Supposing now the space between the two circles to represent in section the luminous matter which flows all round the nucleus, while the surface of the inner circle represents the unilluminated part behind the nucleus, the two tangent lines will represent the lines of sight on either side of the dark region, where as we might expect, we get plenty of light; and we can also understand very well why outside of that the line of sight through the luminous matter (or the chords to our outer circle), getting shorter and shorter, the light of the luminous streaks bounding this part of the tail gets fainter and fainter; but if just inside either of the two tangents, chords are drawn parallel to them, crossing the inner circle, the parts of these chords which lie between the two circles are very nearly equal in length to the tangent lines themselves; and even a common diameter to both circles has, lying between them, two portions together equal to the radius of the outer. Hence, since the line of sight even across the middle of the space behind the nucleus, passes through a considerable range of luminous matter, while a line within but near the outskirts of that space passes through nearly as great a range of luminous matter as one just outside that space, there should be plenty of light where yet to the eye there seems to be something like absolute darkness. Either then the eye is greatly deceived, or else we must find some explanation of darkness existing where considerable brightness might be expected.[D] The matter which forms the tail, seems, as I have said, to be swept off from the envelopes raised by the sun’s action on the nucleus. It seems as though the matter thus raised had undergone in some way a change of character, which caused it no longer to obey the law of gravity as it had done when forming part of the nucleus, but instead of yielding to the sun’s attraction, to submit rather to an intense repulsive action, carrying it at a much greater rate from the sun than, under the action of gravity—starting from rest and free from all perturbing influences—it could have been drawn toward him. Dr. Huggins thus words his account of what seems to happen: “Now is seen to take place a change which is most puzzling—namely, these envelopes of light appear to give up their substance under the influence of a strong repulsive force exerted from the sun, and to be forced backwards.” Sir John Herschel, after his long and careful study of the comet of 1830 (Halley’s at its second return) came to the conclusion that repulsive action exerted by the sun on the matter raised in these envelopes had been distinctly proved.