REPORT OF COMMITTEE ON OBSERVATORIES 37 



to the Committee in response to its request (Appendix B). This 

 would be an immense undertaking, which would seem to demand the 

 cooperation of several agencies before it could be properly under- 

 taken for the entire sky. It would also be desirable to learn more 

 of the practical capabilities of the method proposed before making 

 any recommendations upon the subject. 



The plates which would be taken in the course of the attempt to 

 determine the relative parallaxes of stars known to have sensible 

 motion would themselves offer an opportunity for preliminary tests 

 of the method and of the value of the expected result. It would 

 also be easy to provide this in a more systematic form for restricted 

 areas of the sky by the exposure and development of plates espe- 

 cially for that purpose, provided the measurements and computa- 

 tions for these plates could be arranged under a cooperative plan of 

 limited extent, such as that suggested in the letter of Professor 

 Kapteyn (Appendix B). 



In view of these considerations we are of the opinion that it would 

 be advisable, in connection with the proposed Southern Observatory, 

 to set up a photographic telescope of about 18 inches aperture and 

 30 feet focal length for parallax work on the southern stars, and 

 that it would probablj^ be found desirable to maintain it in constant 

 operation for a period of eight years at least. The services of two 

 skilled observers and a small staff of measurers and computers would 

 be required. 



(4) Measureme7it of Radial Motions. 



In close connection with the three projects already mentioned is 

 another for the measurement of velocities of stars in the line of 

 sight — /. e., radial velocity toward or away from the earth. A 

 knowledge of these velocities is of the utmost importance, and 

 since the accurate measurement of such velocities has become pos- 

 sible, a most valuable source of information for verifying and 

 enlarging the conclusions to be drawn from the discussion of proper 

 motions has been placed in the hands of astronomers. In fact, 

 through an adaptation of the investigations for solar motion in 

 connection with these measurements of radial velocity, it is possible 

 to obtain valuable conclusions as to the distances of various classes 

 of stars having sensible proper motions. When a very large num- 

 ber of such measurements upon stars distributed over the entire 

 sky shall have been obtained, it will be possible to determine the 

 velocity of the solar motion with great precision. It will even be 



