220 LECTURES TO SCIENCE TEACHERS. 



were fixed, the ray would be reflected back in a perfectly 

 definite direction ; but since the mirror is rotating, it will 

 be turned through a sensible angle, and therefore it will 

 reflect the ray back in a slightly different direction. This 

 idea, proposed by M. Arago, was first successfully carried out 

 by M. Foucault, and here we have the apparatus which 

 was used by him in the determination of the velocity of 

 light. This is the instrument he used to give a great 

 velocity of rotation to the revolving mirror. This is 

 simply a syren, an instrument for producing musical notes 

 of different pitch, founded upon a principle first employed 

 by Cagniard De La Tour. It is driven by air or by steam. 

 Foucault employed steam to drive this syren, and the 

 steam or air plays a double part. In the first place, it 

 causes, by an action similar to that of a turbine, the disc 

 to revolve, and with it the associated mirror. In the 

 second place, the air passing through these holes, which 

 allow it to acquire very rapid motion from the air, pro- 

 duces a musical note, and the faster you rotate the mirror, 

 the higher is the tone of the musical note produced. Here 

 is one of a more powerful form, and you will be able to 

 hear that the more rapidly the apparatus is blown the 

 higher will be the note we produce. If we increase the pres- 

 sure you will hear the musical note heighten in pitch. The 

 tone of the note is gradually rising, as we increase the 

 velocity of rotation. Then by means of a tuning-fork 

 M. Foucault was able to tell the number of times this 

 rotated in the course of a second, and the velocity varied 

 from 200 to 800 revolutions. The light was sent from the 

 side just as in M. Fizeau's apparatus. It fell on an inclined 

 mirror, and was reflected through a lens on to a reflector, 

 through that to another reflector, and then back again, so 

 that by the time it got back, the mirror had turned 

 through a sensible angle, and an observer here is able to 

 measure the exact distance through which the beam of 

 light has been deflected. M. Foucault employed here a 

 network of fine platinum wires, eleven to the millimetre, 

 which was illuminated by the light, and consequently when 

 he examined it through this network, he was able to see 

 the network, and he employed glass of such a thickness, 

 that the reflection from the two surfaces of the glass over- 

 lapped; that is to say, that the first reflection of one of 



