Second and Third Order Tests of the "Mther Drift. 3 ' 11 



would be possible. With the former conditions, and allowing 

 a frequency in fluctuation of 10 per second for good resolution 

 to the eye, a difference in speed not greater than one part in 

 a million of the two eclipsing systems would be requisite. 

 While this seems extremely small, experience shows that such 

 an approximation is entirely practicable. Thus Xewcomb* 

 records a " run " in his measurement of the velocity of light, 

 in which his micrometer showed " beautiful bisection " during 

 the greater portion of the duration (2 minutes) of such a " run/' 

 Allowing this setting to one part in ten of his unit, which 

 was 2 //# 4, out of a total deviation of 7500" of arc for a period 

 of 90 seconds, we have a fluctuation in the speed of only one 

 part in 27,000,000. If the two mirrors could be regulated 

 to this degree, we should still have less than one-tenth the 

 fluctuation of our limit, from the two mirrors combined. 

 Hence, so far as speed regulation is concerned, a much higher 

 eclipse frequency, say the forty-million limit, is possible. 

 The other disturbances would be of the same order as 

 encountered by Newcomb over his total " go " and l ' return " 

 distance of 5000 m. (Supposing the eye could detect a dif- 

 ference in intensity of tw r o per cent, between the two fields 

 (under very favourable conditions this sensibility is one-half 

 of one percent.), we should need an interval corresponding to 

 one hundred eclipses to detect a change in velocity equal 

 to the aberration constant, since we have to add the effects 

 from each ray. This would mean a distance between the 

 mirrors of 3000 m. for the lower and 750 m. for the higher 

 eclipse frequency referred to, which is much within the 

 distance given above in Newcomb's experiment. 



Of the other methods proposed f, several do not require a 

 return of the ray in the determination of the velocity constant. 

 Thus the rotation of a polarizing system, such as a half -shade 

 nicol or tourmaline system, could be carried up to 3000 

 revolutions per second. Polishing machines are now run up 

 to 2000 revolutions per second. Allowing a sensibility of 

 0°'01, a distance of 15 kilometres would give a variation in 

 velocity of one part in ten thousand. 



The objection to the above methods and other similar ones, 

 is the great distance required. If we could increase the 

 eclipse frequency, the distance could be reduced accordingly. 

 This can be attained by the use of electric oscillations in con- 

 junction with suitable optical systems. Two methods proposed 



* Astronomical Papers of American Ephenieris, vol. ii. pts. hi., iv. 

 p. 172. 



t All the methods referred to in the paper were devised as far back as 

 1889-90, and have been developed experimentally to a greater or less 

 extent since. 



