90 THE PROBLEMS OF ASTRONOMY. 



pregnant with future discoveries to-day than it ever has been, and it is 

 more disposed to welcome the spectroscope as a useful handmaid, which 

 may help it on to new fields, than it is to give way to it. How useful it 

 may thus become has been recently shown by a Dutch astronomer, 

 who finds that the stars having one type of spectrum belong mostly to 

 the Milky Way, and are farther from us than the others. 



In the field of the newer astronomy perhaps the most interesting 

 work is that associated with comets. It must be confessed, however, 

 that the spectroscope has rather increased than diminished the mystery 

 which, in some respects, surrounds the constitution of these bodies. 

 The older astronomy has satisfactorily accounted for their appearance, 

 and we might also say for their origin and their end, so far as questions 

 of origin can come into the domain of science. It is now known that 

 comets are not wanderers through the celestial spaces from star to star, 

 but must always have belonged to our system. But their orbits are so 

 very elongated that thousands, or even hundreds of thousands, of years 

 are required for a revolution. Sometimes, however, a comet passing 

 near to Jupiter is so fascinated by that jflanet that, in its vain attempts 

 to follow it, it loses so much of its primitive velocity as to circulate 

 around the sun in a period of a few years, and thus to become, appar- 

 ently,* a new member of our system. If the orbit of such a comet, or 

 in fact of any comet, chances to intersect that of the earth, the latter 

 in passing the point of intersection encounters minute particles which 

 causes a meteoric shower. The great showers of November, which 

 occur three times in a century and were well known in the years 1866-67, 

 may be expected to reappear about 1900, after the passage of a comet 

 which, since 1866, has been visiting the confines of our system, and is 

 expected to return about two years hence. 



But all this does not tell us much about the nature and make-up of a 

 comet. Does it consist of nothing but isolated particles, or is there a 

 solid nucleus, the attraction of which tends to keep the mass together? 

 No one yet knows. The spectroscope, if we interpret its indications in 

 the usual way, tells us that a comet is simply a mass of hydrocarbon 

 vapor, shining by its own light. But there must be something wrong 

 in this interpretation. That the light is reflected sunlight seems to 

 follow necessarily from the increased brilliancy of the comet as it 

 approaches the sun and its disappearance as it passes away. 



Great attention has recently been bestowed upon the physical consti- 

 tution of the planets and the changes which the surfaces of those bodies 

 may undergo. In this department of research we must feel gratified 

 by the energy of our countrymen who have entered upon it. Should I 

 seek to even mention all the results thus made known I might be 

 stepping on dangerous ground, as many questions are still unsettled. 

 While every astronomer has entertained the highest admiration for the 

 energy and enthusiasm shown by Mr. Percival Lowell in founding an 

 observatory in regions where the planets can be studied under the 



