MOUNT WILSON OBSERVATORY. 



253 



nents are 4.62 and 3.65 times the solar mass. The observations show 

 the following residuals when compared with the adopted orbit : 



As a fm-ther test of the precision of the method, a longer series of 

 observations should be secured as soon as possible. 



VELOCITY OF LIGHT. 



It is estimated that the uncertainty in the value of this fundamental 

 constant is of the order of 30 km., or 1 part in 10,000. When deter- 

 mined by the method of Foucault with the revolving mirror, the result 

 depends on the measurement of three quantities, namely, the dis- 

 tance between the stations, the speed of the revolving mirror, and the 

 angular displacement of the returning beam by the revolving mirror. 

 The first and second of these quantities can probably be measured to 1 

 part in 250,000, possibly to 1 in 1,000,000; but this can not be affirmed 

 of the angular displacement. This difficulty may, however, be elimi- 

 nated, or rather transferred to the construction of a revolving mirror 

 with angles of 90° or 135°, which has been found possible to an order 

 of accuracy of 1 in 1,000,000. Since the mean of all four (or eight) 

 angles is the determining factor (the effect of these errors being merely 

 to broaden the return image by a very minute quantity), the actual 

 error in the resulting determination due to this source will be still less. 



The method of observation adopted by Professor Michelson is to give 

 the mirror such a speed that the returning light is received on the next 

 succeeding face of the mirror, the image of the slit source being kept on 

 the cross-hair of the eyepiece. The speed of the mirror is measured 

 simultaneously by optical comparison with a tuning-fork, whose rate 

 is ultimately ascertained by comparison with an astronomical clock. 

 Measurement of the zero is eliminated by alternate clockwise and 

 counter-clockwise rotations. It was found possible to obtain a speed 

 of 1 ,500 to 2,000 revolutions per second. With a speed of 1 ,000 per sec- 

 ond and a rotation of 90°, the required distance between the stations is 

 37.5 km., which is somewhat less than that between the Mount Wil- 

 son Observatory and Mount San Antonio. 



