396 MK. W. HUGGINS ON THE SPECTRA OF SOME OF THE NEBULJE. 



intensities of the nebulae in terms of the intensity of the unscreened flame of the sperm 



candle are — 



Nebula No. 4628 . . . . = j^, 



Annular nebula in Lyra . . ^ W(h~S'> 



Dumb-bell nebula . . . =T9^^i- 



It may be remarked, in connexion with these values of the intensity of the light of 

 these nebulae, that nebula No. 4628 gives a spectrum of three bright lines, and also a 

 faint continuous spectrum. The nebula in Lyra and the Dumb-bell nebula give one 

 bright line only. 



§ V. Measures of some of the NehulcB. 



If great physical changes, such as subsidence and condensation, of the magnitude 

 necessary for the conversion of the nebulae into suns are taking place in these objects, 

 an indication of the advance of these processes might perhaps be obtained by measure- 

 ments, taken at considerable intervals, of such of the nebulae as are suitable for this 

 purpose. There are several of the planetary nebulae vi^hich give a gaseous spectrum, 

 which in telescopes of moderate power have disks sufficiently well defined for micro- 

 metric measurement. Measures of these nebulae would be comparable with future 

 measures obtained with telescopes of similar power. 



Some months since I invited the Rev. W. R. Dawes, F.R.S., to take measures of the 

 diameters of several of these objects. Ill health has unfortunately prevented him from 

 measuring more than one nebula, No. 4234, 5S. Mr. Dawes writes, " So bright and 

 yet so imperfectly defined, like the heads of some comets. The moon was near the 

 horizon, yet I found that I could get hold of more of it with high powers than vrith 

 lower. It appeared rather suddenly to fade away at the edges, and to have a sort of 

 faint halo round it, which, however, was not distinctly separate from the brighter centre. 

 I obtained four sets of measures of the diameter which was parallel to the equator; but 

 thought that the form was rather elliptic, the equatorial diameter being the greater. 

 The four sets were obtained with powers 148, 218, 292, and 382; that with 148 

 appearing to be far less certain than the others. The illumination of the field necessary 

 to show the wires with power 148 seemed to diminish the visibility of the nebula more 

 than did the increase of power ; with which the wires were sufficiently seen with much 

 less illumination. The results were — 



With power 148 diam. =14-23 



218 „ =15-76 



292 „ =15-70 



382 „ =16-23 



Meanofalll5"-5. 



"Mean of the three higher powers =15" -9, which I consider worthy of much more 

 confidence. I could not see the fainter halo sufficiently well Avith any illumination to 

 get safe measures of it. These results refer to the brighter disk only." 



