in his equation (6) on page 8. 
In order to explain this, I shall in the first place compare the 
different formulae that may be applied to the propagation of light 
in an absorbing gaseous body, placed in a magnetie field. 
§ 2. The equations of Vorer contain the following quantities: 
1°. The components w,v,w of a vector (the vector of NEUMANN) 
which comes into play in all media, the aether itself included, 
which are traversed by light-waves. 
2°. The components §, 7, © of a second vector (the vector of 
FRESNEL), which is related to the former in the way expressed by 
the equations 
dw dv ey du dw _ dv Ou 
(oe 
S= — zen 
dy de Oe ORT Ry Te ay eae e (1) 
3°. A certain number of vectors P}, Ps, Ps, ...., serving to 
determine the state of the ponderable molecules, and each of them 
corresponding to one of the principal modes of vibration of a mole- 
cule. The components of the vector P, are denoted by U,, V; 
Wy, and the index h is likewise affixed to the constant coefficients 
belonging to these different vectors. | 
40. A vector (£, H, Z), which is defined as follows: - 
av Stee, AS vn Se, Vi, Zl Eer Wi. (2) 
Here, the coefficients « are constants, and v is the velocity of 
light in the aether !), 
Between the vectors (5, H, Z) and (u, v, w) there exists a 
relation, expressed by 
Qu dH OZ do ZZ dF Ww dF OH 
Pree ee? de. de aaa pe 
Finally there are a certain number of equations — three for each 
1) In order to avoid confusion, I shall depart a little from the notation of Vorer 
and from that which I myself have used on former occasions. 
