LABOURS RELATIVE TO THE SOLAR SYSTEM. 297 



the nucleus could be illuminated by the sun. The 

 nucleus then must shine by its own light. 



This is a legitimate inference in the opinion of every 

 one who will allow, on one hand, that the nucleus is a 

 solid body, and on the other, that it would have been 

 possible to observe a phase of - on a disk whose appar- 

 ent total diameter did not exceed one or two seconds 

 of a degree. 



Very small stars seemed to grow much paler when 

 they were seen through the coma or through the tail of 

 the comet. 



This faintness may have only been apparent, and 

 might arise from the circumstance of the stars being 

 then projected on a luminous background. Such is, 

 indeed, the explanation adopted by Herschel. A gase- 

 ous medium, capable of reflecting sufficient solar light to 

 efface that of some stars, would appear to him to pos- 

 sess in each stratum a sensible quantity of matter, and 

 to be, for that reason, a cause of real diminution of the 

 light transmitted, though nothing reveals the existence 

 of such a cause. 



This argument, offered by Herschel in favour of the 

 system which transforms comets into self-luminous 

 bodies, has not, as we may perceive, much force. I 

 might venture to say as much of many other remarks 

 by this great observer. He tells us that the comet was 

 very visible in the telescope on the 21st of February, 

 1808 ; now, on that day, its distance from the sun 

 amounted to 2'7 times the mean radius of the terrestrial 

 orbit ; its distance from the observer was 2-9 : " What 

 probability would there be that rays going to such dis- 

 tances, from the sun to the comet, could, after their 

 reflection, be seen by an eye nearly three times more 

 distant from the comet than from the sun ? " 



13* 



