NOVEMBEB 19, 1909] 



SCIENCE 



r3] 



With r= 10 and the data assumed above, we have 

 P = 1/43,000. Hence near approaches in such a 

 system are extremely rare. 



The period of revolution of a star in a star 

 cluster of approximately homogeneous star dis- 

 tribution, is 



where M is the total mass. Supposing M =^ 5,000 

 times the sun's mass and the other data as above, 

 we find r = 89 X lO'^ years. 

 The greatest velocity is 



V = 



k\'M 



at the center of the cluster. With the data used 

 above, T'=1.04 astronomical units per year; or, 

 in angular measure, as a maximum, 0".01 per 

 year. The apparent angular velocity varies with 

 the three-halves power of the parallax. Conse- 

 quently if the large value taken above is actually 

 ten times too great, the greatest apparent angular 

 velocity is only 0".0003 per year. If these nu- 

 merical assumptions be regarded as reasonable, 

 then sensible relative motions of permanent mem- 

 bers of star clusters are not to be expected until 

 the observations extend over some decades. 



Achromatic and Apochromatic Comparative Tests 

 — Seco7id Communication: E. D. Roe, Jb. 

 This paper presented the final results of the 

 testing of two objectives, which was outlined a 

 year ago in a preliminary communication before 

 the society. The two objectives, two-lens type 

 achromatic and apochromatic telescope objectives 

 of approximately the same aperture and focal 

 length, by Mr. Lundin and Steinheil Sohne, re- 

 spectively, were tested visually on double stars, 

 and in the laboratory the photographic knife edge 

 test was applied to both objectives with satisfac- 

 tory results, while the color curves of the two 

 lenses were ascertained by measurements on extra- 

 and intra-focal spectrograms. The paper will ap- 

 pear in Archiv fiir Optik (Berlin). 



Report of Progress on the Radial Velocity Pro- 

 gram of the Lick Observatory: W. W. Camp- 

 bell. 



The programs of observation for the Mills spec- 

 trograph attached to the 36-inch equatorial, and 

 for the D. O. Mills expedition to the southern 

 hemisphere (Santiago, Chile) have aimed to se- 

 cure at least four spectrograms of every star down 

 to the 5.0 visual magnitude, with 3-prism disper- 



sion if possible, and of somewhat fainter stars, 

 especially in the southern hemisphere, with 2- 

 prism dispersion. Up to June 1, 1909, 3-prism 

 spectrograms of 882 stars had been obtained at 

 Mt. Hamilton ; 200 of these, whose spectra contain 

 broad and poorly defined lines, have been rejected 

 from the main program for observation later with 

 lower dispersion. E.xcepting these, the northern 

 observing program is essentially complete for the 

 good summer months; and if next winter, and 

 especially next spring, have average weather con- 

 ditions, the program should be nearly complete 

 throughout the twenty-four hours of right ascen- 

 sion, by June 1, 1910. The D. 0. Mills expedi- 

 tions, under Astronomers Wright and. Curtis, suc- 

 cessively, have observed altogether 530 stars 

 brighter than 5.01 visual magnitude and about 

 150 stars fainter than 5.00 magnitude, or 680 

 stars in all. Correcting for those stars observed 

 at both Mt. Hamilton and Santiago, the total 

 number of stars whose spectra have been photo- 

 graphed is 1,368. The original Mills spectrograph 

 was succeeded in May, 1903, by a new Mills spec- 

 trograph. All the spectrograms obtained with the 

 original spectrograph, and about three fourths of 

 those obtained with the new spectrograph, have 

 been measured and reduced definitively, and are 

 being rapidly prepared for publication. During 

 the first period of the T>. O. Mills expedition, in 

 charge of Astronomer Wright, covering two years 

 of observation, spectrograms of about 200 stars 

 were secured. Those containing lines suitable for 

 accurate measurement numbered 148, and four 

 plates, on the average, were obtained for each of 

 these. The results, including all the text, are 

 entirely ready for publication in volume form. 

 Of these 148 stars, 29, or one in five, have been 

 found to have variable velocities. Of the plates 

 secured during the second period of the D. 0. 

 Mills expedition, under Astronomer Curtis, about 

 one third have been measured and reduced defini- 

 tively by Dr. Curtis and his assistant, Mr. Pad- 

 dock, while carrying on the work of observing. 

 The remainder have been measured approximately 

 — that is, utilizing only a few of the available 

 lines. It should be said that only a small pro- 

 portion of the spectroscopic binaries discovered at 

 Mount Hamilton and in Chile have been investi- 

 gated. To do this would require several years of 

 observing, measurement and computation. 



The Lick Observatory Double-star Survey — A Re- 

 port of Progress: R. G. Aitken. 

 The program for this survey as originally 



planned contemplated the examination of every 



