THE TWO COMETS OF THE YEAR 1868. 149 



necessary to note, however, of the three features com 

 monly recognised in comets viz. the nucleus, coma, 

 and tail the coma alone is invariably exhibited. A 

 comet which has neither nucleus nor tail presents 

 simply a round mass of vapour slightly condensed 

 towards the centre. The nucleus, when seen, appears 

 as a bright point within the condensed part of a comet. 

 The tail, as every one knows, is a long train of light 

 issuing from the head. 



It was noted in very early times that comets are 

 almost perfectly translucent. This peculiarity has 

 been confirmed by modern and more exact observations. 

 Sir W. Herschel watched the central passage of a comet 

 over the fainter component of a double star ; and he 

 could detect no diminution of the star s brilliancy. 

 Similar observations were made by MM. Olbers and 

 Struve. Sir John Herschel watched the passage of 

 Biela s comet over a small cluster of very faint tele 

 scopic stars. The slightest haze would have oblite 

 rated the cluster, yet no appreciable effect was pro 

 duced by the interposition of cometic matter having a 

 thickness (according to Herschel s estimate) of 50,000 

 miles. And there is another remarkable evidence of 

 tenuity. From recognised optical principles, a star 

 seen through the globular head of a comet, should 

 appear displaced from its true position just as any 

 object seen (non-centrally) through a globular decanter 

 full of water seems thrown out of its true place. The 

 astronomer Bessel made an observation on a star which 

 approached within about eight seconds of the nucleus 



