444 The Bev. Walter Sidgreaves [Jan. 22, 



two bright lines x and y, the more refracted x has vanished at about 

 six hours after this minimum, and does not reappear for some days 

 in the period. It may, therefore, have disappeared through the 

 increment of temperature, brought about at the tidal disturbance of 

 the periastron, as the same line manifestly disappeared in the 

 spectrum of the great Nova of Perseus. The changes in the Nova's 

 spectrum are now projected on the screen. There are four spectra of 

 two very different types following alternately. The dates are written 

 in the margin ; and you will observe that the change from the earlier 

 type to the new type of March 25th and 28th was very rapid. It 

 occurred within the 24 hours from the 27th to the 28th, and also 

 between the 21st and 22nd, for on March 22nd a very good photo- 

 graph was obtained, precisely the same as on the 25th, but the clear 

 sky break in the clouds was of short duration, and the trail on the 

 plate gave too narrow a spectrum for a good reproduction. Confining 

 our attention to the H^ line, we find in it the same changes as in the 

 spectrum of jS Lyrae. It has a broad and bright extension on the 

 more refractive side, on March 25th and 28th, covering the whole 

 interval between H^ and H^ ; and on the 21st and 27th the extension 

 has vanished. There can be no doubt about the apparent extension ; 

 it cannot be of hydrogen origin. This is manifest, first, by its 

 position in the spectrum, and, second, by the distinctly weaker part of 

 the whole band, which is the true H^ line, known by its position and 

 by its relative weakness in the series of hydrogen lines. 



Comparing these spectral changes of the Nova with its light 

 curve, as plotted from the excellent series of measures made at the 

 Eadcliffe Observatory, Oxford, the apparent extension occurs at the 

 minima of the light. And taking these measured magnitudes for our 

 thermometric indications, the extension vanished at the higher tem- 

 peratures and appeared at the lower temperatures. It was a new 

 bright band which appeared for the first time on March 22nd, when 

 the temperature of the star had fallen below the dissociation tempera- 

 ture of the origin, probably cyanogen. The subsequent oscillations 

 of heat were, for a time, crossing the dissociation point, until the 

 temperature of the star had fallen permanently below it, when the 

 earlier spectral type was finally lost. This interpretation you will 

 find confirmed in a remarkable manner by the tabulated com j)ari sons 

 of the star's light-magnitudes with the two types of its spectrum. 

 The table shows (projection on screen), without any exception, that 

 the new bright band appeared only when the magnitude of the star 

 was below 4 • 57 ; and the temperature indicated by this magnitude 

 is probably the dissociation temperature of the origin ; for the plate 

 referred to in the table, as exposed on the night of this magni- 

 tude of the star, shows an undecided sjDectrum, belonging partly 

 to both types, a notice of which was sent to the E.A.S. before 

 the magnitudes were published. 



The oscillations of heat and light may well be exj)lained in the 

 Nova by the oscillating volume of gaseous matter evolved at the 



