in Media of every degree of Transparency. 113. 
We may therefore write in vector notation 
ae woe: D[Ulave > Y [U]ave (7) 
where Mand ¥ denote linear functions.* 
The optical properties of media are determined by the form 
of these functions. But all forms of linear functions would 
not be consistent with the principle of the conservation of 
In all isotropic media not subject to magnetic influence, it is 
probable that @ and ¥ reduce to numerical coefficients, as is 
certainly the case with @ for transparent isotropic media. 
9. Comparing the two values of [E]Jaye, we have : 
e Pot [Ulave — 71 Glave = 2[Ulave + Y[U]ave (8) 
the solenoidal character of the displacements, if we regard them 
a8 necessarily solenoidal, or in connection with that which 
expresses the relation between the electrostatic potential and 
the displacements, if we reject the solenoidal hypothesis, may 
be regarded as the general equation of the vibrations of mono- 
chromatic light, considered as oscillating electrical fluxes. For 
€ symbol Pot, however, we must substitute the symbol rep- 
resenting the operation by which electromotive force is caleu- 
ated from acceleration of flux, with the negative sign, if we 
4 hot satisfied with the law provisionally adopted. 
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rough the medium, will not affect the reasoning by which 
this equation has been established, provided that the nature 
mple in whi ese functi y 
afforded by the phenomena of selective absorption and abnormal dispersion. c 
vector is said to be a linear function of anothe th argent 
of the first omogeneous functions of the first de, the three compone: 
