NA TURE 



{May 29, 1884 



Assume now that light occupies no time in travelling from 

 the lamp to the first mirror, through the first tele-cope, across the 

 space between the two telescope-, and back again after its 

 reflection by the second mirror. Assume, in fact, that the 

 velocity of light is infinite, then it is perfectly clear that an 

 observer would keep on seeing that star of light whether the 

 wheel remained at rest or were put in motion. But now assume 

 that light does take a certain very small time to make the journey 

 spoken of, and that the wheel can be turned with just such a 

 velocity that when the light reaches it on its return it will meet, 

 not an opening, but one of the cogs. Then the light would not 

 be visible ; it would find itself a cog behind, so that, if light 

 travels very fast indeed and the wheel is made to travel with a 

 great and known velocity and the relation existing between the 

 velocities be known, the velocity of light can be measured in this 

 way. That is the way in which Fizeau mea-ured it, and he gave 

 the velocity as being 190,000 miles per sec rnd. 



It may be thought perhaps that this being the fir-t attempt in 

 a matter of this kind it was not very worthy of credit ; but the 

 similarity of the results which have been obtained in all such 

 experiments proves that they are all very worthy of credit, and 

 that this velocity must be accepted as established within narrow 

 limits. 



We come now to Foucault, the man to whose genius science 

 owes the experimental proof of the earth's rotation, to which 

 reference has already been made. He also attacked this ques- 

 tion of the velocity of light. Going to work in quite a different 

 way from Fizeau, he succeeded in enriching science with a 

 method quite as reliable in its operation and as accurate in its 

 results. 



A pencil of light coming from a slit at s (see Fig. 40) impinges 



\ 





ement for determining the velocity of light. 



upon the plane mirror R, which is capable of turning round a ver- 

 tical axis. This mirror reflects the light falling on its surface, and 

 the action of the lens, L, causes an image to be formed on the 

 surface of the concave mirror, M, the centre of which coincides 

 with the axis at R. This concave mirror reflects the image 

 backwards on its path to the slit. Foucault's arrangement, as ha: 

 been said, was to have the mirror, R, made to rotate. If, there- 

 fore, R be turned about its axis while the light from the slit, s, is 

 falling upon its surface, for so long as the light falls on the lens 

 so long will the image of the slit be formed on the surface of 

 the distant mirror. Similarly for so long as the reflected image 

 falls upon the lens, so long will the image be reflected back 

 to the slit. Now if the mirror were made to rotate rapidly, and 

 light were infinite in its velocity, then once during each revolu- 

 tion of the mirror at one particular angle the light would be 

 reflected back to the slit ; but assume that light takes some very 

 small fraction of time to travel over the space between the 

 mirrors, it will be observed that the image will not be reflected 

 back to the slit but will suffer a deflection in one direction or the 

 other according as the mirror turns from left to right or from 

 right to left, and, the velocity of the rotating mirror being 

 known, the amount of this displacement will enable the velocity 

 of light to be determined. 



With two such different methods it might be supposed that 

 the results obtained were very different. Not so, however ; the 

 velocity obtained by Fizeau was, as I have said, 190,000 miles 

 per second, that by Foucault 185,000 per second. 



It so happens that both these methods have been gone over 

 quite recently, Fizeau's method by another Frenchman, M. 

 Cornu, and Foucault's by Mr. Michelson, an officer in the 

 American navy. 



Mr. Michelson modified Foucault's method somewhat, the 

 fault in which was that the displacement obtained was so ex- 

 tremely small, being but the fraction of a millimetre ; and when 

 it is remembered that the image is always more or less indistinct 

 on account of atmospheric conditions and imperfection in the 

 lenses and mirrors employed, it will be seen that it was difficult 



for Foucault to attain to any very great accuracy. Mr. Michel- 

 son therefore used an apparatus which would give him a 

 greater deflection than that obtained by Foucault. As before, s 

 (Fig. 41) was the slit, R the rotating mirror in the principal focus 



Michel on's variation of Foucault's experiment. 



of the lens, but the distant mirror, instead of being concave, was 

 a plane one, and the lens one of great focal length, for a reason 

 that will appear immediately. This lens, in consequence of the 

 smallness of its diameter in comparison with its great focal length, 

 was not entirely convenient. In order that the displacement 

 should be great, it is necessary that the distance between R 

 and M, the distance from the revolving mirror to the slit, and the 

 speed of rotation should be the greatest possible. 



Unfortunately, the second condition clashes with the first, 1 

 for the distance from the revolving mirror to the slit, or the 

 " radius" is the difference between the distances of principal and 

 conjugate focus for the distant mirror M, and the greater the 

 distance the smaller the radius. T«vo methods were employed 

 by Mr. Michelson in overcoming this difficulty : first, he had 

 his lens of great focal length, 150 feet, and he placed the re- 

 volving mirror, not at the principal focus, but fifteen feet within 

 it. He thus managed to get a distance between the mirrors of 

 2000 feet with a radius of thirty feet, and his mirror made 256 

 revolutions per second. He then obtained a deflection of 133 

 millimetres, that being about 200 times greater than the deflec- 

 tion obtained by Foucault. This deflection he measured to 

 within three or four hundredths of a millimetre in each obser- 

 vation. 



Mr. Michelson's experiments were made along an almost level 

 stretch of sea wall at the Naval Academy. 



We are therefore justified in Faying, as the result of these 

 experiments of Fizeau and Cornu, Foucault and Michelson, that 

 light has a velocity of wme iS6,ooo miles per second. 



If that be so, then, if the statement that the earth revolves 

 about the sun be true, this must follow. In Fig. 42 a, />, c, d 



t J^t 



Fig. 4?. — Annual change of a star's p isiiion, due to aberration : a b c d, the 

 earth, in different parts of its orbit ■ a b' c' d' t the corresponding aberra- 

 tion places uf ihe star, varying from ihe true place in the direction of the 

 earth's motion at the time. 



represent the earth in different parts of its orbit around 

 the sun ; the contention is that if there be this revolution 

 of the earth round the sun, and if light really travels with 

 anything short of an infinite vel icity, then the position of a star 

 must change, for the reason that ihe telescope of the astronomer 

 must always be pointed in advance of the star to catch its light 

 in the same way that to catch the falling weight we had to 

 incline the tube in the direction of its motion. 



When any observation is made on any star in the heavens, the 

 telescope of the astronomer must therefore be pointed in advance 

 of the star to catch its light, and taking, as in the diagram, four 

 different points in the earth'.-, orbit, it is obvious that the tele- 

 scope at these four different points must be pointed in four dif- 

 ferent directions with regard to the star. For instance, if we 

 take a point at c, where the earth is travelling in the diiection 

 of the arrow, and the point at w hich the star would be seen if 

 the earth were at rest, or the velocity of light were infinite, 

 be indicated by the star in the figure, c' is the direction in which 

 the star would be seen, and in which the astronomer's 

 telescope must be pointed to catch its light. Similarly with 

 the earth at d the telescope must be pointed to d', and so with 

 the earth at a we must have it pointing towards a'. It was 

 this strange anomaly which puzzled Dr. Bradley in the year 1729. 

 ■ For full details of Michelson's experiments see Nature, vol. xxi. p. 94 

 et seq 



