298 HERSCHEL. 



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 appear to be an element of 

 visibility. If the comet be self-luminous, its intrinsic 

 splendour (its brightness for unity of surface) will 

 remain constant at any distance, as long as the subtended 

 angle remains sensible. If the body shines by borrowed 

 light, its brightness will vary only according to its change 

 of distance 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 ap- 

 parent diameter shall not be diminished below certain 

 limits. 



Herschel finished his observations of a comet that was 

 visible in January, 1807, with the following remark :- 



*' Of the sixteen telescopic comets that I have exam- 

 ined, fourteen had no solid body visible at their centre ; 

 the other two exhibited a central light, very ill defined, 

 that might be termed a nucleus, but a light that certainly 

 could not deserve the name of a disk." 



The beautiful comet of 1811 became the object of that 

 celebrated astronomer's conscientious labour. Large tel- 

 escopes showed him, in the midst of the gazeous head, a 

 rather reddish body of planetary appearance, which bore 

 strong magnifying powers, and showed no sign of phase. 

 Hence Herschel concluded that it was self-luminous. 

 Yet if we reflect that the planetary body under consid- 

 eration was not a second in diameter, the absence of a 

 phase does not appear a demonstrative argument. 



The light of the head had a blueish-green tint. Was 

 this a real tint, or did the central reddish body, only 



