MOUNT WILSON OBSERVATORY. 281 



1921. The measured position angle was then 251 ?9, the distance 

 0''0836. Subsequent measures during March and April showed the 

 distance to be decreasing at the rate of 0''003 per month, and the posi- 

 tion angle about one degree per month. Thus it appears that the com- 

 ponents will approach much closer in the near future. 



About 80 other stars have been examined for change of visibility 

 with position angle of the slits. These include spectroscopic binaries, 

 variable stars, stars with composite spectra, and some bright stars 

 chosen at random. For two or three of these objects probable varia- 

 tions were noted, but for nearly all no change in visibility could be 

 detected. 



VELOCITY OF LIGHT. 



In the autumn of 1920 Mr. Pease and Mr. Ellerman carried out some 

 tests of the apparatus described in last year's report, using the 

 Mount Wilson station and one located on the hills near Whittier. 

 Although the return beam was observed, it became clear that more light 

 was necessary and that the adjustment of the distant mirror was 

 inadequate. It was also evident that the site selected for the distant 

 station was unsuitable on account of the dust and haze prevalent 

 over the San Gabriel Valley at low elevations. 



As a means of avoiding the difficulty of frequent readjustments in 

 securing a return of the light from the distant station, the following 

 modification was devised by Professor Michelson. The light, instead 

 of being returned directly by the distant mirror, which in the earlier 

 plan had a radius equal approximately to the distance between the 

 stations, is brought to a focus by a concave mirror 22 inches in diameter 

 and 30 feet in focal length. The focal image is formed at the surface 

 of a small concave mirror of 30-feet focus, whence the light retraces 

 its path and an unage is formed which would coincide with the slit 

 source where it could be observed by the aid of a half-silvered plane 

 parallel plate. In order, however, to avoid direct illumination of this 

 plate and the revolving mirror, the return beam is received on another 

 plane mirror, which allows it to be reflected from the opposite face of 

 the revolving mirror. 



This apparatus has been tested in a preliminary way between 

 stations at the Pasadena laboratory and in the foothills at Altadena, a 

 distance of 3 miles. As a second station to be used in connection 

 with that on IMount Wilson for measurements of the velocity of light, 

 a point has been selected on the San Antonio ridge near a lookout 

 maintained by the Forest Service. The elevation of this station is 

 6,800 feet, its distance from Mount Wilson 22 miles, and atmospheric 

 conditions appear to be exceptionally favorable. 



THE RELATIVE MOTION OF THE ETHER. 



If a pencil of fight is divided into two parts by a half-silvered plate 

 of glass and these are brought back to the starting-point by suitably 



