238 REPORT—1885. 
of reflexion and refraction on this theory, and after that to make some — 
general remarks on the whole. . 
Chapter III.—Reriexion and ReErraction. 
§ 1. Lommel, so far as I am aware, has not considered the problem 
of the reflexion and refraction of light on his theory. Ketteler, however, 
has discussed it in many of his papers. 
In one of the earlier papers ' the fundamental principles on which he 
intends to work are laid down. They are as follows :— 
I. The conservation of energy. 
Ila. The continuity of the stress parallel to the surface of separation. 
IIb. The continuity of the component of the force on an element 
resolved normal to the surface. 
III. The continuity of the displacement resolved along the surface. 
The reasons given for IIb. in place of the correct principle of the 
continuity of the stress normal to the surface are not very clearly stated. 
No assumption, except such as is implied in I. and III. combined, is 
made as to the displacement normal to the surface. 
The principles are then applied to the general problem, but in express- 
ing them in symbols, except in the case of I., the motion of the matter 
is entirely neglected. Thus the stress considered in II. is only that 
arising from the action of the ether; the part which springs from the 
reaction of the matter is omitted from consideration. Again, in forming 
the equations connecting the amplitudes of the incident reflected and 
refracted rays, IIb. is not employed. 
Ketteler’s work, then, in this paper is not really specially connected 
with his theory of the mutual reaction between the ether and matter. It 
is rather a modification of Fresnel and Green’s work, for which there 
can be no justification assigned. The problem of metallic reflexion is 
discussed, and in a second part? of the same paper that of moving media. 
In the next paper on this subject * the correct principle of the continuity 
of the stress normal to the bounding surface is introduced in place of one 
of the other conditions, but it is supposed that the term involving the 
dilatation disappears in consequence of the incompressibility of the ether ; 
in reality, as Green showed, the coefficient of that term is very large, and 
it must be retained to give correct results. Ketteler fails to see this, and 
hence concludes that the retention of Green’s longitudinal wave is 
unnecessary. He then considers, as Green had done, the problem of total 
reflexion ; and, through not taking into account the continuity of the dis- 
placement normal to the surface, appears to be able to do without the 
jongitudinal waves. The motion of the matter particles does not come 
into consideration. 
Another series of surface conditions are given in the next paper on the 
subject, and the matter particles being treated merely as a sort of 
1 Ketteler, ‘ Beitrige zur einer endgiiltigen Feststellung der Schwingungsebene 
des polarisirten Lichtes,’ Wied. Ann. t. i. p. 206. 
2 Ketteler, Wied. Ann. t.i. p. 556. 
8 Ketteler, ‘Zur Theorie der longitudinalen elliptischen Schwingungen im incom- 
pressiblen Ether,’ Wied. Ann. t. iii. pp. 83, 284. See also Theoretische Optik, p. 130. 
4 Ketteler, ‘ Ueber den Uebergang des Lichtes zwischen absorbirenden isotropen 
und anisotropen Mitteln und tiber die Mechanik der Schwingungen in denselben,’ 
Wied. Ann. t. vii. p. 107. 
