7S Intelligence and Miscellaneous Articles, 



definite position in reference to the mirror and to the sight itself. 

 This position is precisely that of the virtual image of the sight, seen 

 by reflexion in the plane of the mirror. This, at any rate, is what 

 occurs when the apparatus is at rest. 



But when the plane mirror rotates, this image changes its place, 

 seeing that during the time in which the light takes to traverse twice 

 the broken line of the concave mirrors, the rotating mirror continues 

 to rotate, and that the rays, on their return, do not meet it under 

 the same incidence as at the moment of arrival. It follows that the 

 image is displaced in the direction of the rotation, and this displace- 

 ment increases with the velocity of the rotation ; it also manifestly 

 increases with the length of the path, and with the distance which 

 separates the sight from the rotating mirror. The manner in which 

 these various quantities affect the experiment as well as the velocity 

 of light itself, are expressed by a very simple formula, which has 

 been already established, and which I shall only have to recall here. 



Calling V the velocity of the light, n the number of turns of the 

 mirror in a second, I the length of the broken line comprised 

 between the rotating mirror and the last concave mirror, r the 

 distance of the sight from the rotating mirror, and d the observed 

 displacement, we get, by considering the structure of the apparatus, 



rr__S7rnlr 



v — r~' 



an expression which gives the velocity of light by means of quan- 

 tities "which must be separately measured. 



The distances I and r are measured directly by a rule, or by a 

 paper band, which is then referred to the unit of length. The dis- 

 placement may be observed micrometrically ; it remains to be shown 

 how the number of turns (n) is estimated which the mirror makes in 

 a second. 



Let us first describe how a constant velocity is imparted to the 

 mirror. 



This mirror of silvered glass, 14 millims. in diameter, is mounted 

 directly on the axis of a small air turbine of known construction, 

 admirably executed by M. Froment. The air is furnished by a high- 

 pressure bellows made by M. Cavaille-Coll, who has justly acquired 

 a high reputation in the manufacture of large organs. As it is im- 

 portant that the pressure be very constant, the air on emerging from 

 the bellows traverses a regulator, recently devised by M. Cavaille, 

 and in which the pressure of a column of water of 30 centimetres 

 does not vary -J- a millimetre. Hence the fluid emerging from the 

 orifices of the turbine represents a remarkably constant motive 

 power. On the other hand, the mirror, in accelerating, meets with 

 a resistance in the surrounding air, which for a given velocity is 

 also perfectly constant. Hence the moveable body placed between 

 these two constant forces cannot fail to assume and retain a uni- 

 form velocity. Any stop acting on the flow of air renders it pos- 

 sible to regulate this velocity within very narrow limits. 



