518 Mr. W. Huggins and Dr. W. A. Millar on the 



servation, the spectra of the components of ft Cygni were sepa- 

 rately examined. These spectra, especially that of B, are so 

 faint that the lines are seen with difficulty, and scarcely admit of 

 being measured. Since, however, on account of the strongly- 

 contrasted colours of these stars, considerable interest attaches 

 to a comparative examination of their spectra, we have represented 

 in fig. 4, Plate V., the appearances which these spectra present to 

 the eye, though we have not yet measured the lines and bands in 

 them. These figures must be regarded as eye estimations only 

 of the general features of the two spectra. The spectra contain, 

 doubtless, many other lines ; and the positions of the lines in- 

 serted in the drawings, with the exception of b and D, were not 

 measured, but only roughly estimated. The distinctive charac- 

 teristics of these spectra are in accordance with the theory of the 

 origin of the colours of the stars proposed in the foregoing para- 

 graphs. In the case of both stars, the portions of the spectrum 

 which correspond to the colours which are deficient in the light 

 of the star, are those which are most strongly shaded with bands 

 of absorption. Thus in the spectrum of A, the light of which is 

 yellow tinted with orange, the absorption is greatest in the violet 

 and blue ; for the strong lines in the orange and red, since they 

 are narrow, would diminish in a much smaller degree the light 

 of these refrangibilities. The yellow and part of the green are 

 free from strong lines. 



The light of the star B appears to us to be blue, though in 

 some states of the atmosphere the star becomes greenish blue, 

 green, and even greenish white. These changes are probably 

 due to the comparatively greater absorptive action of the vapours 

 in the air upon the more refrangible portions of the spectrum ; 

 in proportion to which absorption the other parts of the spectrum 

 become relatively exalted, and thus predominate more or less in 

 the eye. 



This inequality of the absorptive action of the vapours of the 

 atmosphere upon different parts of the spectrum becomes very 

 evident if the eyepiece of the telescope be put out of focus (out- 

 side of the focus) so as to bring the blue and red rays to a focus in 

 the centre of an expanded image of the star. In the case of B 

 of /3 Cygni, the centre appears purple, surrounded with a margin 

 of green. In proportion to the changes in the atmosphere, by 

 the passage of masses of vapour or thin cloud, will be the varia- 

 tions of these colours. The green becomes greener; but the 

 blue and the violet are affected in a much greater degree, at times 

 fading almost completely ; then the colours resume their former 

 tints and brightness. Several such changes may sometimes occur 

 during one observation. 

 The spectrum B, observed under conditions of atmosphere in 



