I'RESIDENTIAL ADDRESS SECTION A. 47 



changed. It has been shown that all " confornial " transforma- 

 tions in Minkowski's four dimensional space-time leave the forms 

 of the accepted electro-magnetic equations unchanged — i.e., these 

 equations take a [Minkowski vector form. 



At the end of 1915 Einstein completed his generalized 

 *' Relativity " work on the theory of gravitation. The old New- 

 tonian equations are greatly modified by the introduction of 

 factors involving the ratio z'/c. " Mass "' is no longer 

 simple. There is " rest mass " and " motion mass." The mathe- 

 matical work tends to be very complicated, being effected by 

 the theory of tensors (generalized vectors). 



Newton's simple results appear as a first approximation : 

 fortunately, the effects of the next order of approximation on 

 the main results of solar system work is infinitesimal, except in 

 the one outstanding discrepancy — the acceleration of 43 seconds 

 per century in the orbit of ^lercury. The new theory accounts 

 completely for this phenomenon, which has been inexplicable 

 on the old theory. " Relativists " ( who are treated now with 

 much respect owing to their success in solving- this and other 

 hitherto " insoluble '' ]>roblems) claim that their theory will do 

 away with the theoretical necessity of an aether, and that " hence- 

 forth space and time will be reduced to mere shadows, and only 

 a sort of union of the tw'o will retain reality." The new light 

 on things has been com])ared to the enlightenment of dwellers on 

 a cloudy planet, from which the stars are invisible, by their 

 reaching- a conception of the rotation of their framework (the 

 planet), hitherto unsuspected. 



Einstein's results may he summarized thus : Gravitation 

 appears in our mathematical work merely as conditioning the 

 framework: hence the elfect of a strong gravitational field is 

 equivalent to a change of co-ordinates (polar for Cartesian, 

 moving axes for fixed, etc. ) : hence in a str.ong gravitational 

 field light will appear not to travel in a straight line ; moreover, 

 light vibrations will be affected; the spectrum will be shifted to- 

 wards the red. These last two definite results can be tested by 

 observations. No opportunity of a crucial test has yet presented 

 itself; but at the next total eclipse (May 29, 1919) faint stars 

 will be photographed near the sun ; if Einstein is right these will 

 seem to be displaced as much as 1.7" from their usual positions 

 owing to their proximity to the sun. Thus the eclipse will 

 receive from physicists an attention even keener than usual ; 

 instead of interesting details as to the comi>ositi.)n of the sun, 

 the corona, the prominences, etc., the fate of a new intellectual 

 theory, a philosophical push towards Truth, the most ambi- 

 tiotis and promising since Newton, will be in the balance. 



