138 



Mr. J. N. Lockyer. 



[Nov. 17, 



is very remarkable how the lines seen in a Geissler tube under the 

 conditions stated, when either magnesium, or olivine, or other 

 meteoric constituents are made to glow, should appear, one may 

 almost say, indiscriminately among the orders of bodies in the heavens 

 which up to the present time have been regarded as so utterly different 

 in plan and structure as stars and nebulae. 



The records of purely continuous spectra in the case of many nebulae, 

 as for example the Great Nebula in Andromeda, is in all probability 

 an indication of our inability to observe them properly. "For a nebula 

 to give a perfectly continuous spectrum, it is evident that the compo- 

 nent meteorites must be incandescent, but still at a lower temperature 

 than that required to give bright lines. Now the Mg line 500 is seen 

 in some of the faintest nebula} where there is little or no continuous 

 spectrum, and it therefore seems likely that these are at a lower 

 temperature than the nebulae said to give perfectly continuous spectra. 

 This being so, it is difficult to believe that other lines, which require 

 a somewhat higher temperature for their existence than the line at 

 500, do not become visible at this increased temperature. 



There can be little doubt that when our instrumental appliances 

 and observing conditions become more perfeet, it will be found that 

 the so-called continuous spectra are really discontinuous. There is, 

 indeed, an element of doubt as regards some of the existing observa- 

 tions ; thus, the spectrum of the companion to the Great Nebula in 

 Andromeda appears to end abruptly in the orange, and throughout 

 its length is not uniform, but is evidently crossed by lines of absorp- 

 tion, or by bright lines.* 



Again, the Great Nebula in Andromeda is generally regarded as 

 having a continuous spectrum pure and simple, but an observer at 

 Y"ale College (name not stated), has observed three bright lines in its 

 spectrum (' Observatory,' vol. 8, p. 385). The lines are the F line of 

 hydrogen, and two other lines at wave-lengths 5312'5 and 5594*0. 

 The latter two lines are mentioned by the same observer as bright 

 lines in 7 Cassiopeiae and ft Lyrae, and are recorded by Sherman 

 (' Astr. Nachr.' No. 2691) as bright lines in these stars and in Nova 

 Andromedae. No other observations with which I am acquainted 

 give these two lines in 7 Cassiopeiae and j3 Lyrae, but Maunder 

 (' Monthly Notices,' vol. 46, p. 20) gives them as two of the lines seen 

 in Nova Andromedae. It is possible, therefore, that the two lines in 

 question, in the Yale College observations, had their origin in Nova 

 Andromedae ; at all events there is no evidence to show that they are 

 visible in the Great Nebula of Andromeda under normal conditions. 



It is not impossible that the lines at 540 and 580 may be eventually 

 traced in some of the brightest nebulae, since these are apparently the 

 lines next in order as regards temperature, to the Mg line 500. 

 * Huggins, c Phil. Trans.,' vol. 154, p. 441. 



