530 Mr. W. Huggins on the Spectra 



be either that of a hollow spherical shell or of a flat disk pre- 

 sented to us (by a highly improbable coincidence) in a plane pre- 

 cisely perpendicular to the visual ray"*. This absence of con- 

 densation admits of explanation, without recourse to the suppo- 

 sition of a shell or of aflat disk, if we consider them to be masses 

 of glowing gas. For supposing, as we probably must do, that 

 the whole mass of the gas is luminous, yet it would follow, by 

 the law which results from the investigations of Kirchhoff, that 

 the light emitted by the portions of gas beyond the surface visible 

 to us, would be in great measure, if not wholly, absorbed by the 

 portion of gas through which it would have to pass ; and for 

 this reason there would be presented to us a luminous surface 

 onlyf. 



Sir John Herschel further remarks J, " Whatever idea we may 

 form of the real nature of the planetary nebulae, which all agree 

 in the absence of central condensation, it is evident that the in- 

 trinsic splendour of their surfaces, if continuous, must be almost 

 infinitely less than that of the sun. A. circular portion of the 

 sun's disk subtending an angle of 1' would give a light equal 

 to that of 780 full moons, while among all the objects in ques- 

 tion there is not one which can be seen with the naked eye." 

 The small brilliancy of these nebulse is in accordance with the 

 conclusions suggested by the observations of this paper; for, 

 reasoning by analogy from terrestrial physics, glowing or lumi- 

 nous gas would be very inferior in splendour to incandescent 

 solid or liquid matter §. 



* Outlines of Astronomy, 7th edit. p. 646. 



t Sir William Herschel in 1811 pointed out the necessity of supposing 

 the matter of the planetary nebulae to have the power of intercepting light. 

 He wrote: — "Admitting that these nebulae are globular collections of 

 neulous matter, they could not appear equally bright if the nebulosity 

 of which they are composed consisted only of a luminous substance per- 

 fectly penetrable to light Is it not rather to be supposed that a cer- 

 tain high degree of condensation has already brought on a sufficient con- 

 solidation to prevent the penetration of light, which by this means is re- 

 duced to a superficial planetary appearance?" 



" Their planetary appearance shows that we only see a superficial lustre 

 such as opake bodies exhibit, and which could not happen if the nebulous 

 matter had no other quality than that of shining, or had so little solidity 

 as to be perfectly transparent." — Philosophical Transactions, 1811, pp. 314, 

 315. 



X Outlines of Astronomy, 7th edit. p. 646. 



§ The author has made an attempt to determine approximately the in- 

 trinsic brightness of three of the gaseous nebulae. It is probable that these 

 bodies consist of continuous masses of material. In the telescope they 

 present surfaces subtending a considerable angle. As long as a distant 

 object is of sensible size in the telescope, its original brightness remains 

 unaltered. By a suitable method of observation, the intensities of these 



