374 SCIENCE PROGRESS 



The negative results indicated so far by the spectroscopic 

 evidence form at present a serious argument against the com- 

 plete acceptance of Einstein's theory. It may also be men- 

 tioned that mathematically there is a difficulty involved in the 

 boundary conditions at infinity not being satisfied. It is 

 possible that the expression which Einstein obtains for the 

 law of gravitation may be the correct one, but that the method 

 of deriving it may require modification. A careful critical 

 examination will be required to ascertain how much of the 

 theory would be affected if the negative spectroscopic evidence 

 were substantiated : diverse opinions have been expressed on 

 this point, and the writer is not in a position at present to give 

 a considered judgment. 



Meanwhile, we must suspend final judgment and remain 

 satisfied that a definite step forward has been made. The 

 field is now open for the solar spectroscopists on the observa- 

 tional side and for the mathematicians on the theoretical side 

 to provide the further material and analysis which will indi- 

 cate whether, and if so in what directions, the theory requires 

 modification. 



Variability of the Sun's Radiation. — For some fourteen years 

 regular observations of the solar constant of radiation have 

 been made at the Smithsonian Astrophysical Observatory on 

 Mount Wilson, California. Before the observations had been 

 continued for many years, it was noticed that the values 

 obtained showed a variation greater than could be attributed 

 to errors of observation, and this suggested that the sun was 

 a variable star. A two-fold variation was indicated : one 

 was of long period, the sun-spot period, with a range of from 

 3 to 5 per cent. ; superposed on this was a short-period irre- 

 gular variation, running its course in intervals of a few days, 

 weeks, or months, and having a range up to 7 per cent. To 

 test the reality of this supposed irregular variation, an expedi- 

 tion was sent to Bassour, in Algeria, and during 191 1 and 191 2 

 simultaneous observations were carried on at Mount Wilson 

 and Bassour. Whilst these observations did not prove con- 

 clusively that the variations found were true variations of the 

 sun's radiation, they supported one another on the whole and 

 indicated that the variations were due to causes outside the 

 earth's atmosphere. 



To test the matter further, in 191 8 another expedition was 

 sent to a station at Calama, Chile, at an altitude of 7,500 feet, 

 which was selected on account of being one of the most cloud- 

 less regions of the world and practically devoid of rain. Simul- 

 taneous observations have since been carried out at this 

 station and at Mount Wilson, and the preliminary results of 

 the computations are given by C. G. Abbot in Proc. Nat. 



