M. Arago on the Light of Comets. 27 



After this long preamble, a very few words will suffice to 

 shew how, without any observation, of the phases, or any de- 

 rived from polarization, it is possible to ascertain that comets 

 shine with a borrowed light. 



I have, in truth, but recently established (p. 23), that a 

 body luminous of itself ought to have, whether to the eye or in 

 any given telescope, precisely the same brightness, whatever be 

 the distance at which it is placed in relation to the observer. I 

 have also proved, on the other hand, that the visibility of a body 

 does not depend on the angle which it subtends, at least when 

 that angle does not descend below certain limits. So much be- 

 ing established, it only remains for us experimentally to resolve 

 the following questions : In what manner does a comet disap- 

 pear ? Is this disappearance the consequence of an excessive di- 

 minution in the apparent dimensions of the star, arising from a 

 vast increase of its distance from the earth ? May it not rather 

 be attributed to a change in its intensity ? Well, then, all as- 

 tronomers will answer that this last cause of disappearing is the 

 true one. The most part of observed comets, and that of 1680 

 in particular, have disappeared by a gradual enfeebling of their 

 light. They have become, so to speak, extinguished. The 

 evening of the day in which we cease to be able to observe them, 

 they still subtend angles which are very considerable. But this 

 mode of disappearance, I have proved in detail, is irreconcile- 

 able with the existence of an inherent light, and comets there- 

 fore borrow their light from the sun. 



In the various experiments which have led the way to this 

 conclusion, we have admitted that, during its variations of dis- 

 tance, the luminous body under our observation did not change 

 in its physical constitution ; but, it is true that comets do not 

 answer to this description. There is here, then, a substantial 

 objection, and some short reflections will therefore be required. 



Till lately, it was very generally believed that the nebulous 

 matter of a comet gradually was condensed, in proportion as in 

 its elliptical course it was removed from the sun. This con- 

 densation could not fail to produce in the star a brightness su- 

 perior to that which without this change it could have main- 

 tained. 



