RECENT ADVANCES IN SCIENCE 553 



with Einstein's own view as stated in his article in the Times. 

 Jeans, in the opening address referred to above, considers, 

 however, that the difficulty can be avoided by abandoning 

 Einstein's interpretation of ds, the fundamental invariant of 

 the theor}^, as a line-element in a distorted space-time con- 

 tinuum. To the present writer the theory becomes unin- 

 telligible when this view is adopted. On the other hand, 

 on Einstein's conception of force, a displacement of the solar 

 lines of exactly the same amount as required by the generalised 

 relativity theory should be expected on Newton's law of gravi- 

 tation. Those interested in the matter will find that the 

 argument in § 34 of Prof. Eddington's report holds equally 

 well for either law of gravitation, since g u is the same for 

 both. The displacement was actually predicted by Einstein 

 in 191 1, before his generalised theory had been developed. 

 The possible existence of the shift, therefore, ceases to some 

 extent to be a crucial phenomenon of the theory. If it is 

 definitely shown that there is no shift, we should not be free 

 from difficulties even accepting Newton's law. The situation 

 is therefore full of interesting possibilities which must be left 

 for the future to unravel. 



The Sources of Stellar Energy. — It is well known that no 

 explanation has yet been given of the source of energy in the 

 sun (and therefore in any star) which will account for the age 

 of the earth, as deduced from geological evidence. The hypo- 

 thesis that the energy is derived from gravitational contraction 

 gives an age which is altogether too short ; nor are matters 

 improved much when radio-activity is taken into account. 

 Various other suggestions have from time to time been ad- 

 vanced, but none has yet been found capable of providing a 

 satisfactory .explanation. The various possibilities are dis- 

 cussed in a suggestive manner by Prof. H. N. Russell in Pub. 

 Ast. Soc. Pac., 31, 205, 191 9. 



The suggestion has been put forward that the rate of radia- 

 tion from a hot body to space is considerably less than it 

 would be to a material enclosure at the absolute zero. Russell, 

 however, using the Smithsonian Institution data relative to the 

 amount of energy received by the earth from the sun, shows 

 that the earth radiates heat to space at a rate which is cer- 

 tainly not much less than the rate at which it would radiate 

 to a material enclosure at absolute zero. On the other hand, 

 the ratio required to reconcile the probable age of the sun 

 with a purely gravitational supply of energy is of the order 

 of only o-ooi . It is improbable that the difference in behaviour 

 is attributable to any appreciable extent to the difference in 

 wave-length of the emitted radiation, and it is therefore con- 

 cluded that the sun and stars have actually radiated many 



