96 ASTRONOMY 



end of one eclipse and the beginning of the next. Stebbins detected minute changes in 

 this interval which are fairly interpreted to mean that the " dark " body has a feeble 

 light of its own, and is moreover illuminated by the " bright " body so as to exhibit 

 phases. Other similar cases have since been found by R. S. Dugan of Princeton. 



Another addition to the facilities for accurate observation of stellar brightness is due 

 to Schwarzschild, who passes a photographic plate to and fro in its plane so as to obtain 

 from each star over a wide area a patch of uniform density, and has already measured 

 in this way the brightnesses of 3522 stars ranging nearly to the gth magnitude. 



Double Stars. Most of the doubles are revolving so slowly round each other as to 

 suggest that we must wait many decades before we can profitably determine their orbits, 

 or deduce other conclusions from the observations. But Hertzsprung has recently 

 made an ingenious suggestion for reaping some of the fruits by anticipation. Assuming 

 average values for the particulars as yet unknown to us, he deduces from the observed 

 arc of motion, however short, the distance of the double star. Now in a number of 

 cases this distance has been directly determined and can be compared with the above 

 deduction: and the comparison shows that the relation of determination to deduction 

 is nearly constant. Deductions can therefore be used to supply approximate determina- 

 tions in cases where they have not been made; individual instances may be in error, but 

 the average will not be far wrong. The new method has indicated more than one con- 

 clusion of considerable interest. 



At Ottawa it has been found that 7 Geminorum is a spectroscopic binary of the 

 exceptionally long period 6 years, which bridges the gap between spectroscopic and vis- 

 ual doubles, since at least one of the latter has a shorter period. Betelgeux is probably 

 also a long period spectroscopic binary. 



Nebulae. A complete descriptive list of nebulae photographed at the Lick Observa- 

 tory was published in October 1912: the Reynolds reflector at Helwan in Egypt is being 

 successfully devoted to the systematic exploration of more southern nebulae: and the 

 five foot mirror on Mount Wilson has already produced many striking pictures. A 

 general conclusion from all such work is that drawings (made before the days of photo- 

 graphy) are quite untrustworthy. From his photographs of the spectra of spiral 

 nebulae E. A. Fath concluded that they are probably unresolved star clusters with 

 gaseous envelopes; and there is some evidence of a progressive change of spectrum with 

 the form. 



J. W. Nicholson has formulated a hypothetical constitution of certain elements with 

 the simplest possible type of atom, one of them being provisionally called nebulium. 

 He can thus account theoretically for all the observed lines in the spectra of nebulae, and 

 was successful in predicting the existence of one line which had not been previously 

 noticed, but which was found, on searching, by W. H. Wright. 



The Stars in General. A recent discovery of fundamental importance is the relation 

 of the spectral type of a star, and therefore of its probable age, to the rate at which it 

 moves. Briefly expressed, the law is that the older a star is, the quicker it moves. The 

 facts supporting this statement emerged from the mass of observations with the spectro- 

 scope made at the Lick Observatory under W. W. Campbell's direction, and he made 

 the first announcement in his Silliman lectures in January 1910: the detailed information 

 followed later, and confirmation was forthcoming from the quite independent study of 

 proper motions by Lewis Boss. It may be that astronomers are not justified in deduc- 

 ing the relative ages of the stars from their spectra, but in any case they had arranged 

 the stars in a certain order from the appearance of their spectra, which afterwards 

 turned out to be also the order of rapidity of their movements. (It may also be men- 

 tioned that it has recently been found possible to make estimates of the temperature of 

 various stars, which at first were rough, but are beginning to show better accordance. 

 These also indicate the same order as the spectra and the motions.) Campbell's re- 

 markable table illustrating this fact may be given here: 



