PROBLEMS OF ASTROPHYSICS 463 



(6) The adoption of a compact and rigid form of spectrograph 

 mounting designed in accordance with good engineering practice. 



(7) The elimination of flexure effects by supporting the spectro- 

 graph, in connection with the telescope, in accordance with engineer- 

 ing principles. The conventional spectrograph had been supported 

 entirely at its extreme upper end; the instrument projected out into 

 space, unsupported, boldly inviting flexure under the varying com- 

 ponent of gravity. 



(8) The use of a constant temperature case around the instru- 

 ment. 1 



(9) Precautions taken to eliminate many sources of error from the 

 measures of the spectrograms. 2 



Up to December, 1900, the last month of the departing century, 



the speeds of 325 stars had been determined with the Mills spectro- 

 graph in the northern two thirds of the sky. Omitting several stars 

 whose lines could not be measured accurately, and some thirty spec- 

 trographic and visual binaries for whose centres of mass the velocities 

 were still unknown, 280 stars remained available for deducing the 

 relative motion of our solar system. 3 The observational data were 

 distributed symmetrically in right ascension, and the result for this 

 coordinate of the apex agreed with Newcomb's proper-motion result 

 within a small fraction of a minute of arc. The data were extremely 

 unsymmetrical in declination, as there were few observations between 



15 and 30 declination, and none whatever south of 30. 

 The solution placed the apex 15 south of Newcomb's position. The 

 deduced speed, 20 km. per second, is no doubt close to its true value. 



There is a question whether the direction of the solar motion 

 can be determined more accurately from proper motions or from 

 radial velocities, an equal number of stars being available in the two 

 cases; but as to the speed, no doubt of the very marked superiority of 

 the spectrographic method can exist. This, however, is but incidental, 

 for the two methods are in fact mutually helpful and mutually de- 

 pendent: the motion of every star involves both components. 



In this connection two points call for appreciation: First, the 

 motion of the solar system is a purely relative quantity. It refers to 

 the group of stars used in the solution. We could easily select twenty 

 or thirty of these stars whose velocities were such that the deduced 

 motion would be reversed 180 from that given by the entire list of 

 stars. We want to know the solar motion with reference to the entire 

 sidereal system. A satisfactory solution of the problem demands that 

 we use enough stars to be considered as representative of the whole 

 system. Second, the great sidereal problems require that observa- 



: Bulletin Astronomique, xv, 49; Astrophysical Journal, xi, 259, and xv, 172. 

 2 Astrophysical Journal, vi, 424, and many others. 

 Ibid, xni, 80. 



