1354 
~ 
must also be deferred. I shall conclude now by remarking that, as 
an immediate consequence of Hamiiton’s principle, the world-line of 
a material point which is acted on only by agiven gravitation field, 
will be a geodetic line, and that the equations which determine the 
gravitation field caused by material and electromagnetic systems will 
be found by the consideration of infinitely small variations of the 
indicatrices, by which the numerical values of all quantities that 
are measured by means of these surfaces will be changed. 
Physics. — “On Einstein's Theory of gravitation.” Il. By 
Prof. H. A. Lorenz. 
(Communicated in the meeting of March 25, 1916). 
§ 15. In the first part of this communication the connexion 
between the electric and the magnetic force on one hand and the 
charge and the convection current on the other was expressed by 
the equation 
JAR NT FIRE NTO =i feted, aS Le 
which has been discussed in § 138. It will now be shown that this 
formula is equivalent to the differential equations by which the con- 
nexion in question is expressed in the theory of Einstein. For this 
purpose some further geometrical considerations must first be deve- 
loped. They refer to the special case that the quantities gq, have 
the same values at every point of the field-figure. 
If this condition is fulfilled, considerations which generally may 
be applied to infinitesimal extensions only are valid for finite 
extensions too. 
§ 16. The factor required, in the measurement of four-dimen- 
sional domains, for the passage from a-units to natural units has 
now the same value at every point of the field-figure. Similarly, 
when any one-, two- or three-dimensional extension in the field- 
figure that is determined by linear equations (“linear extensions”) 
is considered, the factor by means of which the said passage may 
be effected for parts of that extension, will be the same for all 
those parts. Moreover the factor in question will be the same 
for two “parallel” extensions of this kind, ie. for two extensions 
the determining equations of which can be written in such a way 
that the coefficients of z,,...e, are the same in them, 
