140 



Mr. J. N. Lockyer. 



[Nov. 17, 



probable that all the spectroscopic phenomena of Nova Cygni will 

 admit of explanation on the supposition that it was produced by the 

 collision of two swarms of meteorites. The outliers were first 

 engaged, and at the maximum the denser parts of the swarm. 



Difficulties connected ivith the Discussion. 



An inspection of the maps, on which are shown all the observations 

 already made upon bright lines recorded in the spectra of celestial 

 bodies, will indicate at first sight an apparent variation of the positions 

 of the lines greater than might have been expected. This, however, 

 I think will vanish on the consideration of the whole question ; and 

 for my part certainly all the examinations which I have been able to 

 make have led me to the conclusion that the various observations have 

 been far better than it was almost possible to hope for when the great 

 difficulties of the observations themselves are considered. 



When it is remembered that, in order to get a determination of the 

 position of a bright line, comparison-spectra and prisms are needed, 

 and that, from mechanical considerations alone, the application of 

 these aids to research is very frequently attended with difficulties and 

 uncertainties ; and further, when we consider that many of the 

 observations have been necessarily made without these aids ; the 

 striking coincidences on the maps become of very much greater 

 importance than the slight variations seen between the positions of 

 the same line recorded by different observers in the same star. 



It will be observed, too, that the information in some cases is fuller 

 in the blue part of the spectrum. Here again a reference to what the 

 maps are really intended to show is necessary. The maps do not 

 show the complete spectrum observed, but only the bright lines 

 recorded in it. The actual observations have really consisted in 

 picking out these bright lines from the background of continuous 

 spectrum, whether in stars, nebulae, or comets ; and, as the continuous 

 spectrum will be generally brightest in the yellow and green, so in 

 this part of the spectrum we must expect, first of all, to get the least 

 information, and then, when the information is obtained, to get the 

 greatest uncertainty, on account of the difficulty brought about 

 by the greater luminosity of the background on which the line 

 appears. 



The discussion by Hasselberg and others of the various observa- 

 tions of comets which have been made from time to time indicates that 

 under certain circumstances, where men of the highest skill and with 

 the greatest care have determined the wave-lengths of the carbon 

 bands, discrepancies exist too great to admit of their being attributed 

 to errors inherent in this branch of observation. 



If for a moment we consider alone the two bright flutings visible in 

 the spectrum of carbon, one with its bright edge just more refrangi- 



