inter- 

 mediate orbits are excluded because they would involve 

 fractions of /*, and h cannot be divided. 



But there is one relaxation. When wave-energy is 



