197 
and reduced to ultramicroscopic dimension and as the appearance 
of these particles remains quite the same, we may certainly assume 
that the ultramicrones also are small crystals. 
The double refraction of the Hg sol must, therefore, be attributed 
to the presence of ultramicroscopic tetragonal needles which, when 
the liquid is streaming, arrange themselves parallel to each other. 
Summarizing we thus may say that there exists continuity between 
the crystalline suspensions of Pbl, and HgCl and the colloidal 
solutions of these substances which form in definite circumstances 
and become birefractive when in motion. The double refraction of 
these sols must be attributed to the crystalline structure of the 
ultra-micrones. 
In analogy herewith it is probable that also the ultramicrones of 
the V,O,-sol must be: regarded as micro-crystals. 
Delft. Inorganic and physico-chemical 
Laboratory Technical University. 
“Physics. — “The jield of a single centre in Einsveiy’s theory of 
gravitation, and the motion of a particle in that field” By 
J. Drostz. (Communicated by Prof. H. A. Lorentz). 
(Communicated in the meeting of May 27, 1916). 
In two communications ') | explained a way for the calculation of 
the field of one as well as of two centres at rest, with a degree 
of approximation that is required to account for all observable 
phenomena of motion in these fields. For this | took as a starting- 
point the equations communicated by ErnstEin in 1913 °*). Einsrern has 
now succeeded in forming equations which are covariant for all 
possible transformations’), and by which the motion of the perihelion 
of Mercurv is entirely explained‘). The calculation of the field should 
henceforth: be made from the new equations; we will make a 
beginning by caiculating the field of a single centre at rest. We 
intend to calculate the field completely and not, as before, only the 
terms of the first and second order. After this, we investigate the 
1) Volume XVII p. 998 and vol. XVIII p. 760. 
2) “Entwurf einer verallgemeinerten Relativitätstheorie und einer Theorie der 
Gravitation”, Tevsner. Or: Zeitschrift fiir Mathematik und Physik, vol. 62. 
3) “Die Feldgleichungen der Gravitation” Sitzungsberichte der Kön. Preuss. Akad. 
der Wiss. 1915, p. 844. _ 
4) “Erklirung der Perihelbewegung des Merkur aus der allgemeinen Relativitäts- 
theorie” Sitzungsberichte der Kön. Preuss. Akad. der Wiss. 1915, p. 831. 
