PHYSICAL CONSTITUTION OP COMETS. 



SyN^Oa^^v 



485 



comparison of the above numbers with the spaces included between these suc- 

 cessive imaginary globes, and with the relative facility or difficulty of discern- 

 ing comets in the different situations thus assigned, leads to a demonstration ! 

 that, so far as these hundred and thirty-seven observed cmets can be consid- 

 ered as an indication of the general distribution of comets through space, that 

 distribution ought to be regarded as uniform ; that is, an equal number of com- 

 ets have their least distances included in equal portions of space. 



Adopting, then, this conclusion, M. Arago reasons in the following manner: 

 > The number of ascertained comets which, at their least distances, pass within 

 1 the orbit of Mercury is thirty. Now, our most remote planet, Herschel, is 

 forty-nine times more distant from the sun than Mercury ; consequently, a 

 globe, of which the sun is the centre, and whose surface would pass through 

 the orbit of Herschel, would include a space greater than a similar globe 

 through the orbit of Mercury, in the proportion of the cube of forty-nine to one. 

 Assuming the uniform distribution of comets, it will follow that, for every com- 

 et included in a globe through the orbit of Mercury when at its least distance, 

 there will be a hundttd and seventeen thousand six hundred and forty-nine 

 comets similarly included within the globe through the orbit of Herschel. But 

 as there are thirty ascertained to be within the former globe, there will, there- 

 fore, be three millions five hundred and twenty-nine thousand four hundred arid 

 seventy within the orbit of Herschel. 



Thus it appears that, supposing no comet ranging within the limits of Mer- 

 cury has escaped observation, that portion of space enclosed within the globe 

 through Herschel must be swept by at least three millions and a half of comets. 

 But there can be no doubt that many more than thirty comets pass within the 

 globe through Mercury ; for it would be contrary to all probability to assume 

 that, notwithstanding the many causes obstructing the discovery of comets, and 

 the short time during which we have possessed instruments adequate to such 

 an inquiry, we should have discovered all the comets ranging within that limit. 

 It is, therefore, more probable that seven millions of comets are enclosed within 

 the known limits of the system than the lesser number ! Such is the astound- 

 ing conclusion to which M. Arago's reasoning leads. 



LIGHT OF COMETS. 



The light of comets is an effect of which astronomers have hitherto given 

 no satisfactory account. If any of these bodies had been observed to have 

 exhibited phases like those of the moon and the inferior planets, the fact of 

 their being opaque bodies, illuminated by the sun, would be at once establish- 

 ed. But the existence of such phases must necessarily depend upon the come* 

 itself being a solid mass. A mere mass of cloud or vapor, though not self-lu- 

 minous, 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 con- 

 stituent 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. For a long peri- 

 od this question was sought to be determined by the discovery of phases. M. 

 Arago then proceeded to apply to the question a very elegant mode of investi- 

 gation, depending on a property* by which reflected light may be distinguished 



* Polarization. 



