1890.] Hai/efs Bright-Line Stars in Cygnus. 43 



consider that the spectra of these stars consist of numerous absorp- 

 tion bands and groups of bright lines upon a feeble continuous 

 spectrum, a character of spectrum which seems to point to a probably 

 unstable condition of the atmospheres of these stars. 



The large difference of position of the bands in the two groups of 

 stars is much too great to admit of an explanation founded upon a 

 possible orbital motion of the stars. Besides, the near coincidence of 

 Dr. Copeland's measures of two bright lines common to the stars 

 4001 and 4013 shows that the difference of position of the blue band 

 is not due to motion in the line of sight.* 



If future observations should show that the bright blue groups are 

 variable, we must look, it would seem, to causes of a physical or a 

 chemical nature. 



If the two bright groups, differing in position by about X 0040, 

 belong to different substances, or, less probably, perhaps, to different 

 molecular conditions of the same substance, it is conceivable that one 

 or other substance, or molecular state, may predominate and appear 

 brilliant, according to certain unknown conditions which may prevail 

 in the stars' atmospheres. 



It might be suggested that both bands are due to a long group of 

 bright lines, extending from about X 461 to X 471, and that this long 

 group is cut down by absorption bands ; in one pair of stars an 

 absorption from the green cuts off the less refrangible part of the 

 long group down to about X 467, while in the other two stars the 

 more refrangible part is eclipsed, and the bright group appears as 

 in 4001. 



The appearance of the spectra in our instruments scarcely seems to 

 us to be in accordance with such a view, because, though we did 

 suspect brightenings in the alternate places, the appearance of the 

 spectrum was not such as to suggest a bright group dimmed by 

 absorption, for in that case the amount of absorption needed to all but 

 obliterate a group, as bright as it appears in the other pair of stars, 

 would have blotted out completely the relatively feeble continuous 

 spectrum. This continuous spectrum, though faint, was still dis- 

 tinctly seen. 



More observations are needed, but it appeared to us desirable by 

 these suggestions to invite the attention of observers to the points in 

 question. 



* Dr. Copeland permits me to give the following measures of the bright lines in 



the Wolf-Rayet stars, which were made by him and Mr. Lohse on January 28, 

 1884. 



1st yellow 2nd yellow Bright Faint Large blue 



Star. line. line. line. line. band. 



+ 35 4001 541-2(3) 522-0(1) 469'5(3) 



+ 35 4013 582-4(2) 568 '9 Si) 541-0(2) 465 -4(2) 



+ 36 3956..., 581-0(2) 570-4(2) 523'3(1) 464 -9(2) 



