( 676 ) 
two points of the body being the same in the two cases. The rotation 
would therefore be independent of the translation, always provided 
we compare cases in which the frequency in a point of the body 
has a definite value. 
§ 9. What precedes calls forth two questions. In the first place: 
can a substance, like the one we have supposed, really have the 
rotatory property? And, secondly, if this be so, is the picture we 
have formed of the substance, the only one that agrees with the 
phenomena, or are there others, equally satisfying ? 
The answer to the first question must undoubtedly be affirmative. 
Within the limits of the hypotheses of § 8 there is room fora large 
variety of optical properties, which may depend either on the form 
of the connexion between the electric force and the moment of a 
single particle, or on the relative position of the different particles, 
and a peculiar arrangement may very well produce a rotation of the 
plane of polarization. For this it is only necessary that the structure 
of the system should be asymmetric, i.e. that the system should not 
be in every respect equal to its reflected image. If, in such a case, 
we consider the electric interaction between neighbouring particles, 
we shall have to introduce into the equations certain terms of a 
rotational character. As a simple example of the required structure 
we may take a molecule containing 4 unequal particles situated at 
the angles of an asymmetric tetrahedron, and each of which may be 
electrically polarized. 
As to the second question, it is clear that in real bodies there 
may very well be circumstances, differing from those we have sup- 
posed in § 8. We may e. g. conceive a movable electron, situated at 
one angle of the asymmetrie tetrahedron, to be subject not only to 
the electric action of a moment, situated at one of the other angles, 
but also to a force of some other kind (“molecular” force), issuing 
from that angle. If, in such a case, the action between two elements 
of matter A and B were such that the action on A at the local 
time tf were determined by the state of B at the same local time, 
what has been said about two corresponding states might still be 
true. But this need no longer be so, if the action on A at the time 
t depends on the state of B at that same instant. 
However this may be, it must certainly be deemed possible that 
after all the rotation is not altered by a uniform motion of the active 
substance ; this possibility would however be excluded if we began 
by omitting in the equation (1) the term with &. 
——— 
ee ee ee 
c——  —_. 
Ee — se 
