The smaller comets, such as are visible only in telescopes, or with difficulty by the naked eye, and which are by far the most numerous, offer very frequently no appearance of a tail, and appear only as round or somewhat oval vaporous masses, more dense toward the centre, where, however, they appear to have no distinct nucleus, or anything which seems entitled to be considered as a solid body. This was shown in a very remarkable manner in the case of the comet discovered by Miss Mitchell in 1847, which on the 5th of October in that year passed _centrally_ over a star of the fifth magnitude: _so_ centrally that with a magnifying power of 100 it was impossible to determine in which direction the extent of the nebulosity was greatest. The star’s light seemed in no degree enfeebled; yet such a star would be completely obliterated by a moderate fog, extending only a few yards from the surface of the earth. And since it is an observed fact that even those larger comets which have presented the appearance of a nucleus have yet exhibited _no phases_, though we can not doubt that they shine by the reflected solar light, it follows that even these can only be regarded as great masses of thin vapor, susceptible of being penetrated through their whole substance by the sun-beams, and reflecting them alike from their interior parts and from their surfaces. Nor will any one regard this explanation as forced, or feel disposed to resort to a phosphorescent quality in the comet itself, to account for the phenomena in question, when we consider the enormous magnitude of the space thus illuminated, and the extremely small _mass_ which there is ground to attribute to these bodies. It will then be evident that the most unsubstantial clouds which float in the highest regions of our atmosphere, and seem at sunset to be drenched in light, and to glow throughout their whole depth as if in actual ignition, without any shadow or dark side, must be looked upon as dense and massive bodies compared with the filmy and all but spiritual texture of a comet. Accordingly, whenever powerful telescopes have been turned on these bodies, they have not failed to dispel the illusion which attributes _solidity_ to that more condensed part of the head which appears to the naked eye as a nucleus; though it is true that in some a very minute stellar point _has_ been seen, indicating the existence of something more substantial. [Illustration: Fig. 24.—Head of Comet]