PROGRESS IN ASTROPHYSICS ABBOT. 189 



tioii is 3.85" per year. This is called the mean solar parallactic 

 motion for the star group. Now, of course, the parallactic motions 

 of the stars depend upon their distances from the earth, and natur- 

 ally will be less for the fainter stars than for the brighter ones, 

 since the fainter ones are situated at the greatest distances. 



The mean magnitude of all the stars investigated by Boss was 

 5.7, but he selects 559 stars having large proper motions whose 

 average magnitude was 5.3 and for which the mean solar parallactic 

 motion was 21.58". Prof. Boss then goes on to compare the magni- 

 tude and parallaxes of 130 stars whose parallaxes had been meas- 

 ured, and thereby obtains a formula connecting the parallax and 

 proper motion for stars of the magnitude 5.3. He thus has a meas- 

 ure of the distance of the stars of large proper motion which he is 

 considering, and from this he finds that the velocity of the solar 

 system, in its motion toward the constellation Hercules, is 24.5 kilo- 

 meters (15 miles) per second. Readers will note that this value is 

 derived quite independently from that of Prof. Campbell, and that 

 it is about 25 per cent larger than his. But from the considerations 

 above mentioned (under the caption " Distances of the stars "") prob- 

 ably a reduction of the estimated distances corresponding to given 

 proper ■ motions will be brought about as more determinations of 

 stellar distances become available. Thus Prof. Boss's estimate will 

 be brought down toward that of Prof. Campbell. 



It is found that the fainter stars are on the whole at greater dis- 

 tances from the sun than the brighter ones, so that the star list of 

 Boss relates on the average to a system of stars at a greater distance 

 from the observer than the star list (vf Campbell. A reason has 

 already been assigned for supposing Boss's value of the solar motion 

 too high. It may be on the other hand that the sun's motion is to 

 some extent shared by the stars which are its more immediate neigh- 

 bors, so that its velocity with respect to them is smaller than with 

 respect to the stars which are more remote. 



Prof. Campbell has adopted in his later discussions the round 

 numbers 270° right ascension and 30° declination for the position 

 of the solar apex, and the velocity of 19.5 kilometers as the rate of 

 its motion toward this apex. 



With these quantities determined, it is possible to take from the 

 observed radial velocity of each star a component which depends 

 upon the motion of the sun, and thus to leave to each star its own 

 individual motion with respect to the earth, as the earth would be if 

 fixed in space with reference to the whole system of stars considered. 

 As the sun moves at the rate of 19.5 kilometers per second in a cer- 

 tain direction, so for each of the other stars investigated, there 

 should be a certain velocity and direction of motion. The stars have 

 been classified at the Harvard College Observatory under the direc- 



