562 
ME. W. HUGGINS ON THE SPECTEA 
There is one observation made by Bunsen which appears to stand as an exception to 
the rule that only bodies in the gaseous state give, when luminous, discontinuous spectra. 
Bunsen discovered that solid erbia, when heated to incandescence, gives a spectrum con- 
taining bright bands. It is therefore conceivable, though all the evidence we possess 
from experience is opposed to the supposition, that carbon might exist in some form in 
which it would possess a similar power of giving a discontinuous spectrum without vola- 
tilization. There is the further objection to this hypothesis, that the telescopic pheno- 
mena observed in comets appear to show that vaporization does usually take place. 
However this may be, a state of gas appears to accord with the very small power of 
reflexion which the matter of the coma of this comet possessed, as was shown by the 
great faintness of the continuous spectrum. 
A remarkable circumstance connected with comets is the great transparency of the 
bright cometary matter. The most remarkable instance is that of Miss Mitchell’s 
comet in 1847, which passed centrally over a star of the fifth magnitude. “ The star’s 
light appeared in no way enfeebled, yet such a star would be completely obliterated by 
a moderate fog extending a few yards from the surface of the earth”*. We do not 
know what amount of transparency is possessed by the vapour of carbon, but the absence 
of a continuous spectrum seems to show that, as it existed in the comet, it was almost 
perfectly transparent. The light of a star would suffer, therefore, only that kind and 
degree of absorption which corresponds with its power of radiation, as shown by its 
spectrum of bright lines. As these occur in the brightest part of the spectrum, we 
should expect a noticeable diminution of the star’s light, if it were not for the luminous 
condition of the gas, in consequence of which it would give back to the beam light of 
precisely the same refrangibilities as it had taken, and so enable the part of the field 
occupied by the image of the star to appear of its original brightness, or nearly so. 
This state of things would not prevent an apparent diminution of the star’s light from 
the effect upon the eye of the brightness of the surrounding field. In the case of the 
tails of comets, the great transparency observed is more probably to be referred to the 
widely scattered condition of the minute particles of the cometary matter. 
I may be permitted to repeat here a paragraph from my paper on the Spectrum of 
Comet I., 1866f. 
“ 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 comae and tails of comets are formed from the 
matter contained in the nucleus^. 
* Outlines of Astronomy, p. 373. t Proceedings of the Eoyal Society, vol. xv. p. 5. 
t The head of Halley’s comet in 1835 in a telescope of great power “ exhibited the appearance of jets as it 
were of flame, or rather of luminous smoke, like a gas fan-light. These varied from day to day, as if wavering 
backwards and forwards, as if they were thrown out of particular parts of the internal nucleus or kernel which 
shifted round, or to or fro by their recoil like a squib not held fast. The bright smoke of these jets, however, 
never seemed to be able to get far out towards the sun, but always to be driven back and forced into the tail as 
if by the action of a violent wind setting against them (always from the sun), so as to make it clear that the 
