HARDWICKE'S SCIENCE-GOSSIP. 



269 



have endeavoured, and in some few instances 

 successfully, to find the distances of stars from us by 

 .noting the exact position in the heavens they appear 

 to occupy at a given time and again six months 

 afterwards, that is to say from another standpoint 

 more than 180,000,000 miles removed from the 

 -former one. The resulting annual parallax, as half the 

 difference of the two positions is called, when cleared 

 of all effects produced by other causes, is most minute, 

 and in the case of many stars quite inappreciable. 

 But the number of miles of distance thus represented 

 is bewildering in the extreme, and in their stead the 

 lime occupied by light passing over the interval 

 between the star and our earth is adopted. Light, 

 travelling unceasingly at the rate of close upon 

 190,000 miles in every second, takes more than forty 

 years to reach us from the Pole Star, or, to put the 

 same fact in a different way, if the Pole Star were in 

 some way destroyed to-day, we should not become 

 aware of its destruction till after the expiration of 

 more than forty years. So far as our present know- 

 ledge extends the nearest fixed star is a Centauri, 

 which is 275,000 times as far from us as we are from 

 the sun, and the light from which takes no less than 

 four and a third years to reach us. 



While moving the telescope about in our explora- 

 tions among the stars, we shall not fail to come 

 across numerous misty, cloud-like masses, which 

 .resemble patches of dim light seen on the dark back- 

 ground of the sky. These are the nebula; which are 

 scattered, apparently in accordance with no law, over 

 all parts of the heavens, of all conceivable shapes and 

 of inconceivable magnitude. As early as the tenth 

 century a curious object, evidently not a star, had 

 been observed in the constellation Andromeda by a 

 Persian astronomer named Sufi, but it was unknown 

 to Europeans until discovered in 1612 by Simon 

 Marius. It is, however, so conspicuous as to be 

 easily seen by the naked eye. This nebula has 

 "been traced to a length of 4 and a breadth of 2§°. 

 Some idea of its apparent size may be formed if we 

 consider that the mean apparent diameter of the sun 

 is only half a degree. The great nebula in Orion has 

 been traced to a still greater distance, as far as 5^° in 

 length and 4 in breadth. There sizes are, however, 

 as far as appearances go, quite exceptional. The 

 •dumb-bell nebula in Vulpecula, that in Lyra which re- 

 sembles a ring, and the spiral nebula in Canes Venatici, 

 not to mention hosts of others, are well worthy of 

 attention. Some astronomers have held the opinion 

 that in the nebulas we have evidences of the be- 

 ginnings of worlds, other that they are only enormous 

 aggregations of stars, but at such vast distances as to 

 be irresolvable. It was reserved for the spectroscope 

 to make a more accurate determination of their 

 nature as well as of that of the stars. 



By allowing a ray of sunlight to pass through a slit 

 in the shutter of a darkened room, and to fall upon a 

 prism of glass (preferably flint), it will emerge in 



the form of a coloured band. The simple arrange- 

 ment thus formed may be improved by using a 

 number of prisms so arranged that when the light 

 emerges from one it enters its neighbour, and so on. 

 This secures a greater amount of dispersion, and 

 causes the band to be of much greater length than it 

 would otherwise be. On examining the coloured 

 band with a small telescope, we shall see that it is 

 striped with a multitude of dark lines at right angles 

 to its length. Although many of these lines were 

 noticed by Wollaston in 1802, and examined and 

 mapped out by Fraunhofer and others, it was not till 

 1859 that Kirchoff was enabled to interpret their 

 meaning. This may, perhaps, be made clear by the 

 following simple experiment. Instead of making 

 use of the sun for our source of light, let us take a 

 Bunsen burner, in the flame of which a small portion 

 of the metal sodium has been placed. On viewing 

 the band, or spectrum, thus produced, one is at once 

 struck by its utter dissimilarity to that which is 

 formed when sunlight is employed. It is neither 

 covered with coloured bands not streaked with dark 

 lines, but in a certain position — which is always the 

 same relatively to the length of the spectrum — there 

 is one bright yellow line. Now let us take a second 

 burner, also with a piece of sodium in its flame, and 

 arrange the two lights and the slit in such a manner 

 that all are in one and the same straight line. The 

 coloured streak is gone, and a black line appears in 

 its stead, occupying identically the same position. 

 On performing similar experiments with other 

 substances, it will be invariably found that wherever 

 coloured lines are seen in the first instance, when the 

 two flames are used they will be replaced by dark 

 ones. Spectra with dark lines are known as absorp- 

 tion spectra, for the vapour of, say, sodium, has the 

 power of absorbing rays of the same refrangibility as 

 those emitted by itself. A spectroscope attached to 

 a telescope in lieu of an eyepiece enabled Huggins, 

 " the father of spectroscopy," to assert in 1864 that 

 the light of certain nebula; consisted of glowing 

 vapour. The question of the constitution of — at least 

 many of — the nebula; has thus been answered. The 

 spectroscope has also been successfully applied to 

 such' few temporary stars as have been observed to 

 burst forth with a sudden intensity of brightness and 

 then wane. On May 12, 1866, a star of the second 

 magnitude was noticed in a part of Corona Borealis, 

 where the observer, Birmingham of Tuam, felt certain 

 that it had not, at any rate in its then brightness, 

 formerly existed. That no star of that magnitude 

 was visible there four hours previously was certified by 

 another observer. The application of the spectroscope 

 revealed the fact that its very sudden increase of 

 brightness was due to incandescent hydrogen. The 

 actual result of such a terrible conflagration will, 

 perhaps, remain for ever unknown to us. T. Coronce 

 was literally a nine days' wonder, for at the expira- 

 tion of that time it had become a mere telescopic star. 



