1 82 POPULAR SCIENCE MONTHLY 



some others, and is perhaps true for all. These bright lines, clue to 

 incandescent hydrogen, undergo various modifications during the time 

 in which they are present. The relative intensity of different lines 

 varies greatly in different stars, and also in the same star at different 

 phases. Mrs. Fleming has heen able to arrange them in a series having 

 ten subdivisions, with R Lyncis at one end, with the H /? and H y lines 

 prominent and H 8 wanting, and E Leonis at the other end with H (3 

 wanting, H y faint, and H 8 prominent. There are also corresponding 

 changes in the distribution of the remaining light of the spectrum, a 

 peculiarity which is shared by stars of the same type which are not 

 variable. These characteristics are well shown in Tig. 7. 



Their great range of variation makes many of these stars invisible 

 when near minimum in telescopes of ordinary size. This may account 

 for the custom which has been followed by many observers of measur- 

 ing the light only when the star is near maximum. This is unfortu- 

 nate, since the determination of the length of the period is not sufficient 

 in itself for the solution of the problems involved. On this account 

 special efforts have been made at the Harvard Observatory, where the 

 observations are carried on by Miss A. J. Cannon and Mr. Leon Camp- 

 bell, to get measures of the variables at all the different phases. Even 

 thus it is doubtful if the secrets of the changes can be found, until the 

 research is made to include a more detailed spectroscopic study than 

 has yet been made. A systematic study of a large number of well- 

 selected stars is much needed. This could probably be done best by a 

 photographic reflector of the largest size. Such a scheme of work has 

 been proposed by Professor W. W. Campbell, director of the Lick 

 Observatory, and from it we may expect results of the highest value. 

 It may be well, also, to study the radial motions of these variables, 

 but it is more than doubtful if their variability is in any way asso- 

 ciated with orbital motion, such as would be found in binary systems. 

 The irregularity in the recurrence of the phenomena seems to preclude 

 the possibility of such an explanation. The stars of this class probably 

 contain within themselves the causes of their changes. They are, per- 

 haps, at that critical stage of development where occasional internal 

 disturbances cause tremendous outbursts, especially of incandescent 

 hydrogen, resulting in an enormous increase of light. The commotion 

 slowly dies down only to return again with more or less of regularity. 

 For the details of these disturbances we must await further study. 



IV. Of the 3,000 variables known at present probably the vast 

 majority have short periods, that is, periods of a few days, or a few 

 hours. The periods, also, are uniform; or, at least, if apparent irregu- 

 larity exists at times, this is capable of being expressed by rigorous 

 mathematical formula?, rj Aquila? and /3 Lyrge are well-known examples 

 of this class. Eecent investigations have shown that such stars are 

 binary systems, and that in some way the light phenomena are as- 



