124 



VELOCITY OF LIGHT. 



Arago made an effort, by tlie use of a similar mirror, to institute a comparison 

 between the velocities of light in passing through water and through air. This 

 wa^s suggested by him as an experimentmn crucis between the opposing theo- 

 rie.s current in regard to the nature of light ; in one of which lif^ht was sup- 

 posed to consist of material particles actually thrown off by luminous bodies, 

 while in the other it was assumed to be an effect of undulations propagated 

 through an exceedingly subtle elastic medium pervading all space. If the first 

 were tlic true theory, the velocity of light in a more powerfully refracting 

 mt'dium should be greater than in a less ; and the reverse, if the second were 

 true. Mr. Arago did not carry out his design to its completion, but it has since 

 been successfully executed by both ]\Ir. Fizeau, whose original method has just 

 been given, and by Mr. Foucault, well known for his pendulum demonstration 

 of the earth's rotation. The experiment served at once to compare the veloci- 

 ties of light in air and water, and to determine the absolute velocity. The an- 

 nexed figure may render the method intelligible. Suppose a small beam of 



Fig. 12. 



parallel rays to be admitted into a room otherwise dark, through an aperture R, 

 and to fidl upon an achromatic lens, fixed at C, in the direction of its axis. Let 

 CD be the focal distance of this lens ; and at M let the beam be intercepted by a 

 mirror capable of turning around a vertical axis coincident with its plane. At 

 a distance, ME=MD, in any convenient position not very remote from R, let 

 there be placed a spherical mirror having its centre of curvature in the axis of M. 

 Let there be, further, across the aperture R a fine wire exactly vertical ; 

 and in front of the aperture R a transparent plane mirror AB, inclined 

 to the beam at an angle of 4o^. The mirror M may obviously be turned 

 on its pivot, so that the ray RCjI falling upon it may be reflected to E. If 

 it remain stationary in this position, the light incident on E will be returned to 

 M, and so again to the aperture R. But a portion of this returning light, being 

 reflected by AB towards 0, will enable an observer at that point to see the im- 

 age of the vertical wire. To assist the eye, a magnifying eye-piece may be em- 

 ployed, and this may bo provided with a spider-line micrometer at its focus. If 

 the miri'or be now put very slowly into revolution, the image of the wire will be 

 seen intermittently and .momentarily, once in every revolution. The spider-line 

 of the eye-piece is now to be brought to exact coincidence with this image. 

 Accelerating the revolution, when the number of turns per second becomes as 

 great as ten, the image will be permanent. If, now, a very high velocity be 

 given to the mirror, the image seen by the observer at will no longer coincide 

 with the spider-line of the micrometer, but will be seen at a sensible distance from 

 it in the direction of rotation. Thus, if the arrow represent the direction of 

 rotation, the returning ray which originally met the mirror A B at ;•, will meet it 

 at r' or r", and the image which originally appeared at S wnll be seen at S' or S". 

 This is evid-^'Utly owing to the change in tlae position of the mirror M, while 



