104 ON THE MAGNITUDE OF THE SOLAR SYSTEM. 



Passing now to tlie plioto-tacliymetrical methods, we bave first to 

 glance briefly at the mechanical appliances by which the tremendous 

 velocity of light has been successfully measured. They are of the sim- 

 plest possible character, and are based either upon a toothed wheel or 

 upon a revolving mirror. 



The toothed-wheel method was first used by Fizeau, in 1849. To 

 understand its operation, imagine a gun barrel with a toothed wheel 

 revolving at right angles to its muzzle in such a way that the barrel is 

 alternately closed and opened as the teeth and the spaces between them 

 pass before it. Then, with the wheel in rapid motion, at the instant 

 when a sjiace is opposite the uuizzle let a ball be fired. It will pass out 

 freely, and after traversing a certain distance let it strike an elastic 

 cushion and be reflected back upon its own path. When it reaches the 

 wheel, if it hits a space it will return into the gun barrel, but if it hits 

 a tooth it will be stopped. Examining the matter a little more closely 

 we see that, as the ball requires a certain time to go and return, if 

 during tliat time the wheel moves through an odd multiple of the angle 

 between a space and a tooth the ball will be stopped, while if it moves 

 through an even multiple of that angle the ball will return into the 

 barrel. !Now imagine the gun barrel, the ball, and the elastic cushion to 

 be replaced, respectively, by a telescope, a light wave, and a mirror. 

 Then if the wheel moved at such a speed that the returning light wave 

 struck against the tooth following the space through which it issued, to 

 an eye looking into the telescope all would be darkness. If the wheel 

 moved a little faster and the returning light wave passed through the 

 space succeeding that through which it issued, the eye at the telescope 

 would perceive a flash of light, and if the speed was continuously 

 increased a continual succession of eclii)ses and illuminations would 

 follow each other according as the returning light was stopped against 

 a tooth or passed through a space farther and farther behind that 

 through which it issued. Under these conditions the time occupied by 

 the light in traversing the space from the wheel to the mirror and back 

 again would evidently be the same as the time required by the wheel 

 to revolve through the angle between the space through which the light 

 issued and that through which it returned, and thus the velocity of 

 light would become known from the distance between the telescope and 

 the mirror, together with the speed of the wheel. Of course the longer 

 the distance traversed and the greater the velocity of the wheel the 

 more accurate Avould be the result. 



The revolving-mirror method was first used by Foucault in 1862. 

 Conceive the toothed wheel of Fizeau's apparatus to be replaced by a 

 mirror attached to a vertical axis and capable of being put into rapid 

 rotation. Then it will be possible so to arrange the apparatus that 

 light issuing from the telescope shall strike the movable mirror and be 

 reflected to the distant jnirror, whence it will be returned to the mov- 

 able mirror again, and being thrown back into the telescope will appear 



