122 ASTRONOMY FOR 1889, 1890. 



the blue. The importance of this observation, especially when taten 

 with the report as to the character of the line, is of the highest kind in 

 its bearing on Mr. Lockyer's great meteoritic theory. If the chief neb- 

 ular line is not the remnant of the magnesium fluting the very keystone 

 is knocked away from the arch and the edifice as such falls to pieces. 

 No doubt there would be many isolated fragments of considerable value 

 still left. The structure mighteveu beput together again, hereafter, on 

 a new plan, and with a more lasting result, but the theory as it now 

 stands— the theory as a whole — would be irretrievably wrecked. On 

 the other hand, if the identity which Mr. Lockyer asserts were estab- 

 lished, it would be a victory for him of the first importance. 



It is indicative of the progress of practical spectroscopy that the 

 whole question turns on an almost inappreciable difference of position, 

 the mean value for the wave-length of the nebular line as found by Mr. 

 Keeler from ten nebulae, being 5,005.68 tenth-meters, whilst that of the 

 fluting of magnesium is 5,006.36. In the brightest nebula examined the 

 wave-length obtained was 5,006.13 tenth-meters, only 0.23 distant from 

 the magnesium fluting. As the observations stand they point strongly 

 to the nebular line being slightly but distinctly more refrangible than 

 the edge of the magnesium fluting, and therefore not due to it. But 

 the amount of displacement is not so great as to make it altogether in- 

 conceivable that it is due to tho relative motion of the nebulte and the 

 solar system, for all the ten nebula? observed are in that hemisphere 

 toward which the sun is travelling, and seven of them are within 45° of 

 the apex of the " Sun's Way," so that a correction must be applied 

 which would tend to bring the nebular line nearer to the fluting; how 

 much nearer we cannot, in our ignorance of the speed of the sun's 

 motion in space, at present say, but a rate of 36 miles per second would 

 suffice to make the accord a perfect one. If Mr. Keeler could obtain a 

 series of comparisons of the F line in these nebuhp with hydrogen, the 

 problem would be solved. Or the deter ujination of the place of the 

 line in a number of nebuli« in the hemisphere we are leaving would go 

 far to settle the matter. In the mean time it is still possible that the 

 eventual result may favor Mr. Lockyer's theory. It may be added in 

 reference to Mr. Lockyer's paper, appearing in Ko. 293 of the Proceed- 

 ings of the Eoyal Society, that if we accept Mr. Keeler's measures it is 

 clear that Mr. Lockyer did not eurploy sufficient dispersion to decide 

 the point at issue. 



The second point brought out by Mr. Keeler's measures is the fact 

 that the nebulii? have very distinct movements of their own. As we do 

 not yet know to what substance the chief nebular line is due, and as 

 Mr. Keeler could not make any measures of the blue hydrogen line in 

 the nebuhT at all comparable in accuracy to those he made of the chief 

 line, we can not say that the difference in position of the chief line from 

 any given comparison line is due to the motion of the nebula. All we 

 can do at present is to observe a number of nebulte, adopt the mean 



