HEESCHEL. 219 



There are several of Herschel's memoirs on comets. lu aualyzing 

 tlicui we sliall see that this acute observer could not touch anything 

 without making farther discoveries in regard to it. 



He applied some of his line instruments to the study of the physical 

 constitution of a comet discovered by Mr. Pigott, on the 28th September, 

 1807. The nucleus wasround and well-determined. Some measures taken 

 on the day when the nucleus subtended only an angle of a single second 

 gave as its real diameter j^ of the diameter of the earth. 



Herschel saw no phase at an epoch when only ^-^ of the nucleus could 

 be illuminated by the sun. The nucleus then must shine by its own 

 hght. 



This is a legitimate inference in the opinion of every one who will 

 allow, on the 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 

 apparent 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 faintuess may have only been apparent, and might arise from 

 the circumstance of the stars being then projected on a luminous back- 

 ground. Such is, indeed, the explanation adopted by Herschel. A 

 gaseous medium, capable of reflecting sufficient solar light to efface that 

 of some stars, would appear to him to i)ossess in each stratum a sensi- 

 ble quantity of matter, and to be, for that reason, a cause of real dimi- 

 nution of the light transmitted, though nothing reveals the existence of 

 such a cause. 



This argument, offered by Herschel in favor of the system which, 

 transforms comets into self-luminous bodies, has not, as we nuiy percek^e, 

 much force. I might venture to say as much of several other remarks 

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

 in the telescope on the 21st of February, 1808 f now, on that day, its dis- 

 tance from the sun amounted to 2,7 times the mean radius of the terres- 

 trial orbit; its distance from the observer was 2.9 : " What probability 

 would there be that rays going to such distances, 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 smi I " 



It is only numerical determinations that could give value to such an 

 argument. By satisfying himself with vague reasoning, Herschel did 

 not even perceive that he was committing a great mistake by making 

 the comet's distance from the observer api)ear to be an element of visi- 

 bility. If the comet be self-luminous, its intrinsic splendor (its bright- 

 ness for unity of surface) will remain constant at any distance, as long 

 as the subtended angle remains sensible. If the body shines by bor- 

 rowed light, its brightness will vary only according to its change of dis- 

 tance from the sun ; nor will the distance of the observer occasion any 

 change in the visibility ; always, let it be understood, with the restriction 

 that the apparent diameter shall not be diminished below certain limits. 



