January 18, 1907] 



SCIENCE 



113 



tion, and to usefulness in most lines of astron- 

 omical work. Professor Hale, however, comes 

 forward with the announcement that he is 

 prepared, through the generosity of Mr. John 

 D. Hooker, of Los Angeles, to undertake the 

 construction of a reflecting telescope having 

 an aperture of eight feet and four inches. 

 Such an undertaking is of necessity somewhat 

 in the nature of an experiment, but no one 

 is better able to overcome the technical diffi- 

 culties involved than Professor Eitchey, Mr. 

 Hale's able assistant. 



Meanwhile researches of importance are 

 being pushed with the present equipment. 

 Recent observations of sun-spots, taken in 

 connection with spectroscopic studies carried 

 on in the new laboratory, show that the dif- 

 ferences between the spectra of the photo- 

 sphere and of sun-spots are due to differences 

 in temperature. It has also been shown that 

 certain stars, as Arcturus, have typical sun- 

 spot spectra, and are thus at a lower tempera- 

 ture than our sun. 



POSITIONS OF STARS IN THE GREAT CLUSTER EST 

 HERCULES. LUDENDOBFF 



The positions of 833 stars in the great 

 cluster in Hercules were determined photo- 

 graphically by Dr. Scheiner, of Potsdam, in 

 1892. After thirteen years. Dr. Ludendorff, 

 of the same observatory, has independently 

 determined the positions of 1,136 stars in the 

 same cluster. Of course, the largest part of 

 the stars measured by Scheiner were remeas- 

 ured by Ludendorfi. Both observers deter- 

 mined the brightness as well as the positions 

 of the stars. The accordance between the 

 results obtained by these two observers is in 

 general extremely good. Ludendorff meas- 

 ured the stars on two plates, and from a com- 

 parison of the two determinations an idea can 

 be foi-med of the precision which was attained. 

 Grouped according to distance from the center 

 in R.A., the differences range from — 0.15" 

 to — 0.22". As might be expected, the mean 

 differences increase systematically toward the 

 center of the cluster, where the measurements 

 are most difficult. Among 1,588 differences 

 only 22, or 1 in Y2, amount to more than 1". 

 The probable error of a catalogue position in 



E.A. is given as 0.179", and in declination 

 0.180". These values are somewhat smaller 

 than the corresponding values in the work of 

 Scheiner. Small systematic differences ap- 

 pear, however, between the two determinations 

 by Ludendorff, and especially in the deter- 

 minations of declination between Ludendorff 

 and Scheiner. 



The importance of precise measurements of 

 the positions of the components of such clus- 

 ters as that in Hercules can hardly be over- 

 estimated. Thirteen years may be too brief 

 an interval for the determination of the proper 

 motion of the cluster and the stars which 

 undoubtedly are projected upon it, and much 

 more so of the motions of the individual mem- 

 bers of the group. When a suitable time 

 shall have elapsed, however, these determina- 

 tions should be of high value in the solution 

 of the fascinating problems which are asso- 

 ciated with the globular clusters. 



Ludendorff finds only small changes in the 

 brightness of the two variable stars discovered 

 in this cluster by the writer, and no evidence 

 of the variability of any other stars. 



POSITIONS OF STARS IN THE CLUSTERS h AND X 

 PEESEI. YOUNG 



An investigation similar to the pre- 

 ceding appears also as No. 24 of the ' Con- 

 tributions from the Observatory of Columbia 

 University.' This is an elaborate and valu- 

 able determination of the positions of 145 

 stars in the double cluster in the sword-handle 

 of Perseus, by Anne Sewell Toung. These 

 clusters are little condensed, but are beautiful 

 objects when seen vdth a low power. The 

 plates employed were made by Eutherfurd 

 during the years I87O7I874; they had double 

 exposures of about six minutes. The meas- 

 ured stars were of the tenth magnitude and 

 brighter. The methods of reduction have 

 been in general those of Jacoby. The results 

 appear to be of the highest precision and in 

 good accord with the best work which has been 

 done before. 



RESEARCHES IN STELLAR PHOTOMETRY. PARK- 

 HURST 



A VALUABLE contribution to our knowledge 

 of the variable stars has been made by Mr. 



