REVIEW OF ASTRONOMY PUISEUX. 137 



that the dispersion of a swarm, once formed, would require billions 

 of years— a time much greater than the probable life of a star as a 

 bright star. 



Kesults well worthy of attention have been obtained lately through 

 the formation of tables having as headings the principal character- 

 istics of the stars, spectrum class, annual parallax, magnitude of mo- 

 tion, intrinsic brightness. Thus, Campbell has shown that the white 

 stars {A and B of the Harvard classification) are more numerous than 

 other stars near the Milky Way, have small velocities, great distances 

 from the sun, and great brightness. The red stars are, on the average, 

 nearer the sun and have greater velocities. There is reason for con- 

 cluding, according to Stratton, that the stars have their birth close 

 to the plane of the JSIilky Way and depart from it with time w^ith 

 increasing velocities. H. N. Russell thinks that he can go yet further, 

 laying stress upon the fact that statistics separate the red stars into 

 two classes — one much brighter intrinsically than the sun, and the 

 others decidedly fainter. The former (giant stars) are less advanced 

 in their evolution. Their destiny is to contract, and consequently 

 become warmer and whiter, losing in size and gaining in velocity. 

 They again become red before their final extinction. These correla- 

 tions are valuable for guiding researches, but it will without doubt 

 be necessary to wait until their degree of generality is better estab- 

 lished. 



The existence of a particularly close analogy between certain stars 

 and the sun results from the work of the observers at Potsdam. They 

 have found that in the spectra of Arctunis and Aldebaran we can 

 obserA^e the partial reversal of the H and K lines, that is to say, the 

 formation of a brilliant central line in them such as those seen in the 

 troubled regions of the sun. 



The category of spectroscopic double stars, enriched continually 

 by the work of the Lick and Allegheny observers, presents on the 

 other hand a phenomenon of which we find no analogy in the sun. 

 We laiow now several instances of the fact, noted first in 5 Orionis, 

 that the calcium lines do not follow the periodic oscillations of those 

 due to hydrogen and helium. Possibly clouds of calcium, unconnected 

 with the stars, are interposed in the line of sight. In the Cepheid 

 variables, compared with one another, LudendorlT has noted the 

 existence of a proportionality between the amplitudes of the varia- 

 tions which the brightness and radial velocities respectively undergo. 



It will be useful, in order to interpret these and similar laws, to be 

 able to reach greater precision in the measure of faint magnitudes. 

 All methods, in which the judgment of the eye is utilized, involve a 

 certain inaccuracy of physiological origin. Attempts are being made 

 to substitute for the operator's eye an apparatus of rigorously im- 

 personal measures, indefatigable and of a superior sensitiveness. 



