447 
racemic mixed crystals and racemic compounds. If, for instance, in 
the system of d and /carvoxim the three-phase tension is to be 
determined, one might think, in connection with the above that we 
can perhaps come to a conclusion as to the much discussed question 
whether we are dealing here merely with a maximum in a series of 
mixed erystal or whether we are dealing with a racemic compound 
giving continuous mixed crystal series with the antipodes. Meanwhile 
it is shown on closer investigation that the resolving of the problem 
cannot be expected in that manner. 
6. The peculiar form of the three-phase line and the correlated 
spacial figure give rise to different theoretical considerations. We 
hope to soon revert to the matter, also in connexion with the dis- 
cussion in the previous paragraph. 
Utrecht, June 1916. vAN “T Horr- Laboratory. 
Physics. — “The field of n moving centres in Einstein's theory of 
gravitation”. By J. Droste. (Communicated by Prof. H. A. 
LORENTZ). 
(Communicated in the meeting of June 24, 1916). 
1. If in one or other field of gravitation there is placed a particle, 
Le. a body so small that, though influenced by the field, it does 
not itself exercise any influence on the field, it will move in such 
a way that the first variation of the time integral of 
bs (= Jij vi ay) 
tJ 
calculated after some definite way, is zero. Here v,=t, and so 
v,=1. If a, ,, @, are small with respect tot unity (i.e. nearly the 
velocity of light), g,, will be much larger than say gijn” We 
will call a term one of the first order if, after division by the square 
of a component of a velocity, it gets a moderate value. Now, as in 
Newron’s theory, which accounts for the phenomena very closely, 
it follows from the equation of energy that a term, multiplied by 
the constant of gravity z, is of the same order as the square of a 
velocity, we will call also such a term one of the first order and 
consequently a term, which contains x’, of the second order. 
Our purpose is the calculation of £ up to the terms of the second 
order inclusive. If there is no body whatever that can produce a 
field, we shall have 
