442 



M-ATURE 



[MaKCII 7, IQOI 



time of discovery. Since it has diminished in brightness 

 this has changed, and on the night of February 27 a 

 reddish tinge was observed. 



Although the sky on Monday evening was by no 

 means free from clouds, ten very satisfactory photographs 

 were secured with the three instruments in regular use for 

 stellar spectra. Edwards's isochromatic plates were used, 

 as it was considered desirable to secure a record of the 

 green part of the spectrum. 



Although there has not been time for a complete dis- 

 cussion of these photographs, it may be stated that the 

 spectrum contains numerous dark lines, several of which 

 are associated with bright bands on the less refrangible 

 side. Further, the spectrum, as a whole, greatly 

 resembles that of Nova Aurigae. 



One of the chief features of the principal bright lines is 

 their great width, amounting to 30 tenth-metres, and each 

 is accompanied by a dark line of considerable breadth on 

 its more refrangible side. A comparison spectrum of 

 y Orionis, photographed alongside that of the Nova on 

 one of the plates, indicates that the middle portions of the 

 bright lines are not far from their normal positions ; those 

 of the dark ones, however, are displaced by some 1 5 tenth- 

 metre:s towards the violet, thus indicating a differential 

 movement of something like 700 miles a second. 



Movements more rapid and disturbances more violent 

 than those observed in Nova Aurigae are therefore indi- 

 cated ; both by the greater displacement of the dark lines 

 relatively to those that are bright and the greater breadth 

 of the bright and dark lines. 



tested by inquiring whether other prominent enhanced' 

 lihes of iron so strongly visible in the spectrum of a Cygni 

 were present. 



A comparison with the spectrum of this star photo- 

 graphed with the same instruments suggested that many 

 lines between F and h in the Nova probably correspond 

 with lines in a Cygni. Certainty could not be arrived at 

 in consequence of the great breadth of the lines in the 

 Nova, , ■ 



Hence, as the Nova bore some resemblance to both 

 Nova Aurigae and a Cygni, a reference was suggested 

 to the lines recorded in the spectrum of Nova Aurigae 

 which were observed when the light of that star was on 

 the wane, and when the lines were thinned enough to be 

 easily measurable. I may also add that these observa- 

 tions were made before the work on enhanced lines was- 

 undertaken. 



The importance of this reference was strengthened by 

 the consideration that with such a tremendous outburst 

 we should expect the original invisible swarm to have been 

 (very rapidly) advanced to a considerable condensation^ 

 at the locus of impact, and therefore to resemble some 

 "star" which had (slowly) arrived at a position pretty 

 high up on the ascending temperature curve in the 

 ordinary course of evolution on the meteoritic hypo- 

 thesis.- 



A comparison of the bright Hnes recorded by Campbell* 

 and Vogel ^ in the spectrum of Nova Auriga^ with the- 

 strongest lines of a Cygni — a very detailed record of the 

 spectrum of which star has been recently compiled here 



rl.i 



Fig. I. — Spectrum of Nova and a Persei compared. 



'The comparison of spectra shows us that we are deal- 

 ing with two swarms, one of which, the less dense, gives 

 us broad bright lines and is almost at rest with reference 

 to the line of sight ; the denser swarm, indicated by the 

 dark lines, is in most rapid movement in the line of sight 

 towards the earth. 



An interesting feature of the spectrum is the presence 

 of finfe dark lines down the middle of each of the bright 

 lines of hydrogen and calcium ; these are most probably 

 reversals, and if this be so, they will be of great service 

 for accurate determination 'of the wave-lengths of the 

 other bright lines. " » -^ * 



The dark hydrogen line Hy, and perhaps H/3 and H5, 

 are also possibly reversed. u • • 



'Eye observations showed among the (ihief lin<es*a group 

 of four in the green ; one probably H/3, the others near 

 XX 492, 501 and 517 ; a bright line at or near D, and a 

 brilliant red linfe probably corresponding to Ha. Each 

 of these was accompanied by a dark broad line on its 

 more refrangible side. Other lines of less brightness 

 were observed bo'th in the green and red. 



It at first seemed probable that two of the bright lines 

 in the green (XX 492 and 501) might be due to asterium, 

 while that in the orange was perhaps the helium line D3. 

 Subsequent investigation, however, suggested as an alter- 

 native origin that these lines might be the enhanced lines 

 of iron at X 4924*1 and 5018-6, which are very nearly in 

 the sarnie positions as the asterium lines. This view was 



NO. 1636, VOL. 63] 



— shows that there is a close agreement between the twO' 

 sets of lines. These strong a Cygni lines are almost 

 without exception the representatives of " enhanced " 

 lines of some of the metals, chiefly Fe, Ti, Cr, Ni, Ca,. 

 Sr, and Sc. If we exclude the lines of hydrogen from 

 those which were recorded in the spectrum of Nova 

 Aurigae, there remain forty-four lines for comparison. 

 Thirty of these, or about 70 per cent., agree approxi- 

 mately in position with either strong isolated lines or 

 groups of lines in the spectrum of a Cygni. 



It may be assumed that, taking into consideration the 

 broad nature of the Nova lines, if there be any genuine- 

 connection between them and the lines of a Cygni, any 

 close groups of , separately distinguishable lines in the 

 latter spectrum would be thrown together in the Nova 

 spectrum, and appear as broad bands. A good instance 

 of this appears in Campbell's list. He records a band 

 extending from XX 4534 to 4501. In the spectrum of a 

 Cygni there is' a strong line at each of the positions- 

 given, and between them there occurs a strong quartet 

 of lines. The former are well enhanced lines of titanium,, 

 and the latter of iron. It seems extremely likely^ 

 therefore, that the six lines thrown together produce the 

 apparently continuous band observed by Campbell. ^ ' 



If the stage of a Cygni has really been reached, the 

 following considerations come in : — ^ 1 . , 



1 Ast.-Phys. Jowr., vol. xi. p. 807, 1892. 



2 Ast.-Phys. Jour., vol. xii. p. 912,. 1^93. 



Oi X-.W 



