(156 ) 
200 nh Sh=bs, everything is much simpler. If the motion 
of light is given e.g. by a point P of the electric part of 
the wave-surface, which is not the point of intersection with the 
X-axis, we find that D lies in the plane through P and the X-axis, 
and touches therefore the meridian through P; DS lies in a plane 
parallel to the Y Z-plane and so touches the parallel-ellipse. The 
reverse takes place when P lies on the magnetic part of the wave- 
surface. If however, P lies on the X-axis, i.e. in the point where 
the two parts of the wave-surface touch each other, the ray is the 
X-axis and the wave-front the Y Z-plane, © and D® being now 
indeterminate in the wave-front (they must, however, be doubly 
conjugate to each other). 
If finally 2; = 6, = bs, the two parts of the wave-surface coincide. 
In this case to every wave-front belongs one ray and vice versa, 
D and DS being always indeterminate in the wave-front. 
Physics. — Mr. Pu. KorNsraMM and Mr. B. M. van DALFSEN: 
“Vapour-tensions of mixtures of ether and chloroform’. 
(Communicated by Prof. J. D. VAN DER WAALS). 
For our determinations of vapour-tensions for mixtures of ether 
and chloroform we have made use of the dynamical method, i.e. 
we have determined the boiling-point at a certain pressure. As the 
methods of the determination of the vapour-tension, and specially 
the apparatus used by us, will be the subject of an extensive com- 
munication by one of us, which will appear before long, it seems 
superfluous to discuss these two points at length. Yet we will point 
out, specially to show how far our values are to be trusted, that we 
found it impossible to attain an accuracy greater than 1 m.m. mereury 
for dynamical determinations of vapour-tensions of mixtures. The values 
given are therefore at the utmost only in so far accurate ; the errors of 
some of the observations can even become three or four times the 
amount. This is specially due to two sources of error, first the 
hydrostatie pressure of the boiling liquid, the influence of which was 
already pointed out by Dr. Sarrs in the reports of these proceedings '), 
and secondly the superheating. We have tried to annul the disturbing 
1) Volume II p. 475. 
