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 



