16S ANNUAL OF SCIENTIFIC DISCOVERY. 



ly be visible. A comet, however, will be visible when its distance 

 from us is many millions of miles. 



What conclusion can we draw from these facts ? Do they not indi- 

 cate that comets do not shine wholly by reflected light ? On the 3d* 

 of July, 1819, Arago made an attempt to analyze the light of comets, 

 by applying his polariscope to the great comet of 1819. The instru- 

 ment showed unmistakable signs of polarized light, and, therefore, of 

 reflected sunlight. Similar observations on Halley's comet, in 1835, 

 more clearly indicated the existence of polarized light. "These beau- 

 tiful experiments still leave it undecided, whether, in addition to this 

 reflected solar light, comets may not have light of their own. Even in 

 the case of the planets, as, for instance, in Venus, an evolution of in- 

 dependent light seems very probable." * 



"The variable intensity of light in comets cannot always be ex- 

 plained by the position of their orbits, and their distance from the 

 sun. "I After mentioning Arago's observations, with his polariscope, 

 on Halley's comet, in 1835, Mr. Hind says : " Still, the variation in the 

 intensity of light is not universally such as should follow if the comet 

 merely reflected the sun's rays under certain permanent conditions, 

 and we are under the necessity of looking to physical causes inherent 

 in the body itself for an explanation of some few observations which 

 appear irreconcilable with the theory of reflected solar light. V J "The 

 molecular condition of the head or nucleus, so seldom possessing a 

 definite outline, as well as the tail of the comet, is rendered so much 

 the more mysterious from the fact that it causes no refraction. " 



I have collected these facts together to show that reflected solar 

 light cannot completely explain, at present, all the phenomena of the 

 light of comets. Besides the. above observations, it may be added 

 that the visibility of comets in the daytime, and even when near the 

 sun, also indicates a light-generating process in the comet ; for other- 

 wise we must suppose them capable of reflecting more light than the 

 planets. Indeed, it is difficult to see how a body can maintain its 

 gaseous or nebulous state without being kept at a high temperature, 

 and therefore, having within itself a light-producing cause. Assum- 

 ing, then, that there is a light-generating process that is active in 

 comets, let us see what use can be made of it, and whether it will 

 afford us any light on the subject of the visibility of nebulous matter. 



It is now a well-established fact that the heads of comets contract 

 in dimensions as they approach the sun. This was first noticed by 

 Hevelius, but was not established till manv vears afterwards. AVhat 



*' " 



is the cause of this condensation of cometary matter as the comet ap- 

 proaches the sun? Whatever may be the cause of it, we know that it 

 has a great effect on the visibility of the comet. Is it not possible 

 that the solar rays, acting chemically or otherwise, excite in the 

 comet those principles which cause it to send out in greater abundance, 

 direct light? We know that a comet will increase in brightness with 

 great rapidity as it approaches the sun ; and also decrease in brilliancy 

 with equal rapidity in general, as it recedes from the sun, so that the 

 fainter comets disappear in the best telescopes, when, their apparent 

 dimensions only being considered, they ought to remain visible. The 



* Cosmos, vol. i. pp. 90, 91. Bohn's edition. f Cosmos, vol. i. p. 91 



J Hind's Treatise on Comets, p. 24. Cosmos, vol. iv. p. 54j>. 



