382 ME. W. HUGGINS ON THE SPECTEA OF SOME OF THE KEBFL^. 



nexion with these observations. The intensity of tlie brightest line of the gaseous 

 nebula? is in most cases greater than the intensity of the light of the same refrangiUlity 

 of those nebulae and clusters which furnish a spectrum which is apparently continuous. 

 The superior intensity of the light may indicate a more intense heat. It may be there- 

 fore that of all the objects usually classed under the denomination of nebulae and 

 clusters, those which give a gaseous spectrum are, as a class, to be regarded as the 

 hottest. — June 18G6.] 



The continuous spectra of some of the nebulae and clusters are irregularly bright in 

 some parts of the spectrum ; but when the width of the slit was reduced, the bright 

 portions did not appear to become more defined, as would be the case with a spectrum 

 containing bright lines *. This irregularity of brightness may perhaps be due in some 

 cases to the probable mode of formation of these continuous spectra. In the case, at 

 least, of the clusters which the telescope resolves into stellar points, the spectrum must 

 be composed of the blending together of the spectra of the constituent bright points. 

 Now it is not improbable that these component spectra, like the spectra of the stars, 

 differ from each other in the relative brightness of their different parts. 



The positions in the spectrum of the bright lines of the gaseous nebulae described in my 

 former paper were determined by a simultaneous comparison in the instrument of these 

 lines with the bright lines of nitrogen, hydrogen, and barium. The bright lines of the 

 gaseous nebulae referred to in this paper were not compared directly with any terrestrial 

 spectra, partly because of the great faintness of most of these objects, and partly because 

 the former comparisons were found to be injuriously fatiguing to the eye. 



The value of this application of spectrum analysis appears to me to consist chiefly 

 in the assistance which this method of observation may afford us in ascertaining the 

 true nature of the nebulae, and the relation which they sustain to the other orders of 

 the heavenly bodies. I have therefore added to my prismatic observations such of the 



* The peculiar appearance of the continuous spectra of some of the nebulse and clusters has suggested to me, 

 from my first examination of them, that possibly the luminous points into which the telescope resolves some of 

 these objects may not be of the same nature as the true stars. My observations of the great nebula in Andro- 

 meda and of its small but bright companion in August 1864, were recorded thus: — " The spectrum appears to 

 end abruptly in the orange ; and throughout its length is not uniform, but is evidently crossed either by lines 

 of absorption or by bright Hnes"^. [The same characters have since been found in several of the brighter 

 nebulae and clusters. It is possible to explain the absence of the less refrangible rays, which are wanting in 

 these spectra, by supposing them to have been intercepted by absorbent vapours. The apparently complete want 

 of light in this part of the spectrum, and the unequal, mottled appearance of the brighter parts of the spectrum, 

 suggest rather that the light may have emanated from a gaseous source, and that the spectrum consists of 

 numerous hrighX lines. The faintness of these spectra has prevented me from using a slit sufficiently narrow for 

 the determination of their true nature. Some quite recent observations, which are not yet complete, appear 

 t« support the \-iew that the bright points of some clusters may possess a physical constitution which is not 

 analogous to th^t of the sun, and the brighter of the separate stars. — June 1866.] 



' Philosophical Transactions, 1864, p, 442. 



