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SCIENCE. 



[N. S. Vol. III. No. 72. 



ard for the University of Mississippi, Alvan 

 G. Clark discovered a faint companion to 

 Sirius. Tliis proved to be in the exact 

 position required by Peters' calculations. 

 For Procyon no companion has been found. 

 Recently there have been found evidences of 

 an unseen body in the system of 70 Ophi- 

 uchi, a wide double. These and similar 

 investigations indicate that there may be 

 myriads of systems in space similar to our 

 own. Painstaking observations and exact 

 calculations will, no doubt, reveal many 

 hundreds of these systems during the next 

 century, even before new inventions have 

 increased our seeing power. 



The thoughtful observer is struck by the 

 fact that the light of most of the stars does 

 not appear to change ; they remain each of 

 them apparently of the same brightness 

 year after year, and so far as we can judge 

 from previous accounts, century after cen- 

 turj'. The stars are so far away that 

 changes in their light-giving power are in 

 most cases invisible to us. There are, 

 however, now known nearly 400 stars which 

 show a variation in light. Some stars 

 change their brightness slowlj^ and continu- 

 ously; others fluctuate irregularly, like the 

 wonderful star vj Argus in the southern heav- 

 ens, which was neai'ly as bright as Sirius in 

 1843 and decreased in brightness down to 

 the 7th magnitude in 1865. It remained 

 at that magnitude until 1888 and has since 

 been increasing in brightness. Dr. Gill, at 

 the Cape of Good Hope, has been studying 

 the star by the aid of photogi-aphy during 

 the past few years. 



Then there is a class of variables called 

 'temporary stars.' These blaze out sud- 

 denly and then disappear. Such variables 

 are styled, sometimes, ' new stars.' Pick- 

 ering gives a list of 14 ' new stars ' discov- 

 ered since the time of Tycho Brahe in 1572. 

 In this list all but four belong to this cen- 

 tury, no temporary star being recorded be- 

 tween 1670 and 1848. Six new stars are 



recorded as having been discovered since 

 1886. Two of these in 1895. There is no 

 doubt that a more careful study of the 

 heavens will reveal many more such cases. 

 Several of the stars of this class were 

 brightly visible to the naked eye. They 

 remained visible with fading light for dif- 

 ferent periods of time. Future investiga- 

 tion may show that these stars will appear 

 again, and thus indicate that they are vari- 

 ables of long period showing such light 

 changes as to place their minima beyond 

 the power of the telescope or even of the 

 photographic plate. Then there are known 

 to be a considerable number of variables 

 whose periods of light changes are well de- 

 termined. These are most interesting to 

 observe. One class has a period of several 

 months, another class a period which is 

 quite short, and stUl another class ' in which 

 the variation is like what might be pro- 

 duced if the star were periodically eclipsed 

 by some intervening object.' Great use is 

 now made of photography both of the stars 

 and of their spectra in studying variables. 

 At the Harvard College Observatory the 

 plates taken in Arequipa, Peru, have shown 

 on examination many variables. Pickering 

 states that in two photographs of the clus- 

 ter Messier 5 taken August 9, 1895, only 

 two hours apart, forty-six variables of short 

 periods wei'e found ! In the photographs 

 of stellar spectra the presence of bright 

 hydrogen lines in conjunction with dark 

 lines or dusky bands has led to the dis- 

 covery of numerous variables. The sub- 

 ject is of growing interest and the further 

 prosecution of the work will add, no doubt, 

 many thousands to the present list which 

 has recently been brought down to date by 

 Dr. S. C. Chandler, of Cambridge. 



The study of the sidereal systems presents 

 many problems for themathematical astrono- 

 mer, but let us consider some unsolved prob- 

 lems connected with our own system. " The 

 profoundest question growing out of the the- 



