1869.] Astronomy. 517 



Tlius we may infer that the structure of the prominences is more 

 complex than had hitherto been supposed. It is to be hoped that 

 the new hues, and possibly others, may admit of being rendered 

 visible by the spectroscope without the aid of an echpse. Possibly 

 when Mr. Huggins' new telescope is at work, we may learn the 

 position of the newly-discovered lines in the accurate manner which 

 the particular mode of observation we refer to renders jwssible. 

 It would be interesting to discover what are the elements to which 

 the lines belong. 



The darkness during the ecHpse was much greater than during 

 the eclipse of last year, though the totahty did not last so long by 

 nearly tlu-ee minutes. Some of the observers searched, but without 

 success, for intra-Mercurial planets. 



Although Professor Tyndall's recent investigations of the phe- 

 nomena presented by comets are too closely associated with his 

 physical researches to be described in fall in our astronomical 

 Chronicle, yet there are certain points connected with his new 

 theory which it belongs especially to astronomy to deal with. 



Passing over the physical considerations on which the theory 

 depends, and which serve to distinguish it from most of the hypo- 

 theses hitherto put forward (based as these are on no known 

 experimental laws), we may describe the theory as follows : — 



The tail of a comet is not matter projected from the head, but 

 matter precipitated on the -solar beams which have traversed the 

 head. Tyndall has shown that such precipitation may occur either 

 with comparative slowness along the beam, or with the velocity 

 with which the beam actually traverses space. Thus the amazing 

 rapidity with which a comet's tail is sometimes developed is accounted 

 for " without invoking the incredible motion of translation hitherto 

 assumed."* As the comet sweeps round the perihelion, the tail is 

 not composed of the same matter, but new matter is precipitated on 

 the solar beams, the part of the old tail which is not protected (so 

 to speak) by the head of the comet being dissipated by the sun's 

 calorific rays ; and the dissipation not being necessarily instan- 

 taneous, " the tail leans towards that portion of space last quitted 

 by the comet — a general fact of observation being thus accounted 

 for." Occasional lateral streamers are explained as possibly due to 

 the temporary mastery of the actinic rays in parts of the cometary 

 atmosphere not screened by the nucleus. Lastly, the shrinking of 

 the head as the comet approaches the sun is due to the beating 

 of the heat-rays against the attenuated fringe of the head, which 

 is thus dissipated. 



Although the theory, as at present put forward, fails to account 



* Here, in passing, we may notice that Professor Tyndall is in error as to the 

 opinions received among astronomers, for Sir John Herschel long since ^jroved that 

 no such assumption is permissible. 



