COMETS. 



their satellites, which are projected upon them, when the latter 

 are interposed between them and the sun. These tests are inap- 

 plicable to comets. They exhibit no phases, and are attended by 

 no bodies to intercept the sun's light. But, unless it could be 

 shown that a comet is a solid mass, impenetrable to the solar 

 rays, the non-existence f phases is not a proof that the body does 

 not receive its light from the sun. 



A mere mass of cloud or vapour, though not self-luminous, but 

 rendered visible by borrowed light, would still exhibit no effect 

 of this kind : its imperfect opacity would allow the solar light 

 to affect its constituent parts throughout its entire depth so 

 that, like a thin fleecy cloud, it would appear not superficially 

 illuminated, but receiving and reflecting light through all its 

 dimensions. With respect to comets, therefore, the doubt which 

 has existed is, whether the light which proceeds from them, and 

 by which they become visible, is a light of their own, or is the 

 light of the sun shining upon them, and reflected to our eyes 

 like light from a cloud. Among several tests which have been 

 proposed to decide this question, one suggested by Arago merits 

 attention. 



It has been already shown in our Tract on "the Eye" (43), 

 that the apparent brightness of a visible object is the same at all 

 distances, supposing its real brightness to remain unchanged. 

 Now if comets shone with their proper light, and not by light 

 received from the sun, their apparent brightness would not 

 decrease as they would recede from the sun, and they would cease 

 to be visible, not because of the faintness of their light, but 

 because of the smallness of their apparent magnitude. Now the 

 contrary is found to be the case. As the comet retires from the 

 sun its apparent brightness rapidly decreases, and it ceases to be 

 visible from the mere faintness of its light, while it still subtends 

 a considerable visual angle. 



85. It will doubtless excite surprise, that the dimensions of a 

 comet should be enlarged as it recedes from the source of heat. 

 It has been often observed in astronomical inquiries, that the 

 effects, which at first view seem most improbable, are never- 

 theless those which frequently prove to be true ; and so it is in 

 this case. It was long believed that comets enlarged as they 

 approached the sun ; and this supposed effect was naturally and 

 probably ascribed to the heat of the sun expanding their dimen- 

 sions. But more recent and exact observations have shown the 

 very reverse to be the fact. Comets increase their apparent 

 volume as they recede from the sun ; and this is a law to which 

 there appears to be no well-ascertained exception. This singular 

 and unexpected phenomenon has been attempted to be accounted 

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