158 Intelligence and Miscellaneous Articles, 
According to the Newtonian hypothesis, or corpuscular theory of 
light, a ray moves in a fluid of higher refractive power, as water, 
quicker than in air, in a ratio expressed by the direct relation of 
the sine of the angle of incidence to the sine of refraction ; whilst 
on the undulatory theory, light traverses a liquid much slower than 
air, and in the inverse ratio of these sines. To test the truth of either 
of these hypotheses, all that is therefore necessary, is to cause tw^o 
rays of light to be incident on a rapidly- revolving mirror, in the 
same vertical line, the lowest beam traversing air only, whilst the 
upper one passes through a tube filled with water, or other refracting 
liquid. Under these circumstances, if the corpuscular theory of light 
be correct, the upper ray will reach the mirror before the lower one, 
and consequently the reflected images will no longer be in the same 
vertical line : if the undulatory hypothesis be the true one, then the 
lower ra}?- will reach the mirror before the upper one, and the verti- 
cal position of the reflected images will become deranged. Let us 
suppose that the mirror revolves in a direction from right to left ; 
then, if the upper reflected image appear to the left of the lower, 
light consists of moving corpuscules ! but if the upper image appear to 
the right of the lower — light is produced hy ethereal undulations ! To 
submit this proposed experiment to the test of experimental investi- 
gation is obviously a difficult matter, for, as tubes of water of any 
moderate length can but very slightly affect the velocity of a ray 
of light, it is obvious that the rotation of the revolving mirror must 
be excessively rapid to produce a deviation of the reflected images 
sufficient to admit of accurate observation. — M. Aragois inclined to 
consider that a deviation of a minute of a degree produced by two 
positions of the reflecting plane inclined half a minute of a degree 
upon each other will be sufficient for this purpose. 
From computations deduced from the known velocity of light, it 
appears that in ^ 2 ,ono.oo ^ second (the time during which 
a revolving mirror moving by mechanism prepared by M. Gambey 
moves through half a minute of a degree) a ray of light traverses a 
portion of space corresponding to 7 '07 metres, or, in round numbers, 
23 English feet. Hence if the mirror perform 2000 rotations in a 
second of time, the tube of water through which one of the rays 
passes must be 23 feet in length to produce, on the theory of emis- 
sion, an angular separation of the reflected images corresponding to 
one minute. As a velocity of the revolving mirror of the enormous 
quantity of 2000 turns in a second is extremely difficult to obtain, 
the angular deviation produced by reflection from one revolving 
mirror performing 1000 rotations in a second, may be increased by 
viewing the images afterwards in a second revolving mirror. In this 
manner, by repeated reflexions from several revolving mirrors, the 
angular deviation of the images v/hich eventually reach the eye of 
the observer may be increased to a sensible quantity. 
If, as M. Arago has thought probable, an angular separation of the 
reflected images equal to half a minute is perceptible to the eye, a tube 
only 1 1 j feet long, full of water, will be sufficient to produce such an 
alteration in the velocity of the transmitted ray, as to render the an- 
gular separation of the images very obvious, after reflection from 
