oil the Velocity of Light. 545 



shutter, and to fall upon the plane sui-face of a reflecting mirror 

 set perpendicular to the direction of the light, the latter will be 

 sent back along the path by which it entered. If the reflecting 

 siu'face be oblique to the direction of the light, the latter will be 

 reflected in some other direction ; supposing a second reflecting 

 mirror to be set perpendicular to this latter direction, the light 

 will be reflected from this in the direction of the perpendicular, 

 will again strike the other mirror, and be flnally sent back by 

 the latter through the apertm-e by which it entered. In this 

 case the ray suffers two reflexions from the intermediate mirror ; 

 and if it be true that light requires time in passing from one 

 point to another, these two reflexions cannot occur contempo- 

 raneously. A certain portion of time, however small, will be 

 required for the journey to and fro from one mirror to the other. 

 Supposing, for instance, the mirrors to be placed six feet apart ; 

 the light proceeding from the aperture is received upon the first 

 mirror and reflected by it on to the other ; from this it is reflected 

 back again, and thus accomplishes a journey of twelve feet between 

 its two reflexions by the first mirror. To this journey, as has been 

 said, time is necessary. If the aperture and the two mirrors be 

 perfectly motionless, the path of the light in coming will coincide 

 with its path in returning ; but if, while on its route between the 

 two mirrors, we conceive the position of the first mirror to be 

 changed, that, for instance, it has become more inchned to the 

 direction of the ray, the latter will not be reflected in the line of 

 its approach, but will be thrown against the window-shutter 

 more or less to the side of the apertiu'e. This change in the 

 position of the mirror during the almost infinitesimal portion of 

 time occupied by the light on its twelve-feet journey is accom- 

 plished by imparting to the mirror a high angular velocity, say 

 a thousand revolutions in a second. We here find ourselves in 

 possession of a means of comparing the velocity of light in air 

 with its velocity in water. When the mirror rotates, the ray 

 sent back does not strike upon the aperture, but more or less to 

 the side of it. The less time occupied by the light in performing 

 its double journey between the two mirrors, the less ought this 

 divergence to be, and vice versa. Hence, if the Newtonian theory 

 be true, the introduction of a column of water six feet long ought 

 to bring the reflected image of the aperture nearer to the aper- 

 ture itself; and if the undulation theory be true, the introduc- 

 tion of such a column ought to make the divergence greater. 

 These speculations have been recently submitted to the test of 

 experiment, and the result has pronounced in favour of the theory 

 of undulation. 



Of course such experiments, though easily described and 

 simple enough in principle, demand considerable delicacy of ma- 



