392 MB. W. HIJGGINS ON THE SPECTRA OF SOME OF THE NEBULA. 



in constitution, invests these objects with a new importance to the theories of cosmical 

 science. A first consideration of these nebulae would suggest that we have now evidence 

 from observation of the existence of that primordial nebulous matter required by the 

 theories of Sir William Heeschel and Laplace *. But though it should be found ulti- 

 mately that, in some of its forms, the theory of the development of aeriform matter into 

 suns and planets is a true representation of the mode of formation of the universe, still 

 it would show a want of the strict caution which experimental science demands in the 

 interpretation of observed facts, to explain the phenomena presented by the gaseous 

 nebulae in connexion with the requirements of a theory which at present is not more 

 than a speculation. In a paper " On the Spectrum of the Great Nebula in Orionf ," I 

 stated, as the result of the observations which I had then made, the provisional opinion 

 that the gaseous nebulae may belong possibly to an order of cosmical bodies distinct from 

 that represented by the sun and fixed stars. In this connexion it may be remarked that 

 my examination of the light of Comet I. 1866 J shows that a close relation probably 

 exists between nebular and cometary matter. 



As further contributions towards a future determination of the true rank and cosmical 

 relations of these nebulae, I proceed to give the results of some observations on the 

 intensity of their light, and also measures of some of the planetary nebulae. 



§ IV. On a Mode of determining the Brightness of some of the NebulcB. 



As long as the nebulae were regarded as aggregations of discrete stars separately 

 invisible, it was not possible by any photometric estimation of the light from them 

 which reaches the earth, to ascertain the intrinsic brilliancy of the suns of which the 

 nebulae were supposed to consist. For since these stars have no sensible magnitude 

 even when separately visible, their intrinsic splendour could not be estimated from their 

 brightness to an observer on the earth, so long as their distance from our system 

 remained unknown. 



Now, however, that the application of prismatic analysis to the light of the nebulae has 

 shown that some of these objects consist of luminous gas existing in masses which are 

 probably continuous, though, indeed, in some cases, this aeriform matter appears to be 

 aggregated into portions of unequal brilliancy, the intrinsic brightness of these nebulae 

 may be estimated from the earth, though their distance from us is unknown. The 

 nebulae are not points without sensible magnitude in the telescope, but present sur- 

 faces, in some cases, subtending a considerable angle. The brilliancy of a luminous 

 surface, when beyond the earth's atmosphere, does not vary with its distance from the 

 observer, except as it may be diminished by a possible power of extinction belonging to 

 celestial space. For the diminution of brightness of a luminous surface, as it becomes 



• A cosmical theory, which may perhaps be described as the converse of the nebular hypothesis, has been 

 recently suggested by Professor E. W. Bbatlet : see Proceedings of the Eoyal Society, vol. xiv. p. 120. 

 t Proceedings of the Royal Society, vol. xiv. p. 39. % Ibid. vol. xv. p. 5. 



