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case, comets' tails ought in many instances to present the appearance of transverse streaks. As a matter of fact, the tail of Donati's comet did present such an appearance; a fact which seems to supply somewhat remarkable evidence in favour of Tyndall's theory.
It may be mentioned that Benedict Prévôt long ago suggested a view so closely resembling Professor Tyndall's (though inferior in the all-important respect that it was a mere speculation, not an hypothesis founded on observed relations) that the same arguments available against one may be urged with apparently equal force against the other. He considered that the head of a comet is converted by the sun's heat into invisible vapour extending to an enormous distance from the head in all directions. Behind the head this vapour is cooled, because it is sheltered from the sun's heat. It therefore condenses into cloud, which reflects light, and forms the comet's tail. This cloud he assumed to be dissipated precisely as Professor Tyndall assumes the old tails to be destroyed.
Dr. Huggins, F.R.S., whose spectroscopic researches have given us the first real facts we have obtained respecting the structure of comets, remarks that Prévôt's theory is obviously inconsistent with the observed appearances and forms of the tails, and especially with the rays which are frequently projected in a direction different from that of the tail, with the absence of tail immediately behind the head, and with the different degrees of brightness of the sides of the The two last peculiarities seem wholly inexplicable on Tyndall's hypothesis, and therefore it may seem unnecessary to consider the first. I may as well remark, however, that there is a possibility of explaining the existence of subsidiary tails in certain directions, as due to the refractive power which irregularities in the head may exert on rays passing through it; or we may even suppose that the brighter planets (which undoubtedly reflect actinic rays, since it has been found possible to photograph these bodies) may in certain cases have caused these smaller tails by pouring their rays through the head of the comet in the same manner as the sun is supposed to do according to the theory, though with less energy.
The existence of subsidiary tails or multiple tails generally is indeed at least as inconsistent with the idea of a repulsive force exerted by the sun, as with the negative shadow' theory. We can understand that light should be so refracted in its passage through the head of a comet (with its envelopes within envelopes and central spherical nucleus) as to be sent off, according to the part of the head on which it fell, in the various directions actually observed in several instances; whereas a repulsive action exerted by the sun on the matter thrown off from the head seems wholly inconsistent with subsidiary tails stretching directly from the comet's head at a considerable angle with the principal tail.
That the luminous envelopes have the power of absorbing or reflecting certain rays and suffering
others to pass through them is accordant with observation. It is certain, for instance, that the brilliant comet called Donati's (which appeared in 1858) did not reflect the actinic rays, since Dr. De la Rue was unable to photograph this object. He exposed a sensitised collodion plate to the action of the comet's light, in the focus of his 13-inch reflector, for three minutes, without obtaining the slightest trace of an image, though a small star which happened to be close to the comet left its impression twice over the clockwork having received a slight disturbance). And again, after exposure for fifteen minutes, during which time the faint luminosity of the sky had appreciably affected the collodion-plate, the comet obstinately refused to leave any trace of its figure. We see then that in this case (and doubtless in many others, if not in all cases) the actinic rays passed freely through the matter which reflected the light-waves to us, and so rendered the comet visible.
We must not forget the evidence which the spectroscope has afforded respecting the structure of comets. We have learned, by means of Dr. Huggins's observations with this instrument, that the nucleus of a comet consists (at least in every case yet observed) of selfluminous gas. In one case it has even been found possible to determine the exact nature of the gas, and thus we are able to pronounce that Winnecke's comet (which appeared in 1868) consists of the luminous vapour of carbon. The coma—that is, the faint light
See the preceding paper.
around the nucleus—is found, on the other hand, to shine in part by reflecting solar light. Of the tails of comets we have as yet learned nothing, and we must wait for the appearance of a brilliant and long-tailed comet before hoping for definite information respecting the nature of these appendages.
Another fact, which must not be left out of consideration in forming a theory of comets, is that which was discovered in 1866–67 by the united labours of Peters, Tempel, Schiaparelli, Adams, and Leverrier, but must be held to be more intimately associated with the name of Professor Adams than with that of any other astronomer. I refer to the remarkable correlation between comets and meteor-systems, according to which meteoric bodies are found to travel in the same orbits as certain comets. How it comes about that the track of vaporous bodies like the comets should be followed by numbers of minute solid bodies such as the meteors, it would be difficult to explain in the present state of our information respecting comets. But no theory of comets can be considered complete in which this relation is left unaccounted for.
It is evident that he who would form a consistent and satisfactory theory of comets will have no easy task. In the absence of definite information on many points, it seems at present even hopeless to attack the question. Doubtless, as Dr. Huggins has remarked, ' we must wait for further positive knowledge of the nature of cometary phenomena, until the searching method of analysis by the prism can be applied to the series of changes presented by a brilliant comet.' Then we require further knowledge respecting the relation between meteors and comets, and between both these classes of bodies and that strange phenomenon the zodiacal light, the peculiarities of which will be found, I venture to predict, to be much more intimately associated with cometic phenomena than is at present commonly supposed. Yet again, we must make an approach towards mastering the relations which exist between the sun's action as a centre of many forms of force, and the phenomena of terrestrial magnetism, looking upon these phenomena as indicative of processes which affect the whole solar domain. When we remember that the appearance of intensely brilliant light-patches on the sun's orb, has been found to be accompanied by an instantaneous thrill of the whole magnetic frame of the earth, presently followed by the appearance of auroral lights in both hemispheres, we recognise the action of solar influences which must be capable of largely affecting such bodies as the comets.
But again, in forming a theory of comets, account must be taken of every phenomenon of importance which these bodies have exhibited to the telescopic observer. The jets of light which the nucleus seems to throw out towards the sun, the mode in which the envelopes are formed round the head, the peculiar distribution of light and shade across the breadth of the tail, the dark space behind the head, the strange configuration of the tail, and the occurrence of mul