22 



t«(/,;,/, = -- 6h<^ ^ (abcf) g^f> Facf l^f ^ {abc) g^^ Par' Fbh', 



where for the sake of simplicity it has been assumed that 1^ — ff=^- 

 Further we have 



/ ab \ rdb -> 



If now our formulae (96) and (97) are likewise simplified by the 

 assumption ^^ — ^^=1 (so that Q becomes equal to G), we may 

 expect that t' will become identical with t^j^y This is really so in 

 the case (/«/, ^ for a =/= b ; by which it seems very probable that 

 the agreement will exist in general. 



In the preceding paper it was shown already that the stress- 

 energy-components t/,e do not form a "tensor", but what was called 

 a "complex". The same may be said of the quantities \'h^ defined 

 by (96) and (97) and of the expressions given by Einstein. If we 

 want a stress-en ergy-^(^?2.s'or, there are only left the quantities t^Q^ 

 defined by (86) and (57), the values of which are always equal and 

 opposite to the corresponding stress-energj^-components Ja^ for the 

 matter or the electromagnetic field. 



It must be noticed that the four equations 



^ ie) ^ (Ih + t%)h ) = 



always express the same relations, whether we choose t^oA, th^, ^'h^ 

 or ^^{E)h as stress-energy-components "^^(r^h of the gravitation field. 

 If however in a definite case we want to use the equations in order 

 to calculate how the momentum and the energy of the matter and 

 the electromagnetic field change by the gravitational actions, it is 

 best to use t'^h or t^[E)h, just because these quantities are homo- 

 geneous quadratic functions of the derivatives gab,c- 



Experience namely teaches us that the gravitation fields occurring 

 in nature may be regarded as feeble, in this sense that the values 

 of the gab'^ are little different from those which might be assumed 

 if no gravitation field existed. For these latter values, which will be 

 called the "normal" ones, we may write in orthogonal coordinates 



Ö'ii =.9,2=5'8s = — 1' .^44=c^ gab = ^, for a=,= b. (98) 

 In a first approximation, which most times will be sufficient, the 

 deviations of the values of the ^«^'s from these normal ones may 

 be taken proportional to the gravitation constant 3c. This factor 

 also appears in the differential coefficients gab,c; hence, according to 

 the character of the functions t' jf mentioned above (and on account 



