SCIENCE. 



[N.S. Vol. XVI. No. 394. 



The dispersion in air was selected, and a 

 combination of the arrangements of both 

 Fizeau and Foucault which had occurred 

 to the writer before was adopted. The 

 essential condition in Fizeau "s method is to 

 produce an intermittence in a beam of light. 

 This is done meehauieally by the rotation 

 of a toothed wheel.* It is quite clear that 

 if the wheel were fixed and the ray rotated 

 the condition of intermittence would be 

 fulfilled. It would then be merely a matter 

 ■of arranging a suitable optical system to 

 maintain a fixed direction for the ray while 

 in transit between the two stations. Any 

 such optical system would avoid the inertia 

 inherent in a mechanical system and would 

 thus allow of much greater speed and con- 

 sequent sensibility. Through the courtesy 

 of the Secretary of the Navj-- and the active 



seemed to be well suited for a preliminary 

 experiment on account of the rectangular 

 shape of the rotating mirror and the num- 

 ber of reflecting faces available. Figures 1 

 and 2 show the arrangement used and the 

 path of the ray. The original apparatus 

 was changed slightly, the two telescopes T 

 and T' being shifted so that their axes 

 passed through the middle of one set of 

 faces when these stood at 45° to the same. 

 The other additions were the lens L the 

 grating G and the plane mirror M. The 

 instrument was originally mounted so 

 as to use the concave mirror belonging to 

 the instrument itself, which had a radius of 

 curvature of 3,000 meters. Owing to im- 

 provements about the campus it became 

 necessary to remove the piers and discon- 

 tinue the experiment for several years. It 



Fig I 



assistance of Professor Newcomb, the photo- 

 tachometer of the latter was secured to 

 carry out this experiment at the University 

 of Nebraska instead. This instrument 



* In the March number of the Philosophical 

 Magazine for this year, Professor Miohelson de- 

 scribes a similar arrangement which occurred to 

 him independently during his experiments on 

 the motion of the ether. 



was again finally mounted in the basement 

 of the Physical Laboratory on a much 

 smaller scale and the flat mirror M used at 

 a distance of about fifteen meters from the 

 rotating mirror R. The lens L was a tele- 

 scope lens of one meter focus and ten centi- 

 meters aperture. The concave mirror 6 

 had a radius of curvature of one meter and 



