150 ESSAY ON THE VELOCITY OF LIGHT. 
cf emission and that of vibration or undulation. We cannot do better than 
here allow Arago to speak for himself. ‘The following is what he says in the 
notice printed in the proceedings of the mecting :* 
‘*T propose to show in this communication how it is possible to decide, unequivocally, 
whether light be composed of little particles emanating trom radiating bodies, as Newton 
supposes, and as the greater part of modern geometers admit; or whether it is simply the 
result of the undulations of a very rare and very elastic medium which physicists have 
agreed to call ether. The system of experiments which I am about to describe will no 
longer permit, it seems to me, to hesitate between these two rival theories. It will decide 
mathematically, (L use designedly this expression ;) it will decide mathematically cne of the 
grandest and most debated questions of natural philosophy. 
‘‘ Besides, my communication is the fuifilling of a sort of engagement to the Academy I 
accepted at one of its last secret sittings. piel omelet 
“fT discussed the admirable method, by the aid of which Mr. Wheatstone attempted the 
solution of the problem of the velocity of electricity over metallic conductors. I had hardly 
terminated the enumeration of the important results obtained by that ingenious physicist, 
when several of our members, whose names are authority in such matters, stated that my 
report was far too approbative. ‘In supposing it well determined, the inferior limit assigned 
by Mr. Wheatstone to the velocity of electricity will not have,’ said one, ‘any marked 
influence on the progress of the sciences; besides, limits of the same order, and even more 
extensive, can be deduced indirectly from various electric or magnetic phenomena, As to 
che method of the revolving mirrors, it does not seem to be susceptible of application, but to 
the simple questions already studied by the inventor.’ I tried to refute this last opinion. I 
believed myself that the new instrument, suitably modified, would lead to results that Mr. 
Wheatstone was not aware of. I already foresaw that, even in supposing it enclosed in 
the narrow limits of a small room, it could serve to measure the comparative velocities of 
light moving through air and through a liquid. I was not slow in learning, and without 
hhaving hardly the right to be astonished or to complain that my assertion had been received 
with incredulity. Nevertheless, I intend to vindicate it to-day in all its parts. 
‘‘Principle of the method: Let a ray of light fall upon a plane polished mirror; it will 
be reflected, as every one knows, in forming with the surface of the mirror an angle of 
reflection exactly equal to the angle of incidence. 
‘*Let us now suppose that the mirror turns through an are @ around the point of its 
surface from which the reflection takes place. If this motion, for example, increases by the 
quantity a, the original angle of incidence, it will diminish as much the original angle of 
reflection. The latter will, therefore, after the displacement of the mirror, be smaller than 
the first by the quantity 2a; thus it must be increased 2a to render it equal to the new angle 
of incidence; hence that angle increased 2a will give the direction of the reflected ray in the 
second position or the mirror; and thus the incident ray remaining the same, an angular 
motion @ of the mirror occasions a double angular motion in the reflected ray. 
‘This mode of reasoning applies as well to the case where the motion of the mirror, 
acting in a contrary direction, would diminish the first angle of incidence. The principle 
is, therefore, general; and it is also that of all reflecting nautical instruments. 
‘The redection from the plane mirrors can serve to project the luminous rays in all parts of 
space, without, however, altering the relative positions; two rays parallel before reflection 
will be parallel after their reflection; those at first inclined to cach other 1 minute, 10 
minutes, or 20 minutes, &c., will form precisely the same angle after the reflection has 
deviated them. ‘ 
‘‘ Instead of a single ray, let us consider two horizontal rays setting out from two neigh- 
boring points situate in the same vertical. Admit that they strike on two points of the 
median line (also vertical) of a plane vertical mirror. Suppose that this mirror revolves on 
itself uniformly and in a continuous manner around a vertical axis whose prolongation 
coincides with the median line just mentioned, the direction in which the twe horizontal 
lines will be reflected will depeud evidently upon the moment they may reach the mirror, 
since we have supposed that it turns. If the two rays have set out simultancously from the 
two contiguous radiating points, they will also reach simultaneously the mirror. ‘Their 
reflection will take place at the same instant; consequently in the same position of the 
turning surface; consequentiy as if that surface was stationary with respect to them. 
Therefore their primitive paraliclism will not be changed. 
“Tn order that the rays which primitively were parallel may diverge after their reflection, 
it is necessary that one of them should arrive at the mirror later than the other. It is 
necessary that in its course from the radiating point to the reflecting and turning surface, 
the velocity of the ray should be accelerated, or what will be precisely the same thing, it is 
necessary (the velocity of the tirst ray remaining constant) that that of the second should 
experience a diminution. It is necessary, finally, that the two rays should be reflected one 
after the other; and, consequently, from two distinct positions of the muror. forming with 
each other a sensible angle. 
* Comptes Rendus des Séances de l’Académie des Sciences. t. vii, p-. 94. 
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