194 THE QUANTUM THEORY 



in hydrogen are calculated by classical laws; but one 

 of the purposes of the calculation is to verify the 

 association of energy and period in the unit /*, which is 

 contrary to classical laws of radiation. The whole 

 procedure is glaringly contradictory but conspicuously 

 successful. 



In my observatory there is a telescope which con- 

 denses the light of a star on a film of sodium in a photo- 

 electric cell. I rely on the classical theory to conduct 

 the light through the lenses and focus it in the cell; then 

 I switch on to the quantum theory to make the light 

 fetch out electrons from the sodium film to be collected 

 in an electrometer. If I happen to transpose the two 

 theories, the quantum theory convinces me that the light 

 will never get concentrated in the cell and the classical 

 theory shows that it is powerless to extract the elec- 

 trons if it does get in. I have no logical reason for 

 not using the theories this way round; only experience 

 teaches me that I must not. Sir William Bragg was not 

 overstating the case when he said that we use the classi- 

 cal theory on Mondays, Wednesday and Fridays, and 

 the quantum theory on Tuesdays, Thursdays and Satur- 

 days. Perhaps that ought to make us feel a little sym- 

 pathetic towards the man whose philosophy of the uni- 

 verse takes one form on weekdays and another form on 

 Sundays. 



In the last century — and I think also in this — there 

 must have been many scientific men who kept their 

 science and religion in watertight compartments. One 

 set of beliefs held good in the laboratory and another set 

 of beliefs in church, and no serious effort was made to 

 harmonise them. The attitude is defensible. To discuss 

 the compatibility of the beliefs would lead the scientist 

 into regions of thought in which he was inexpert; and 

 any answer he might reach would be undeserving of 



