TKANSACTIONS OF THE SECTIONS. 21 



overlooked in the 20-feet reflector had it existed. The amount of change thus in- 

 dicated implies such prodigious movements in this nebula, as to make it pro- 

 bable that its distance from us is less than was supposed by astronomers, and point 

 it out as deserving most careful -watching, with circumstances as litte varied as 

 possible. It may even be hoped that the spectroscope will give some such evi- 

 dence of its motion as Mr. Iluggins has obtained with Sirius. 



0)1 Comets*. By Prof. P. G. Tait, F.B.S.E. 



The principal object of this paper was to investigate how far the singidar phe- 

 nomena exhibited by the tails of comets and by the envelopes of their nuclei, the 

 shrinking of their nuclei as they approach the sun, and vice- versa, as well as the 

 diminution of period presented by some of them, can be explained on the probable 

 supposition that a comet is a mere cloud of small masses, such as stones and frag- 

 ments of meteoric iron, shining by reflected light alone, except where these masses 

 impinge on one another, or on other matter circulating- round the sun, and thus 

 produce luminous gases along \A\h considerable modiflcations of their relative 

 motion. Thus the gaseous spectrum of the nucleus is assigned to the same impacts 

 which throw out from the ranks those masses which form the tail. Some of the 

 most wonderful of the singular phenomena presented by comets, such as the almost 

 sudden development of tails of many millions of miles in length, theoccm-reuce of 

 comets with many tails, and the observed fact that there is no definite relation of 

 direction between a comet's tail and its solar radius-vector, were here looked upon 

 as due to the diflerences of motion of these discrete fragments relatively to the 

 earth in a manner somewhat analogous to the appearances presented by a distant 

 flock of seabirds flying in nearly one plane, and only becoming visible as a long 

 streak when the plane of the flock passes approximately through the spectator's 

 eye. The so-called envelopes are compared with the curious phenomena presented 

 by tobacco-smoke (which seem, however, to be emitted in a form apparently re- 

 semblingthincontinuous films of smallparticles of carbon) ,and the so-called "gaseous 

 jets " which appear to be projected from the nucleus and to be repelled from the sun, 

 are not diflicult of explanation, the author considered, from the general points of view 

 here taken. Investigation, mainly conducted by quaternions, show how a group 

 of discrete masses, so small that their mutual pertm-bations are not of great mo- 

 ment except in the case of actual impact, gradually changes its form, as it revolves 

 about the sim, independently of any hypothesis as to the cause, planetary attraction 

 or othei-wise, by which it was first introduced into the solar system. The ideas 

 here brought forward occurred to the author more than two years ago, on his being 

 made aware of the identity of the orbits of the August meteorites and of Comet II., 

 1862 ; but they seemed so obviously to follow from that identity that it was only 

 on reading Dr. Tyndall's recent speculations, and on being informed by Prof. New- 

 ton that the question of tails, envelopes, and " gaseous jets " had been treated by 

 Schiaparelli, as proving the existence of a repulsive force, that he ventured to pro- 

 duce an explanation so apparently simple and yet so inconsistent with what appears 

 to be held by the majority of astronomers. 



Optics. 



On the Influence of Annealing on Crystalline Structure. 

 By Chakles Brooke, M.A., F.B.S. 



The author having been recently engaged in the construction of a rock-salt 

 spectroscope for observations on the heat-spectrum, met v.nih an unexpected diffi- 

 culty in the construction of the prisms. In some of the optically best specimens 

 of rock-salt in his possession, it was found that, in forming equilateral prisms one 

 side of which was cut parallel to a cleavage plane, the opposite angle intended to 

 be the working angle of the prism was repeatedly traversed by spontaneous clea- 



* Vide Proceedings of the Eoyal Society of Edinburgh, 1868-69, p. 553. 



