1889.J of the various Species of Heavenly Bodies. 169 



attributed to faulty observation ; but this is certainly not so in all 

 cases. 



The following, which I quote from Dr. Copeland's discussion of 

 observations on comet spectra, is important in its bearing upon this 

 point : " We cannot omit to say a few words about the first 

 yellowish-green band. It is generally described as similar to the two 

 other bands, beginning brightest towards the red, and fading 

 gradually away towards the violet. It is true the dispersive power 

 of the instrument greatly modifies the appearance, but we must say, 

 that under high dispersion we have never seen the first band like the 

 others : it always faded away on both sides, and had seldom a very 

 marked maximum, sometimes it had two, and, perhaps, more, and it 

 seems to be the only band which shows an essentially different 

 appearance in different comets, and, therefore, deserves always a 

 special examination. Unfortunately, it is nearly always the faintest 

 band, and difficult to deal with, and only in Comet III, 1881, traces of 

 what may be bright lines were recognisable ; that the iron lines have 

 any connexion with it is very doubtful, since E falls outside of it."* 



Again, Professor Young remarks : 



" It is hardly necessary to say that the evidence as to the identity 

 of the flame and comet spectrum is almost overwhelming. The 

 peculiar, ill-defined appearance of the cometary bands at the time of 

 the comet's greatest brightness is, however, something which I have 

 not succeeded in imitating with the flame spectrum. The comet 

 spectrum on July 25th certainly presented a general appearance quite 

 different from that of the later observations as regards the definition 

 of the bands."t 



Other observers have also remarked this variability in the citron 

 band. 



A discussion of the recorded observations shows that this varia- 

 bility is perfectly regular, and depends chiefly on the distance of the 

 comet from perihelion. When carbon first makes its appearance in 

 the spectrum as the comet approaches the sun, the wave-length of 

 the citron band agrees with that of the carbon fluting at 564. As 

 the comet gets nearer perihelion the changes begin, and I now proceed 

 to show that the irregularities are produced by a special case of 

 masking due to the addition of the radiation of manganese or of 

 manganese and lead. 



In the Bakerian Lecture (page 63) I showed that in the spectra of 

 some " stars " the characteristics of the spectra of many substances 

 are considerably modified by what I called " masking." Thus in the 

 early species of Group II we have manganese indicated, not by the 

 first fluting at 558, but by the second at 586. This is due to the 



* ' Copernicus,' vol. 2, p. 243. 



f ' Amer. Joum. Sci.,' 3 series, vol. 22, p. 157- 



