Septembek 23, 1898.] 



SCIENCE. 



391 



The writer has examined the cluster M 

 5 with the great telescope of the Yerkes 

 Observatory and has visually verified a 

 number of these variables. The brighter 

 of them appear to vary slowly in their 

 light, while many of the smaller ones are 

 extremely rapid, passing through their en- 

 tire light changes in a few hours. In the 

 discovery of such objects, photography 

 offers special advantages, since on the dif- 

 ferent photographs a thousand or more 

 stars can be rapidly and accurately com- 

 pared with each other and any variation 

 in their light at once detected, while such 

 comparisons in the actual sky, visually, 

 would be limited to a very few stars. By 

 the aid of the Harvard photographic plates 

 over five hundred variable stars have been 

 discovered in these clusters. It must be 

 said, however, in speaking of the variables 

 in the cluster M 5, that the two most prom- 

 inent ones were really discovered visually 

 nearly ten years ago by Mr. D. Packer with 

 a very small telescope. These two seem to 

 have been the first of the variable stars 

 found in this cluster. 



The shortest period variable so far dis- 

 covered in the globular clusters — indeed, 

 the shortest known variable — is a small 

 Star in the great southern cluster Omega 

 Centauri, whose period is seven hours. 

 These cluster variables seem to form a dis- 

 tinct class from the ordinary variable stars. 

 It is very interesting to watch one of these 

 small stars in a powerful telescope and to 

 see with what quickness it passes through 

 its light variation. One of the small stars 

 in M 5, whose period is 12 h. 31 m., seems 

 to be dormant for a large part of the time, as 

 a very faint star, invisible in ordinary tele- 

 scopes. It begins to brighten, and in two 

 or three hours has risen nearly two magni- 

 tudes and faded again to its normal condi- 

 tion, while another and larger star quite 

 near it seems to require a month or more 

 to go through its light fluctuation. 



Frequent reference has been made to the 

 photographic work of the Harvard College 

 Observatory. It is to be regretted that 

 time does not permit a more detailed ac- 

 count of this work. No other observatory 

 is so active in the application of photog- 

 raphy to the various departments of astron- 

 omy. Not content with the available sky 

 as seen from the northern hemisphere. 

 Professor Pickering wisely established a 

 branch observatory at Arequipa, in Peru, 

 where a thorough photographic survey of 

 the southern skies has been made, and a 

 vast amount of work of high value has been 

 accomplished, which has resulted in many 

 important discoveries among the southern 

 stars. 



In dealing with the ordinary stars of the 

 sky it has been shown that measures of the 

 relative positions of the photographic im- 

 ages are strictly comparable with the best 

 meridian circle work, while the number of 

 stars that can be measured is vastly greater. 

 The Pleiades, the cluster of Perseus, Prsesepe 

 in Cancer, etc., have all been measured with 

 the micrometer, the heliometer and by 

 photography. The comparisons have shown 

 that photography has many advantages 

 over the older methods, and the results are 

 possibly even more accurate. These ob- 

 jects, however, are loose clusters, and the 

 stars are not thickly crowded, and, more- 

 over, the small scale of the photographic 

 plate in such cases does not seriously inter- 

 fere with the work. The great globular 

 clusters of the sky, however, from the ex- 

 traordinarily crowded condition of their 

 stars, would almost forbid any attempt to 

 deal with the individual positions by pho- 

 tography, except in outlying regions, where 

 the stars are thinly scattered. No compari- 

 son between photographic and visual meas- 

 ures of such objects has yet been made, be- 

 cause no visual measures exist. The great 

 cluster of Hercules is, perhaps, the easiest of 

 these objects, both visually and photograph- 



