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 two 

 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 ray 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 minor 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 corpuscvles ! but if the upper image appear to 

 the right of the lower — light is produced by ethereal undulations ! To 

 submit this proposed experiment to the test of experimental investi- 

 gation is obviously a diflicult matter, for, as tubes of water of any 

 moderate length can but very slightly aff"ect 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 

 sufiicient 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 sufl&cient for this purpose. 



From computations deduced from the known velocity of light, it 

 appears that in :ry2,oT7o.o^o*'^ P''^'^^ °^^ 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 min-or 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 which 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^ 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 



