a eo 
-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 ;3, 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?” 
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