September 5, 1884.] 



SCIENCE. 



201 



failure to recognize it in other cases creates in my 

 mind no presumption against the wisdom of nature, 

 or against the correctness of an hypothesis otherwise 

 satisfactory. It merely suggests human limitations 

 and ignorance. How can one without sight under- 

 stand what a telescope is good for? 



At the same time, perhaps we assume with a little 

 too much confidence, that, in free space, radiation 

 does take place equally in all directions. Of course, if 

 the received views as to the nature and conduct of the 

 hypothetical ' ether ' are correct, there is no possibil- 

 ity of questioning the assumption ; hut, as Sir John 

 Herschel and others have pointed out, the proper- 

 ties which must be ascribed to this ' ether,' to fit it 

 for its various functions, are so surprising and almost 

 inconceivable, that one may be pardoned for some 

 reserve in accepting it as a finality. At any rate, as 

 a fact, the question is continually started (the idea 

 has been brought out repeatedly, in some cases by 

 men of real and recognized scientific and philosophic 

 attainment), whether the constitution of things may 

 not be such that radiation and transfer of energy 

 can take place only between ponderable masses ; and 

 that, too, without the expenditure of energy upon 

 the transmitting-agent (if such exist) along the line 

 of transmission, even in transitu. If this were the 

 case, then, the sun would send out its energy only to 

 planets and meteors and sister-stars, wasting none 

 in empty space ; and so its loss of heat would be enor- 

 mously diminished, and the time-scale of the life of 

 the planetary system would be correspondingly ex- 

 tended. So far as I know, no one has ever yet been 

 able to indicate any kind of medium or mechanism 

 by which vibrations, such as we know to constitute 

 the radiant energy of light and heat, can be trans- 

 mitted at all from sun to planet under such restric- 

 tions. Still one ought not to be too positive in 

 assertions as to the real condition and occupancy of 

 so-called vacant space. The ' ether ' is a good work- 

 ing hypothesis, but hardly more as yet. 



I need not add, that a most interesting and as yet 

 inaccessible problem, connected with the preceding, 

 is that of the mechanism of gravitation, and, indeed, 

 of all forces that seem to act at a distance : as for 

 that matter, in the last analysis, all forces do. If 

 there really be an ' ether,' then it would seem that 

 somehow all attractions and repulsions of ponder- 

 able matter must be due to its action. Challis's 

 investigations and conclusions as to the effect of 

 hydrodynamic actions in such a medium do not seem 

 to have commanded general acceptance ; and the field 

 still lies open for one who will show how gravitation 

 and other forces can be correlated with each other 

 through the ether. 



Meteors and the comets, seeming to belong neither 

 to the solar system nor to the stellar universe, pre- 

 sent a crowd of problems as difficult as they are inter- 

 esting. Much has undoubtedly been gained during 

 the last few decades, but in some respects that which 

 has been learned has only deepened the mystery. 



The problem of the origin of comets has been sup- 

 posed to be solved to a certain extent by the re- 

 searches of Schiaparelli, Heis, Professor Newton, 



and others, who consider them to be strangers com- 

 ing in from outer space, sometimes ' captured ' by 

 planets, and forced into elliptic orbits, so as to be- 

 come periodic in their motion. Certainly this theory 

 has strong supports and great authority, and proba- 

 bly it meets the conditions better than any other yet 

 proposed. But the objections are really great, if not 

 insuperable, — the fact that we have so few, if any, 

 comets moving in hyperbolic orbits, as comets met 

 by the sun would be expected to move ; that there 

 seems to be so little relation between the direction of 

 the major axes of cometary orbits, and the direction 

 of the solar motion in space; and especially the 

 fact, pointed out and insisted upon by Mr. Proctor in 

 a recent article, that the alteration of a comet's natu- 

 ral parabolic orbit to the observed elliptic one, by 

 planetary action, implies a reduction of the comet's 

 velocity greater than can be reasonably explained. 

 If, for instance, Brorsen's comet (which has a mean 

 distance from the sun a little more than three times 

 that of the earth) was really once a parabolic comet, 

 and was diverted into its present path by the attrac- 

 tion of Jupiter, as generally admitted, it must have 

 had its velocity reduced from about eleven miles a 

 second to five. Now, it is very difficult, if not out of 

 the question, to imagine any possible configuration 

 of the two bodies and their orbits which could result 

 in so great a change. While I am by no means pre- 

 pared to indorse as conclusive all the reasoning in 

 the article referred to, and should be very far from 

 ready to accept the author's alternative theory (that 

 the periodic comets have been ejected from the plan- 

 ets, and so are not their captives, but their children), 

 I still feel that thedifficulty urged against the received 

 theory is very real, and not to be evaded, though it 

 may possibly be overcome by future research. 



Still more problematical is the constitution of these 

 strange objects of such enormous volume and incon- 

 ceivable tenuity, self-luminous and transparent, yet 

 reflecting light, the seat of forces and phenomena 

 unparalleled in all our other experience. Hardly a 

 topic relating to their appearance and behavior can 

 be named which does not contain an unsolved prob- 

 lem. 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 ex- 

 planation 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 inter- 

 planetary space, and the behavior of matter when re- 

 duced to lowest terms of density and temperature. 



Time forbids any adequate discussion of the nu- 

 merous problems of stellar astronomy. Our work, 

 in its very nature incessant and interminable, con- 

 sists, of course, in the continual observation and 

 cataloguing of the places of the stars, with ever- 

 increasing precision. These star-places form the 

 scaffold and framework of all other astronomical 



