There are few phenomena which have given rise to more speculation than those presented by the tails of comets. Astronomers who, in dealing with other matters, have exhibited the soundest judgment, and the most logical accuracy of argument, seem to feel free to indulge in the most fanciful speculations when dealing with this subject.
A favourite theory with the earlier astronomers was founded on the observed peculiarity that the tails of comets are usually turned directly from the sun. It was supposed that the tail is not a really existent entity, but merely indicates the passage of the solar rays through space, after their condensation by the spherical head of the comet. Just as a light received into a dark room through a small aperture appears as a long ray extending in a straight line through the room, so, according to this theory, the sun's light, concentrated by the comet's head, throws a long luminous beam into space. Unfortunately for this view there is a want of analogy between the two cases thus brought into comparison. The light shining into a room produces the appearance of a ray, because it illuminates the air and the small particles of floating dust which it encounters in its passage. There is nothing corresponding to this in the interplanetary spaces. If there were, the sky would never appear black, since the sun would always be shining on matter capable of reflecting his rays.
Kepler was the first to form a reasonable hypothesis respecting comets' tails. He supposed that the action of the solar heat dissipates and breaks up a comet's substance. The rarer portions are continually swept away, he imagined, by the propulsive energy of the solar rays, and are swept in this way to enormous distances from the comet's tail. The denser portions remain around the nucleus and form the coma.
The modern theory respecting light (according to which there is no propulsion of matter from the sun, but a simple propagation of wave-like motion), does not affect Kepler's hypothesis so much as might be imagined. Whatever theory of light we adopt we are forced to assume an extreme tenuity in the matter which forms the tails of comets. And when once we have made this assumption, we are enabled to admit that even the propagation of a wave-like motion through the ether which is supposed to occupy the interplanetary spaces, might suffice to carry off the attenuated nebulous matter with tremendous rapidity.
The defect of Kepler's theory is that it appears insufficient to account for those anomalous tail-formations which were referred to in our paper on Comet I.
Newton's hypothesis respecting comets' tails was somewhat different. He supposed that the intensely heated comet communicated its heat to the surrounding ether, which thus grew rarer and ascended in the solar atmosphere - that is, flowed away from the sun-precisely as heated air ascends from the earth. The ether thus displaced would carry away with it the rarer portions of the comet's substance, just as smoke is carried upwards by a current of heated air.
It will be seen at once that Newton's theory, like Kepler's, affords no explanation of lateral tails, or of tails turned towards the sun.
In modern times a theory has been founded on the supposition that cometic phenomena may be due to electrical agency. The German astronomer Olbers was one of the first to propound this view, and many eminent astronomers-amongst others the younger Herschel - have looked with favour upon the theory. As yet, however, we do not know enough respecting electricity to accept with confidence any theory of comets founded upon its agency.
The comet respecting which I now have to treat was discovered in the middle of June 1868, by Win--necke. At first it was a telescopic object, but it gradually increased in brilliancy until it became visible to the unaided eye. In the telescope, at the end of June, the comet appeared as a circular cloud rather brighter in the middle, where there was a roundish spot of light. A tail could be traced to a distance of about one degree from the nucleus.
Dr. Huggins quickly subjected the new arrival to spectroscopic analysis. The result, at first sight, seemed to differ little from that which had been noticed in the case of Brorsen's comet. Indeed the astronomers at the Paris observatory and the Padre Secchi at Rome were led to pronounce the spectra of the two comets to be absolutely identical. The more powerful spectroscopic appliances employed by Dr. Huggins, however, exhibited important differences.
The spectrum consisted of three bands of light separated by dark intervals. Of these bands two were greenish blue, the other greenish yellow. The two former were tongue-shaped, the last was narrowed off at both extremities.
From what I have said above respecting the nature of spectroscopic analysis, it will be understood that the distribution of the comet's light along the length of the spectrum is the most important circumstance to be attended to in endeavouring to form an estimate of the substance of the comet. But as we see that there are, in this instance, peculiarities affecting the breadth of the spectrum, it will be well briefly to consider their meaning. The matter is, in reality, simple enough, but requires a little attention.
The breadth of the spectrum corresponds to the breadth of the object which is the source of light. If that object is uniformly bright the spectrum is also uniformly bright across its breadth, whatever variations may exist in the direction of its length. But if the object is brighter in some parts of its breadth than in others, the spectrum will show corresponding variations of brilliancy across its breadth. Hitherto we have been assuming that all the light from the object is of the same kind, however it may vary in brilliancy. Suppose, however, that the light from the middle of the object gives one kind of spectrum, the light from the outer parts another; then the spectrum will vary in character as well as in brilliancy across its breadth. Suppose for example, that the middle of the object is gaseous while the outer parts are solid or liquid, then the appearance presented would be two thin streaks of rainbow-tinted light, separated by a dark space 1 across which would be seen the bright lines belonging to the gaseous central part of the luminous object.