1245 



If a system of coordinates is chosen for which p=|=l, both t/ 

 and the other t/ aie different from zero. Einstein deduced approxi- 

 mated values for the t,/ in a system of coordinates in which tiie 

 velocity of the light is independent of the direction of propaga- 

 tion '), and tiiese expressions show that the r/ are not zero. In this 

 case 1/ can also be calculated by means of formulae (16) and (18). 

 For that system of coordinates in which the velocity of light is in- 

 dependent of the direction of propagation, we have according to 

 Droste"^) for u, p, w outside the body 



u = p^ll +-], w^ = 1 ^ . . (22) 



These expressions can be introduced into (18). The further cal- 

 culation will not be given here. 



The property of the components t,/ that they can be made to 

 vanish by a transformation of coordinates when the field possesses 

 spherical symmetry, is evidently connected with the circumstance, 

 that not for every transformation of coordinates these quantities 

 behave like tensor-componer)ts. It is remarkable, that the change of 

 the system of coordinates, necessary to make the components t/ 

 vanish is undetectably small for really existing gravitation fields. 



This circumstance renders the conception of the quantities t,/ as 

 stress-energy-components somewhat less sympathie, and supports the 

 conception of the gravitation energy enunciated by Lorentz. Whatevei- 

 may be however our opinion on this subject we have no ground to 

 banish the quantities t,/ from Einstein's gravitation theory. As is 

 evident from our considerations in I § 2, several general theorems 

 may be derived in a simple way with the aid of these quantities. 



1) A. Einstein, Naherungsweise Integration der Feldgleichungen der Gravitation, 

 Berl. Bar. 1916, p. 688. 



2) Droste, Hel zwaartekrachtsveld, p. 20, equation (31). 



Proceedings Royal Acad. Amsterdam. Vol. XX. 



86 



