1877.] 



On Bright Lines in the Spectra of Nebulae. 



179 



convex side is screened off, so as to let the light shine only on the con- 

 cave, continuous rotation is produced at the rate of one revolution in 

 6*95 seconds, the concave side being apparently attracted. These experi- 

 ments show that the repulsive action of radiation on the convex side is 

 about equal to the attractive action of radiation on the concave side, and 

 that the double speed with which the fly moves when no screen is inter- 

 posed is the sum of the attractive and repulsive actions. 



14. Radiometer. — A two-disk, cup-shaped, aluminium radiometer, lamp- 

 blacked on the concave surfaces. In this instrument the usual action of 

 light is reversed, rotation taking place, the bright convex side being 

 repelled, and the black concave attracted. "When the light shines only 

 on the bright convex side, no movement is produced ; but when it shines 

 on the black concave side, this is attracted, producing rotation. 



15. Radiometer. — A cup-shaped radiometer similar to the above, but 

 having the convex surfaces black and the concave bright. Light shining 

 on this instrument causes it to rotate rapidly, the convex black being 

 repelled. No movement is produced on letting the light shine on the 

 bright concave surface, but good rotation is produced when only the 

 black convex surface is illuminated. 



16. Radiometer.— A multiple-disk, cup-shaped, turbine radiometer, 

 bright on both sides, working by the action of warm water below and the 

 cooling effect of the air above. 



17. Radiometer.— A four-armed, metallic radiometer with deep cups, 

 bright on both sides. 



18. Radiometer. — A four-armed radiometer, the vanes consisting of 

 mica cups, bright on both sides. 



19. Radiometer. — A four-armed radiometer having clear mica vanes, 

 the direction of motion being determined by the angle formed by the 

 mica vanes with 'che inner surface of the glass bulb. 



IV. " On the Inferences to be drawn from the Appearance of 

 Bright Lines in the Spectra of Irresolvable Nebulae/' By 

 William Huggins, D.C.L., LL.D V F.R.S. Received April 

 26/1877. 



In a paper recently read before the Eoyal Society, Mr. Stone attempts 

 to show that the fact that the spectra of some of the irresolvable nebulae 

 consist mainly of bright lines does not warrant the inference that these 

 bodies are of a constitution different from our sun and the generality of 

 the fixed stars, and consist mainly of glowing gas, so far, at least, as the 

 light-giving portion of them is concerned. 



Waiving for the present the objections which may be urged against 

 Mr. Stone's reasoning, let us consider the question in the light of the 

 results afforded by actual observation. 



