6 



Mr. Huggins on the Spectrum 



[Jan. 18, 



I, 1866. The telescope and spectrum-apparatus which I employed are 

 described in my paper " On the Spectra of some of the Nebulse " *. 



The appearance of this comet in the telescope was that of an oval nebu- 

 lous mass surrounding a very minute and not very bright nucleus. The 

 length of the slit of the spectrum-apparatus was greater than the diameter 

 of the telescopic image of the comet. 



The appearance presented in the instrument when the centre of the 

 comet was brought nearly upon the middle of the slit, was that of a broad 

 continuous spectrum fading away gradually at both edges. These fainter 

 parts of the spectrum corresponded to the more diffused marginal portions 

 of the comet. Nearly in the middle of this broad and faint spectrum, and 

 in a position in the spectrum about midway between b and F of the solar 

 spectrum, a bright point was seen. The absence of breadth of this bright 

 point in a direction at right angles to that of the dispersion showed that 

 this monochromatic light was emitted from an object possessing no sensible 

 magnitude in the telescope. 



This observation gives to us the information that the light of the coma 

 of this comet is different from that of the minute nucleus. The nucleus 

 is self-luminous, and the matter of which it consists is in the state of 

 ignited gas. As we cannot suppose the coma to consist of incandescent 

 solid matter, the continuous spectrum of its light probably indicates that 

 it shines by reflected solar light. 



Since the spectrum of the light of the coma is unlike that which cha- 

 racterizes the light emitted by the nucleus, it is evident that the nucleus is 

 not the source of the light by which the coma is rendered visible to us. It 

 does not seem probable that matter in the state of extreme tenuity and 

 diffusion in which we know the material of the comes and tails of comets 

 to be, could retain the degree of heat necessary for the incandescence of 

 solid or liquid matter within them. We must conclude, therefore, that 

 the coma of this comet reflects light received from without ; and the only 

 available foreign source of light is the sunf. If a very bright comet were 

 to visit our system, it might be possible to observe whether the spectra of 

 the coma and the tail contain the dark lines which distinguish solar light. 

 If the continuous spectrum of the coma of Comet 1, 1866, be interpreted 

 to indicate that it shines by reflecting solar light, then the prism gives no 

 information of the state of the matter which forms the coma, whether it be 

 solid, liquid, or gaseous. Terrestrial phenomena would suggest that the 

 parts of a comet which are bright by reflecting the sun's light, are pro- 

 bably in the condition of fog or cloud. 



* Phil. Trans. 1861, p. 421. 



t This conclusion is in accordance with the results of observations on the polariza- 

 tion of the light of the tails of some comets. Some of these observations appear to 

 have been made with the necessary care. See J. P. Bond's "Account of the Great 

 Comet of 1858," Annals of the Astronomical Observatory of Harvard College, 

 vol. iii. pp. 305-310. 



