30 



Mr. J. N. Lockyer. On the Classification [Apr. 12, 



liave gone, in the case of each star. I shall return to the line of 

 evolution of these bodies in a later paper. 



Mashing of Radiation Effects produced by Variations of Interspacing. 



I have already stated that carbon bands are apt to mask the appear- 

 ance of other spectral phenomena in the region of the spectrum in 

 which they lie. In this way we can not only account for the apparent 

 absence of the first manganese fluting, while the second one is visible, 

 but it is even possible to use this method to determine which bands 

 of carbon are actually present. There is another kind of masking 

 effect produced in a different way, and this shows itself in connexion 

 with sodium. It is well known that when the temperature is low, D 

 is seen alone, and if seen in connexion with continuous spectrum the 

 continuous spectrum is crossed by either dark or bright D, according 

 to the existing circumstances. 



I showed some years ago that the green line of sodium (but not the 

 red one) is really visible when sodium is burned in the bunsen burner. 

 It is, however, very much brighter when higher temperatures are used, 

 although when bright it does not absorb in the way the line D does. 



Now, if we imagine a swarm of meteorites such that in the line of 

 sight the areas of meteorite and interspace are equal, half the area 

 will show D absorbed, and the other half D bright ; and in the 

 resulting spectrum D will have disappeared, on account of the 

 equality, or nearly equality, of the radiation added to the absorption 

 of the continuous spectrum. The light from the interspace just fills 

 up and obliterates the absorption. 



But if the temperature is such that the green line is seen as well as 

 D ; in consequence of its poor absorbing effect there will be no dark line 

 corresponding to it in the resulting spectrum, but the bright green 

 line from the interspace will be superposed on the continuous spec- 

 trum, and we shall get the apparently paradoxical result of the green 

 line of sodium visible while D is absent. This condition can be partly 

 reproduced in the laboratory by volatilising a small piece of sodium 

 between the poles of an electric lamp. The green line will be seen 

 bright, while D is dark. 



In the bodies in which these phenomena apparently occur — for so 

 far I have found no other origin for the lines recorded as 569, 570, and 

 571, the wave-length of the green sodium line being 5687 — such as 

 Wolf and Rayet's three stars in Cygnus and in 7 Argus, the continu- 

 ous variability of D is one of the facts most clearly brought out by 

 the observations, and it is obvious that this should follow if from any 

 cause any variation takes place in the distance between the mete- 

 orites. 



In all meteoric glows which have been observed in the laboratory, 

 not only D but the green line has been seen constantly bright, 



