234 Royal Society : — On a Comet-Spectrum. 



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

 nehulous mass surrounding a very minute and not very bright nu- 

 cleus. 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 

 characterizes 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 

 comae and tails of comets to be, could retain the degree of heat ne- 

 cessary 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 sun*. 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 conti- 

 nuous 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 probably in the condition of fog or cloud. 



We know, from observation, that the comas and tails of cornets are 

 formed from the matter contained in the nucleus. 



The usual order of the phenomena which attend the formation of 

 a tail appears to be that, as the comet approaches the sun, material 

 is thrown off, at intervals, from the nucleus in the direction towards 



* This conclusion is in accordance with the results of observations on the 

 polarization of the light of the tails of some comets. Some of these observa- 

 tions 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 Obser- 

 vatory of Harvard College, vol. hi. pp. 305-310. 



