(49) 
at that side, which is covered during the nignt, has been confirmed 
by the photobacteria-method respecting the secretion of oxygen. Bo, 
the clover-leaf closes at night by putting the upper surfaces of the 
leaflets against one another: hence these surfaces must exhibit a 
more energetic secretion of water-vapour, and in the light, of 
oxygen, then the under surfaces, which has been confirmed by the 
experiment. 
For Robinia pseud-acacia, where at night the under surfaces cover 
each other, the most vigorous secretion of oxygen is to be expected 
in the light from the underside, which is likewise confirmed by the 
experiment. But with Robinia the difference is less considerable than 
with clover. 
Physics. — Mr. Frep. Scrum on: “Plane waves of light in an 
homogencous, electrically and magnetically anisotropic dielectric.” 
(First part). 
1. If P, Q and R are the components of the electric force €, 
and /, g and & the components of the electric induction D (4% times 
the dielectric displacement) both expressed in electric units, then we 
have, in the case of an electrically anisotropic medium, the relations: 
Pari hy ehh geer lit ee 
Q = hye f + ky g + hyeh » « « « « « « (2) 
tee = Koo va “+ hy g a foe h 3 . . . . . ° (3) 
in which kay == kyr, hoz = hen, yz = key. The electric (potential) 
energy per unit volume is: 
ee 
U = ae di bose ays: Sas ette eee) 
The surface: 
hoa #® A Ieyy y® 4 hog 2? + 2 keys ye + 2 hye me A Blo wy = 1 
is an ellipsoid (with O as centre), because U > 0, if f, gandh are 
not all zero. I shall call this the electric ellipsoid. If D is a radius 
1 BA’ 
vector of this ellipsoid, then Ue = et € is then normal to the 
TU 
diametral plane conjugate to 2, which we shall call the electrically 
conjugate diametral plane. The radius vector 7, of this ellipsoid 
has the following meaning: 
4 
Proceedings Royal Acad. Amsterdam, Vol. IV. 
