In answer to this objection, M. Arago has submitted to examination the rate 

 at which comets increase their dimensions as they recede from the sun, ao- 

 cording to Valz ; and calculates the corresponding diminution of intrinsic 

 splendor which would arise from such a cause. The question then is, wheth- 

 er, by such a diminution of splendor, the brightest comets would be invisible 

 beyond the orbit of Jupiter ? This question he proposes to decide by the fol- 

 lowing experimental test, to be applied to some future comet. 



Let a telescope be selected having a large opening and low magnifying 

 power, by the aid of which the comet may be observed in every part of its 

 visible course. Let the body be observed with this instrument at some deter- 

 minate distance from the sun, such as, for example, the distance of Venus. 

 M. Arago shows how, by applying different magnifying powers to the teles- 

 cope under these circumstances, the image of the comet may be made to as- 

 sume different degrees of brightness. He shows, also, how the magnifying 

 power may be regulated, so as to exhibit the image of the comet with just that ( 

 degree of brightness with which it would appear at any given increased dis- < 

 tance to the lowest magnifying power ; on the supposition of its being a self- 5 

 shining body, losing brightness by reason of the enlargement of its dimensions. ' 

 In this way, he shows that the actual brightness which the comet ought to have \ 

 at any given distance from the sun, when looked at with any given magnifying j 

 power, may be predicted. He proposes, then, that, this observation being pre- ) 

 viously made, the comet should be observed subsequently at the proposed dis- * 

 tances. If it appear with that degree of brightness which it ought to have in } 

 correspondence with such previous observations, then there will be a presump- 

 tion that it shines with its own light. But if, as is probable, and perhaps near- 

 ly certain, the splendor of the comet at increased distances will be greatly less 

 than it ought to be, and that it will be wholly invisible at distances at which it 

 ought to be seen, then there will be conclusive proof that it is a body not self- 

 luminous, but. one which derives its light from the sun ; and that its disappear- 

 ance, when removed to any considerable distance from that luminary, arises 

 from the extreme faintness of the light which its attenuated matter reflects. 



It will, of course, be perceived, that the enlargement of the volume of the 

 comet will produce a diluting effect upon its reflected light, as much as it 

 would if it shone with direct light ; and this furnishes an additional reason for 

 its rapid disappearance as it recedes from the sun. 



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 soem most improba- 

 ble, are nevertheless 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 dimensions. But more recent and exact observa- 

 tions have shown the very reverse to be the fact. Comets increase their 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 for in several ways. Valz ascribed it to the 

 pressure of the solar atmosphere acting upon the comet ; that atmosphere, being 

 more dense near the sun, compressed the comet and diminished its dimensions ; 

 and, at a greater distance, being relieved from this coercion, the body swelled 

 to its natural bulk. A very ingenious train of reasoning was produced in sup- 

 port of this theory. The density of the solar atmosphere and the elasticity ot 

 the comet being assumed to being such as they might naturally be supposed, 

 the variations of the comet's bulk were deduced fay strict reasoning, and showed 

 a surprising coincidence with the observed change in the dimensions. But 



