PENDING PROBLEMS OF ASTRONOMY. 65 



an unsolved problem. The varying intensity, polarization, and spec- 

 troscopic character of their light ; the configurations of the nucleus 

 and its surrounding nebulosity ; and especially the phenomena of jets, 

 envelopes, and tail — all demand careful observation and thorough dis- 

 cussion, I think it may be regarded as certain that the explanation 

 of these phenomena when finally reached, if that time ever comes, 

 will carry with it, and be based upon, an enormous increase in our 

 knowledge as to the condition, contents, and temperature of interplan- 

 etary space, and the behavior of matter when reduced to lowest terms 

 of density and temperature. 



Time forbids any adequate discussion of the numerous problems 

 of stellar astronomy. One work, in its very nature incessant and in- 

 terminable, consists, of course, in the continual observation and cata- 

 loguing of the places of the stars, with ever-increasing precision. 

 These star-places form the scaffold and framework of all other astro- 

 nomical investigations involving the motions of the heavenly bodies : 

 they are the reference-points and bench-marks of the universe. Ulti- 

 mately, too, the comparison of catalogues of different dates will reveal 

 the paths and motions of all the members of the starry host, and bring 

 out the great orbit of the sun and his attendant planets. Meanwhile, 

 micrometric observations are in order, upon the individual stars in 

 different clusters, to ascertain the motions which occur in such a case ; 

 and the mathematician is called upon again to solve the problem of 

 such movement. 



Now, too, since the recent work of Gill and Elkin in South Africa, 

 and of Struve, Hall, and others elsewhere, upon stellar parallax, new 

 hopes arise that we may soon come to some wider knowledge of the 

 subject ; that, instead of a dozen or so parallaxes of doubtful precision, 

 we may get a hundred or more relating to stars of widely different 

 brightness and motion, and so be enabled to reach some trustworthy 

 generalizations as to the constitution and dimensions of the stellar 

 universe, and the actual rates of stellar and solar motion in space. 



Most interesting, also, are the studies now so vigorously prosecuted 

 by Professor Pickering in this country, and many others elsewhere, 

 upon the brightness of the stars, and the continual variations in this 

 brightness. Since 1875 stellar photometry has become almost a new 

 science. 



Then there are more than a myriad of double and multiple stars 

 to watch, and their orbits to be determined ; and the nebulae claim 

 keen attention, since some of them appear to be changing in form 

 and brightness, and are likely to reveal to us some wonderful secrets 

 in the embryology of worlds. Each star also presents a subject for 

 spectroscopic study ; for although, for the most part, the stars may be 

 grouped into a very few classes from the spectroscopic point of view, 

 yet, in detail, the spectra of objects belonging to the same group differ 

 considerably and significantly, almost as much as human faces do. 



VOL. XXTI. — 5 



