26 COMETS. 



the strange changes which have been seen to take place 

 in cometary bodies, even whilst they were passing near 

 the earth, as the division of Biela's comet and the 

 ultimate formation of a second nucleus from the de- 

 tached portion, strongly tend to support the probabi- 

 lity of the idea that attenuated matter has, in the pro- 

 gress of time, been condensed into solid masses, and 

 that nebulous clouds must still exist in every state of 

 tenuity in the regions of infinite space,* which, in the 



after sunset, at the spring, and before sunrise, at the autumnal 

 equinox ; not only because the direction of its apparent axis lies 

 at those times more nearly perpendicular to the horizon, but also 

 because at those epochs we are approaching the situation when it 

 is seen most completely in section. 



" At the vernal equinox the appearance of the zodiacal light is 

 that of a pretty broad pyramidal, or rather lenticular, body of light, 

 which begins ito be visible as soon as the twilight decays. It is 

 very bright at its lower or broader part near the horizon, and, if 

 there be broken clouds about, often appears like the glow of a dis- 

 tant conflagration, or of the rising moon, only less red, giving rise, 

 in short, to amorphous masses of light such as have been noticed 

 by one of your correspondents as possibly appertaining to the comet 

 At higher altitudes, its light fades gradually, and is seldom trace- 

 able much beyond the Pleiades, which it usually, however, attains 

 and involves, and (what is most to my present purpose) its axis at 

 the vernal equinox is always inclined (to the northward of the 

 equator) at an angle of between 60 and 70 to the horizon, and it 

 is most luminous at its base, resting on the horizon, where also it 

 is broadest, occupying, in fact, an angular breadth of somewhere 

 about 10 or 12 in ordinary clear weather." 



* " The assumption that the extent of the starry firmament is 

 literally infinite has been made by one of the greatest of astrono- 

 mers, the late Dr. Olbers, the basis of a conclusion that the celes- 

 tial spaces are, in some slight degree, deficient in transparency j 

 so that all beyond a certain distance is, and must remain for ever, 

 unseen; the geometrical progression of the extinction of light far 

 outrunning the effect of any conceivable increase in the power of 

 our telescopes. Were it not so, it is argued, every part of the celes- 

 tial concave ought to shine with the brightness of the solar disc, 

 since no visual ray could be so directed as not, in some point or 

 other of its infinite length, to encounter such a disc;' Edinburgh 

 Review, p. 185, for January, 1848 ; Etudes d 1 Astronomic Stellaire* 



