527 
of curves at the one and n-+ 1 bundles of curves at the other side. 
The (J/)-bundle contains four curves at least. 
In following communications we shall apply those rules in order 
to deduce the types of the P,7-diagram, which may occur in the 
different systems of m components. 
Leiden. Inorg. Chem. Lab. 
(To be continued). 
Mechanics. — “On the relativity of rotation in Einstein's theory.” 
By Prof. W. pr SITTER. 
(Communicated in the meeting of September 30, 1916). 
Observations have taught us that the relative accelerations of 
material bodies at the surface of the earth differ from those which 
would be caused by Nrwron’s law of gravitation only. The difference 
is explained by Newton's law of inertia, combined with the rotation 
of the earth relatively to an ‘absolute space’. Newton') quite deli- 
berately introduces tbis absolute space, and also absolute time, as 
an element of his explanation of observed phenomena. Many objections 
have been raised against it, all of which are based on the logical 
claim that a true causal explanation shall involve only observable 
quantities. It has been tried to replace the absolute space by the 
fixed stars, by the “Body Alpha’, ete. All these substitutes are, 
however, entirely hypothetical and quite as objectionable, or more 
so, as the absolute space itself. 
Einstein’) also rejects the absolute space, but he apparently still 
clings to the “ferne Massen”. It appears to me that Einstein has 
made a mistake here. The “Allgemeine Relativitätstheorie” is in 
fact entirely relative, and has no room for anything whatever that 
would be independent of the system of reference. The need for the 
introduction of the distant masses arises from the wish to have the 
gravitational field zero at infinity in any system of reference. This 
wish, however well founded in a theory based on absolute space, 
is contrary to the spirit of the principle of relativity. The best way 
to show this clearly is to consider the fundamental tensor gj. We 
will, to simplify the argument, negleet the mass of the earth. This 
does not affect the fundamental question, if only we imagine the 
1) Principia, Definitiones, Scholium. 
2) Die Grundlagen der allgemeinen Relativitdtstheorie, Annalen der Physik, 
Band 49, p. 772 (separate edition p. 9). 
