395 



vapour curve of lliis equilihrium a similar point of the vapour 

 saturation curve under its own vapour pressure. 



If one of the pi'operties mentioned under 'i. or 2. appears, then, 

 reasoning backwards, the three-phase reaction, mentioned above, 

 will also take place in the four-phase equilibrium. 



Above we have considered onl}' the case that the volume increases 

 at the congruent or incongruent melting of the complex F -\- F' \ 

 corresponding properties make their appearance when the volume 

 decreases. These the reader can easily deduce for himself. 



In the same way in which we deduced above the properties oi 

 two saturation- and vapour saturation curves under their own vapour 

 pressures, the reader can consider the appearance of two boilingpoint 

 curves and their corresponding vapour curves. 



Astronomy. — ^'On the constancy of the velocity of Uglit". By 



Prof. W. DE vSlTTER. 



In my communication to the meeting of February of this year 

 (see these Proceedings, Vol 15, page 1297) I pointed out that the 

 existence of spectroscopic doubles whose motion obeys the laws of 

 Kepler, is incompatible with the theory of Ritz, while in agreement 

 with that of Lorentz. 



Since then Messrs. P. Guthnick ') and E. Freündlich ') have 

 brought forward the hypothesis that the velocity of light might 

 depend on the velocity of the source in a manner differing from the 

 simple addition postulated by the theory of Ritz. The most simple 

 hypothesis would be 



V r=i c ■■{- x?/., 



where v is the velocity of light emitted by a source having the 



velocity u. The problem then is no longer to decide whether ?« = 



or 3c = l, intermediate values being excluded, but to assign an 



upper limit to x. 



We have then, using the notations of my former paper 



A 

 a -— -a — . 



If the true orbit is a circle, then the equation (1) becomes : 



u — u^cos — {t-f,) (1) 



If 5c is very small we find for the equation (2) the following 

 approximate expression 



1) Astronomische Kriterien fur die Unabhangigkeit der Fortplanzungsgcschwindig- 



keit des Lichtes von der Bewegung der Lichtquelle, Astr. Nachr. 4670 (195, 265). 



~) Zur Frage der Konstanz der Lichtgeschwindigkeit, Physik. Zeitsclir. 14, 835. 



