244 ANNUAL OF SCIENTIFIC DISCOVERY. 



" Last January a small t«>lescopic comet was visible. It was a 

 nearly circular, very lalut, vaporous mass. Neaily in the centre, 

 a small and rather dim nucleus was seen. When this object was 

 viewed in the spectroscope, two spectra were distinguished : — a 

 very faint continuous si)ectrum of the coma, showing that it was 

 visible by retiecting solar light: — about the middle of this faint 

 spectrum a bright point was seen. This bright point is the spec- 

 trum of the nuch'us, and shows that its light is different from that 

 of the coma. This short bright line indicates that the mieltius of 

 this comet was self-luminous, and, further, the position of this line 

 of the spectrum suggests that the material of the comet was 

 similar to the matter of which the gaseous nebulas consist. 



*' Measures of the Iidrin.sic Brujlitness of the Nehulce. — It ap- 

 peared to me that some information of the nature of the nelnilaj 

 might be obtained from observations of another order. If ph^'si- 

 cal changes, of the magnitude necessary for the conversion of the 

 gaseous bodies into suns, are now in progress in the nol)ula3, 

 surely, this process of development would be accompanied by 

 marked changes in the intrinsic brightness of their light, and in 

 their size. 



" Now, since the spectroscope shows these bodies to be continu- 

 ous masses of gas, it is possible to obtain an ai)proximate measure 

 of their real briglitness. It is known that as long as a distant 

 object remains of sensible size, its brightness remains unaltered. 

 By a new photometric method, I found the intrinsic intensity of 

 the light of three of the gaseous nebula3, in terms of a sperm 

 caudle burning at the rate of 158 grains per hour : — 



Nebula No. 4,628 < ros*'^ P^* °^ *^® intensity of the candle. 



^6032 



Annular Nebula Lyra < d " " " 



^ 6032 



Dumb-bell Nebula )_J_th " " " 



i 19604 



"These numbers represent, not the apparent brightness only, 

 but the true brightness of these luminous masses, except so far as 

 it may have been diminished by a possible power of extinction 

 existing in cosmical space, and by the absorption of our atmos- 

 phere. It is obvious that similar observations, made at consider- 

 able intervals of time, may show whether the light of these ol:)jects 

 is undergoing increase or diminution, or is subject to a periodic 

 variation . * 



" If the Dumb-bell Nebula, the feeble light of which is not more 

 than the one-twenty-thousandth part of that of a candle, be, in 

 accordance with popular theory, a sun-germ, then it is scarcely 

 possible to jjut into an intelligible form the enormous number of 

 times by which its light must increase, before this faint nebula, 

 feebler now in its glimmering than a rushlight, can rival the 

 dazzling sjjlendor of our sun. 



" Measures of the Nehulce. — Some of the nebulae are sufficiently 



