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SCIENCE. 



[N. S. Vol. VIII. No. 195. 



ically. It requires, however, a powerful 

 telescope to measure the individual stars. 

 Dr. Scheiner has given a catalogue of 833 of 

 the stars of this cluster measured on photo- 

 graphs taken with the 13-inch refractor of 

 the Potsdam Observatory. The stars that 

 were measured all lie between the magni- 

 tudes 11.7 and 14. As a matter of compari- 

 son with visual measures, the writer has 

 taken up the measurement of a few of the 

 stars contained in Scheiner's catalogue. A 

 rough inspection of the results so far ob- 

 tained shows a close agreement between the 

 visual and the photographic work. These 

 observations also show that no appreciable 

 change has taken place in the position's of 

 any of the stars in the past six years, which, 

 perhaps, is surprising, since one would ex- 

 pect a possible rapid change in some of the 

 positions of the individual stars when they 

 are massed so close together. They, how- 

 ever, seem to be as stable in their relative 

 positions as are the stars elsewhere in the 

 sky. A more remarkable object with a 

 great telescope is the cluster Messier 5, in 

 which the stars are more closely compressed 

 and irresolvable than in the cluster of Her- 

 cules. This object has already been men- 

 tioned in speaking of the variable stars 

 discovered at the Harvard College Obser- 

 vatory. The measurement of nearly 100 

 of these small stars has been undertaken 

 with the great telescope of the Yerkes Ob- 

 servatory. Many of them are apparently 

 in the very heart of the cluster, where the 

 compression is the greatest. It is doubtful if 

 at this time photographs can be made of this 

 cluster upon which the crowded individual 

 stars can be accurately measured. 



It has been frequently photographed, but 

 no measures have been made of the great 

 mass of stars in the center of the cluster. 

 It has been already stated that the ac- 

 curacy of the photographic positions of in- 

 dividual stars is as great as the best 

 meridian observations. The facility and 



ease with which the photographic positions 

 are obtained is well shown in a report 

 by Professor H. H. Turner, who is making 

 the Oxford portion of the great Astro- 

 graphic catalogue. An average of 3,951 

 measures per week is obtained. Over 150 

 stars per hour each can be measured by 

 those most skilled in this work. 



In the discovery of nebulae, variable 

 stars and asteroids, the photographic plate 

 has done a great work which is still 

 being carried on. The number of known 

 asteroids has been doubled in the past few 

 years (as many as nine have been found 

 in a single night), and now it has become a 

 matter of impossibility to keep track of 

 them all, and they are found and turned 

 adrift again unless they show some striking 

 peculiarity of orbit. 



Up to the present time but two comets 

 have been discovered by photography. The 

 first of these was discovered on a photo- 

 graphic plate taken by the writer on 

 October 12, 1892, with tlie 6-inch Willard 

 lens of the Lick Observatory, and was sub- 

 sequently verified visually and observed at 

 the different observatories. The second 

 was photographed at the same Observatory 

 by Mr. Coddington, with the same instru- 

 ment, in July, 1898. 



In photographing the sky it is found that 

 the short focus portrait lens, from its small 

 scale and large field, will show faint nebu- 

 losities beyond the reach of the larger pho- 

 tographic telescopes. This results from 

 various causes. The action of these lenses 

 upon the Milky Way, comet's tails and the 

 great nebulosities of the sky does not 

 seem to be strictly subject in practice to 

 the law of the ratio of aperture to focus ; 

 or, if it is, this law must be somewhat 

 modified in effect. The action seems to 

 be quicker with the short focus lens than 

 it should be. Probably, however, much 

 of this is due to the small scale and 

 the consequent compression of the image 



