BARNARD— SOME PECULIARITIES OF THE NOV^. 105 



This was verified spectroscopically by Moore and was increasing in 

 size. 



In a previous paper {Monthly Notices of the Royal Astronomical 

 Society, Vol. 72, p. 673, June, 1912) I have criticized some of the 

 theories concerning the novae ; the encounter with a nebula, the col- 

 lision theory, etc. In this connection was also criticized the theory 

 that the apparent recession of nebulous matter from Nova Persei was 

 due to the successive illumination of the details of a nebula in which 

 the star was placed, by the outgoing light from the nova. This did 

 not seem satisfactory because it was shown that certain details, such 

 as the " arrow head " in the photographs by Ritchey and Perrine, 

 were in actual motion away from the star and could not be due to 

 light reflection from the nebula. This light reflection theory still 

 seems to hold with some astronomers. There is much question, how- 

 ever, as to its correctness. Recent photographs of Nova Persei have 

 been made by Lampland at the Lowell Observatory, not of the orig- 

 inal nebulosities, for they are gone, but of the new nebulosity of 1916, 

 December 16, which, as shown by the photographs by Professor 

 Lampland, is moving out very slowly from the star. This slow mo- 

 tion, if its slowness is not due to motion nearly in the line of sight, 

 seems to have no relation to the rapid movement of the masses in the 

 earlier photographs of 1901, and it certainly can not be identified with 

 any of the details of those pictures. Nor can the present nebulous 

 disc about the star as photographed at Mount Wilson— so like a 

 planetary nebula — be traced on the earlier photographs. 



The most satisfactory theory to account for the novae, especially 

 with respect to Nova Persei, is that the forces of equilibrium of the 

 interior of the star were disturbed, and that there was an outburst in 

 the nature of an explosion, which, though it produced the great bril- 

 liancy in 1 90 1, apparently neither destroyed the star nor very seri- 

 ously affected its variability. 



Summary. 



The novae usually appear suddenly in the sky. Their advent is 

 entirely unheralded and it is only in recent years, through photog- 

 raphy, that anything has been known of their former existence. The 

 reason for their sudden appearance is unknown. It is probable that 



