210 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



standing, and their results prove beyond a doubt the reality of the 

 predicted effect. 1 



IV. NEW THEORY BASED ON POSITIVE RESULTS. 



The older form of the theory of relativity was based upon the 

 result of very precise observations, but upon negative results — upon 

 the failure to find things which ought to have been found, and easily 

 found, provided that the older theories had been correct. 



But the new extension of the theory is based upon positive re- 

 sults—the presence of an effect, in the case of the planet Mercury, 

 which though long known baffled all explanation, and in the case of 

 the eclipse observations, upon the presence of an unquestionable and 

 very remarkable influence whose existence no one anticipated or 

 imagined until it was predicted by the theory. 



It therefore appears to be very strongly established. 



It is true that the original form of Einstein's theory also predicted 

 that the position of the lines of any element, such as iron, in the 

 solar spectrum should be slightly different from those produced by 

 the same element in the laboratory. At the present time it is very 

 hard to say whether this effect has been observed or not. 



The positions of the lines in the spectrum can indeed be measured 

 very accurately. But there are a variety of influences at work on 

 the sun's surface which may shift the positions of these lines, such 

 as the pressure in the sun's atmosphere, actual motion of this atmos- 

 phere, and possibly a host of other things, so that different lines of 

 the same element are shifted by different amounts and in spite of 

 years of investigation of this exceedingly complex problem it is not 

 possible yet to explain all the things that have been observed. 



It is, therefore, still uncertain whether, after these other causes 

 are allowed for, it would be found that the lines in the sun's spec- 

 trum were shifted or not. It seems probable, however, that Ein- 

 stein's theory could be modified in such a manner as to account for 

 the other effects already observed without demanding the existence 

 of this one. Hence this can hardly be called at the present time a 

 failure of the Einstein theory. 



The mathematical expression of this last portion of Einstein's 

 theory is the part which is so intricate and difficult. 



Mathematicians, whose minds are saturated with conceptions with 

 which the layman is utterly unfamiliar, find that these mathematical 

 expressions may be (to them at least) most simply described in terms 

 of space of four dimensions, or even of five dimensions in certain 

 cases. 



1 See "A determination of the deflection of light by the sun's gravitational field, from 

 observations made at the total eclipse of May 29, 1919," by Dyson, Eddington, and David- 

 son. Smithsonian Report for 1919, pp. 133-176. 



