208 Mr. J. N. Lockyer. On the Classification [Jan. 10, 



Returns of Encke's Comet, showing Reduced Period of Revolution. 



There is still another point. If the luminosity were due entirely 

 to internal collisions brought about by the increase of solar action, 

 then large comets, or those best visible, should begin to be brilliant 

 long before smaller or more distant ones. But this does not seem to 

 be so. Mr. Hind has pointed out that proximitv to the earth is not 

 so important a condition for visibility of a comet in the daytime as 

 close approach to the sun* ; and M. Faye is the authority for fche 

 statement that no comet has been seen beyond the orbit of 

 Jupiter.t " It is assuredly not on account of their smallness 

 that they thus escape our notice in regions where the most distant 

 planets, Saturn, Uranus, and Neptune, shine so clearly with the 

 light which they borrow from the sun ; this is because the rare and 

 nebulous matter of comets reflects much less light than the solid and 

 compact surfaces of the planets of which we speak, much less even 

 than the smallest cloud of our atmosphere." 



On the latter part of this quotation it may be remarked that it is 

 not necessary to assume that comets at a great distance from 

 the sun, any more than nebulae, are visible by means of reflected 

 light. 



Olbers, Faye, and others have attributed the production of comets' 

 tails to solar repulsion. Away from the sun, as we have seen, comets 

 are tailless. 



The tail of a comet usually grows with its approach to the sun. 

 This is not merely an apparent increase due to diminished distance, 



* ' Nature,' vol. 10, p. 286. 

 t ' Nature,' vol. 10, p. 228. 



