THE ADVANCE OF SCIENCE 93 



history backwards for more than a dozen years by refer- 

 ence to his plates. This is a wonderful new method, a 

 mode of keeping record of present movements and changes 

 which promises much for the future of astronomy. By 

 the photographic method hundreds of new variable stars 

 and other interesting objects have been discovered. New 

 planets have been detected by the hundred. Up to 1881 

 two hundred and twenty were known. In 1881 only one 

 was found ; namely, Stephania, being No. 220, discovered 

 on May 19. Now a score at least are discovered every 

 year. Over 500 are now known. One of these Eros 

 (No. 433) is particularly interesting, since it is nearer to 

 the sun than is Mars, and gives a splendid opportunity for 

 fixing with increased accuracy the sun's distance from the 

 earth. Two new satellites to Saturn and two to Jupiter 

 have been discovered by photography (besides one to 

 Jupiter in 1892 by the visual telescope of the Lick Obser- 

 vatory). One of the new satellites of Saturn goes round 

 that planet the wrong way, thus calling for a funda- 

 mental revision of our ideas of the origin of the solar 

 system. 



The introduction of photography has made an 

 immense difference in spectroscopic work. The spectra 

 of the stars have been readily mapped out and classified, 

 and now the motions in the line of sight of faint stars 

 can be determined. This ' motion in the line of sight,' 

 which was discernible but scarcely measurable with 

 accuracy before, now provides one of the most refined 

 methods in astronomy for ascertaining the dimensions 

 and motions of the universe. It gives us velocities in 

 miles per second instead of in an angular unit to be inter- 

 preted by a very imperfect knowledge of the star's 

 distance. The method, initiated practically by Huggins 

 thirteen years before, was in 1881 regarded by many 



