APPLIED TO THE HEAVENLY BODIES. 



207 



of the nebulae. In 18G4 Donati found that the spectrum of a comet visible in 

 that year consisted of bright lines. 



Last January a small telescopic comet was visible. Its appearance in a large 

 telescope is represented on the screen. It was a nearly circular, very faint va- 

 porous mass. Nearly 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 spectrum of the coma shoAving that it was visible by 

 reflecting solar light. About the middle of this faint spectrum a bright point 

 was seen. This bright point is the spectrum of the nucleus, and shows that ita 

 light is different from that of the coma. This short bright line indicates that 

 the nucleus of this comet was self-luminous, and further, the position of t'lis line 

 of the spectrum suggests that the material of the comet wan similar to the tnat- 

 ter of which the gaseous nehulce consist. 



MEASURES OF THE IXTRINSIC BRIGHTNESS OF THE NEBUL.E. 



It appeared to me that some information of the nature of the nebulse might 

 be obtained from observations of another order. If pliysical changes of the 

 magnitude necessary for the conversion of the gaseous bodies into suns are now 

 in progress in the r buh^e, surely this process of development would be accom- 

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

 size. 



Now since the spectroscope shows these bodies to be continuoius masses of 

 gas, it is possible to obtain an approximate measure of their real brightness. 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 intens- 

 ity of the light of three of the gaseous uebulaj in terms of a sperm caudle burn- 

 ing at the rate of 158 grains per hour : 



Nebula No. 4,G28, j-^^^ part of the intensity of the candle. 



Annular nebula, Lyra, -^-q^^ P^""*' ^^ ^^^ intensity of the candle. 



Dumb-bell nebula, tq^oT P^^'* ^^ ^^^ intensity of the candle. 



These numbers represent nut the apparetit brightness only, but the true 

 brightness of these luminous masses, except so far as it may have been dimin- 

 ished by a possible power of extinction existing in cosmical space, and by the 

 absorption of our atmosphere. It is obvious that similar observations, made at 

 considerable intervals of time, may show whether the light of these objects 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 one 

 twenty-thousandth part of that of a candle, be in accordance with popular theory 

 a sun-germ, then it is scarcely possible to put in an intelligible form the enor- 

 mous 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 splendor 

 of our sun. 



MEASURES OF THE NEBULA. 



Some of the nebulce are sufficiently defined in outline to admit of accurate 

 measurement. By means of a series of micrometric observations, it may be 

 possible to ascertain whether any considerable alteration in size takes place in 

 nebulse. 



METEORS. 



Mr. AI-3xander Herschel has recently succeeded in subjecting another order 

 of the heavenly bodies to prismatic analysis. He has obtained the spectrum of 

 a bright meteor, and also the spectra of som'^. of the trains which meteors leave 

 behind them. A remarkable result of his observations appears to be that sodium, 

 in the state of luminous vapor, is present in the trains of most meteors. 



