1889.J On the Spectrum of the Great Nebula in Orion. 59 



doubtless rhythmically connected, appear to me to possess great 

 interest, especially if it should come to be found from future photo- 

 graphs that these groups are characteristic of the most tenuous part 

 of the nebula. At present, I am not able to make any suggestion as 

 to their chemical origin, but the suggestion presents itself that we 

 may have to do with some molecule of low vapour-density. 



The pair of lines on the more refrangible side of the line at X 3724, 

 may possibly be connected with the state of the nebula as it exists in 

 the neighbourhood of the stars. — April 26.] 



General Conclusions. 



It seems to me premature until we can learn more of the signifi- 

 cance of the new groups of lines, and especially of their connexion 

 with the neb alar matter generally, or with certain condensed parts 

 only, to express more than provisional suggestions as to the nature of 

 these nebulae. It may be that they represent an early stage in the 

 evolutionary changes of the heavenly bodies. 



As some physical importance, in the relation of these nebulas to 

 each other, has been given to my inability, in consequence of insuffi- 

 cient optical means in my original observations in 1864, to see all 

 three of the bright lines in some faint nebulas, I may mention that 

 in the case of one object, the Ring Nebula in Lyra, in which at that 

 time the light appeared monochromatic, as only the brightest line 

 could be certainly seen, as soon as larger means were placed at my 

 disposal by the loan of the Royal Society telescope in 1870, I had no 

 difficulty in seeing all three lines on any night of sufficient clearness. 

 There is little doubt that the same cause prevented me from seeing 

 more than the brightest line in Nebula 4572 of Herschel's ' General 

 Catalogue.' Yogel saw two lines.* 



These bodies may stand at or near the beginning of the evolutionary 

 cycle, so far as we can know it. They consist probably of gas at a 

 high temperature and very tenuous, where chemical dissociation 

 exists, and the constituents of the mass, doubtless, are arranged in 

 the order of vapour- density. As to the conditions which may have 

 been anterior to this state of things, the spectroscope is silent. We 

 are free, so far as the spectroscope can inform us, to adopt the 

 hypothesis which other considerations may make most probable. 

 On Dr. Croil'st form of the impact theory of stellar evolution, which 

 begins by assuming the existence of stellar masses in motion, and 

 considers all subsequent evolutional stages to follow from the energy 

 of this motion converted into heat by the collision of two such bodies, 

 these nebulae would represent the second stage in which these existing 



* ' Beobachkmgen zu Bothkamp,' 1872, p. 59. 

 f * Stellar Evolution,' 1889. 



