W. HarTcness — Magnitude of the Solar System. 243 



returning light wave struck against the tooth following the 

 space through which it issued, to an eye looking into the tele- 

 scope all would be darkness. If the wheel moved a little faster 

 and the returning light wave passed through the space succeed- 

 ing that through which it issued, the eye at the telescope would 

 perceive a flash of light ; and if the speed were continuously 

 increased a continual succession of eclipses and illuminations 

 would follow each other according as the returning light was 

 stopped against a tooth or passed through a space further and 

 further 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 evi- 

 dently 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 velo- 

 city 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 would 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 

 mirror, whence it will be returned to the movable mirror again 

 and being thrown back into the telescope will appear as a star 

 in the center of the field of view. That adjustment being 

 made, if the mirror were caused to revolve at a speed of some 

 hundred turns per second it would move through an appre- 

 ciable angle while the light was passing from it to the distant 

 mirror and back again, and in accordance with the laws of 

 reflection, the star in the field of the telescope would move 

 from the center by twice the angle through which the mirror 

 had turned. Thus the deviation of the star from the center of 

 the field would measure the angle through which the mirror 

 turned during the time occupied by light in passing twice over 

 the interval between the fixed and revolving mirrors, and from 

 the magnitude of that angle together with the known speed of 

 the mirror, the velocity of the light could be calculated. 



In applying either of these methods the resulting velocity is 

 that of light when traversing the earth's atmosphere, but what 

 we want is its velocity in space which we suppose to be desti- 

 tute of ponderable material, and in order to obtain that ,the 

 velocity in the atmosphere must be multiplied by the refractive 

 index of air. The corrected velocity so obtained can then be 



Am. Jour. Sci.— Third Series, Vol. XLVIII, No. 285.— Sept., 1894. 

 16 



