270 



HA RD WICKE'S SCIENCE- G OS SI P. 



In many almanacks we find, against certain clays, 

 an intimation that meteors may then be expected. 

 Hardly an hour elapses on a clear night without one 

 or more of these small erratic bodies being seen to 

 pass athwart the sky, and sometimes leave a trail of 

 light, occasionally also bursting into many fragments. 

 In fact, many thousands of meteors enter the earth's 

 atmosphere in the course of the year. The Leonids, 

 so called because they appear to emanate from the 

 constellation Leo, have been for very many years 

 noticed on the nights intervening between November 

 11 and 15. The brilliancy of the shower varies in 

 different years, and reaches a maximum once in 

 thirty-three years ; the last display of this nature, 

 when " the heavens seemed to be on fire," occurred 

 in 1866. The radiant points of about a hundred of 

 these meteor showers have been determined with a 

 very considerable amount of accuracy. Schiaparelli 

 found, in 1866, that the path pursued by the August 

 meteors, the Perseids, was identical with that of the 

 orbit of a bright comet which had been seen in 1862. 

 Here, then, was a discovery which seemed to bear 

 on the nature, not of meteors only, but of comets as 

 well. Other astronomers followed the example thus 

 set them, and the identity of the November meteors 

 and Tempel's comet of 1866 was soon established, as 

 well as that of the April Lyraids and a comet which 

 had appeared in 1S61. As meteor-showers reappear 

 after a certain time, it is evident that they are parts 

 of our system, and pursue very elongated elliptic 

 paths, as do many comets, unless attracted from their 

 course into hyperbolic or parabolic paths by some 

 more powerful body than our sun. 



All comets do not present the same appearance in 

 the telescope, but they have several features in 

 common. Most of them have tails, though they are 

 sometimes inconspicuous in consequence of their 

 distance or size, or their position relatively to the sun 

 and earth ; while nearly all have a head, which, 

 when examined in the telescope, usually appears 

 surrounded by envelopes. AVhen a comet first comes 

 into visibility, it generally appears small, and looks 

 somewhat like a misty star — a hairy star, as the 

 name implies. As it gets nearer the sun, a tail is 

 thrown out, increasing in length until the comet is 

 lost to sight from its proximity to the sun, to re- 

 appear on the other side of the orb of day, grow 

 fainter, and gradually disappear in the realms of 

 space. The most striking comet of recent years was 

 undoubtedly Donati's, named after the astronomer 

 who discovered it at Florence, June 2, 1858. Not 

 till nearly three months after its discovery did a tail 

 begin to appear, or the comet itself to be visible to 

 the naked eye. It was not finally lost sight of till 

 March 4, 1859. Whatever comets are composed of, 

 their mass is very slight, and their constituent matter 

 of such a nature as to allow even faint stars to be 

 seen through them without loss of brightness. The 

 spectroscope shows that comets do not shine, as was 



formerly supposed, merely by light reflected by them* 

 from the sun, but that they are partly self-luminous, 

 and that their gaseous surroundings contain, at any 

 rate, hydrogen and carbon. 



In our somewhat desultory inspection of the 

 heavens, we have viewed some of all the different 

 sorts of celestial bodies, but the two with which we 

 are most intimately concerned have been purposely 

 reserved to the last. 



When observing the sun telescopically, it is of the 

 utmost importance that the eye should be protected 

 by some special contrivance from the excessive heat 

 and light concentrated in the focus of even a small 

 mirror or object-glass. The simplest plan is to attach 

 to the end of the eye-piece nearest the eye a " dark 

 head," or glass of some dark colour. The disadvan- 

 tages attending this method of viewing the sun lie 

 chiefly in the fact that the only means of adjusting 

 the amount of light received is to change the dark 

 head in use for another of a greater or less degree of 

 transparency. The operation, though slight, takes 

 time, and what might otherwise have proved a valu- 

 able observation is marred. Sir John Herschel 

 suggested the use of a diagonal eye-piece, which 

 consists of a tube, open at both ends, sliding into the 

 eye-end of the telescope, and having another shorter 

 tube at right angles to itself. In the latter is placed 

 the eye-piece, opposite which, in the longer tube, a 

 piece of plain unsilvered glass is fixed at an angle of 

 45 , or, better still, a small prism. By these means 

 about 53 of the light, and very little of the heat, is 

 reflected. The diagonal eye-piece is also a great 

 comfort when viewing objects near the zenith with a 

 refractor. A polarising eye-piece is a still greater 

 luxury in solar observations, since it permits the 

 degree of illumination of the image to be adjusted to 

 a nicety. When light falls on a plane glass surface 

 at a certain angle, the reflected rays are polarised, 

 and if then permitted to fall perpendicularly on a 

 plate of tourmaline, capable of being rotated, the 

 quantity of light transmitted can be varied at will. 

 Dawes' solar eye-piece is another contrivance for the 

 same purpose. Briefly, it consists of three circular 

 plates, each of which is capable of independent 

 rotation. In one is a series of circular apertures 

 varying from T ) )D to 5 inch in diameter ; in the next 

 some single lenses ; and in the outermost dark glasses 

 of different shades. The lenses have to be focussed. 

 by a rack-and-pinion movement. It will be seen that 

 the instrument is rather complicated. A solar and 

 sidereal diaphragm eye-piece of simpler form, devised 

 by the writer some years ago, will be found described 

 in the " Monthly Notices " of the Royal Astronomi- 

 cal Society. In this the variable size of the aperture 

 is produced by a single screw movement. The sun 

 may also be viewed with very great comfort and ease 

 by projecting its image on a piece of card attached to 

 the telescope at right angles to the line of sight. 



Having fitted one or other of these contrivances to 



