76 Prof. D. B. Brace on Negative Results of 



determining the difference of time required by two inter- 

 fering rays to traverse circuits in opposite directions, would 

 require a path one kilometre square in a horizontal plane to 

 give a displacement equal to *7 of a band for latitude 45°. 

 This would necessitate a verv high degree of refinement 

 indeed over any previous attempts at interference. The 

 method of determining the velocity of light which I devised 

 as far back as 1889, and tried a number of years ago*, and 

 which consisted of an eclipsing system made up of a rotating 

 double mirror and a grating, thus combining the principles 

 of the toothed wheel and rotating mirror methods of Fizeau 

 and Foucault, would be delicate enough to show a variation 

 of one part in ten thousand, providing synchronism could be 

 maintained during a short interval of time. With a suitable 

 system of mirrors and observing telescopes, a single observer 

 would be able to bring into his field of view both beams of 

 light after their passage through the eclipsing systems. If 

 now one of the rotating mirrors were either gaining or losing 

 on the other, the observer would see, alternately, eclipses of 

 one ray and the other, if their times of transit were different. 

 If their times of transit were the same, then the two fields 

 would maintain a constant relative intensity, each going 

 through its maximum and minimum simultaneously as the 

 relative phases of the eclipsing systems varied. The latter 

 would correspond to the condition of a moving, the former to 

 that of a quiescent aether. Thus the experiment would be 

 possible, even if perfect synchronism were not attainable, but 

 only sufficiently so to make the frequency of the successive 

 maxima in the field of view less than a few times a second, 

 or slow enough for the eye to resolve the fluctuations of 

 intensity. If, by means of mirrors, a common source of light 

 were employed, the half-shade principle in the field of view 

 could be used which would be ver} r sensitive in showing any 

 difference in intensity (even if rapidly fluctuating) between 

 the two portions of the field due to any slight difference in 

 the time of propagation of the two rays with and against the 

 aether drift. If we take the conditions in the experiment 

 referred to f , namely, an aperture of 2*5 cm. and a distance of 

 '02 cm. between the lines of the reflecting grating, with a 

 radius of 1 m. and 250 revolutions per second, 10,000,000 

 eclipses per second could be obtained; and, if we carried this 

 to the limit in speed and resolving power, four times as many 



* Vice-Pres. Address, Sect. B, Amer. Assoc. Adv. Sci., Pittsburg 

 Meeting, 1902 ; Science, July 18, 1902. 



t This apparatus was the photo-tachometer, modified for the purpose, 

 which Newcomb used in determining the velocity of light. 



