268 



HARDWICKE'S SCIENCE-GOSSIP. 



magnitude of which is to be determined, through a 

 wedge of darkened glass which can be moved to and 

 fro until a part is reached, through which the light of 

 the star can no longer penetrate. The position of 

 the wedge is then read on an appropriate scale. 

 This "method of extinctions" has been found very 

 trustworthy. 



Again, it will be noticed that many of the stars, 

 which to the naked eye, or even when viewed in the 

 finder, appear to be single, are not so in reality, but 

 double, triple, and even quadruple. In some in- 

 stances this appearance is due to the fact that one of 

 the stars is nearer to us than its apparent com- 

 panion, but in almost the same straight line. These, 

 then, are only optically double, but many are really 

 double, being physically connected with each other, 

 though separated by an enormous interval, each 

 circulating round their common centre of gravity. 

 Till about a hundred years ago only four such pairs 

 were known, whereas now a complete list would 

 contain about ten thousand. As the relative positions 

 of those that are not merely optically double is 

 constantly varying, it is of great importance to 

 determine them precisely, and it may be interesting 

 to the reader to know how this is accomplished. 

 The eye-pieces we have hitherto been using are of 

 such a construction that the focus is between the two 

 component lenses, and are called negative eye-pieces. 

 We shall now have to substitute for them another 

 sort — the positive — the plano-convex lenses of which 

 have their convex surfaces facing each other. The 

 peculiarity of this construction is that the focus of 

 the combination is beyond the field-lens ; this enables 

 us to observe distinctly not merely the image of a 

 star, but also an arrangement of delicate spider lines 

 placed in the common focus of the object-glass, or 

 mirror, and the eye-piece. As it is necessary for all 

 observers to measure accurately, and in the same 

 manner, the angle which the components of a double 

 star make with each other, there must needs be some 

 fiducial line to start from as the fixed arm of the 

 angle. This is a line joining the true north and 

 south points in the field of view, and indicated 

 therein by a spider-line, technically called a wire, 

 which can be placed in that direction, as shown by a 

 divided "position circle," usually engraved on the 

 eye-end of the telescope. This circle has, of course, 

 its verniers and reading microscopes, together with a 

 mechanical arrangement for imparting a slow rotary 

 motion to the eye-piece and its wire. Having, then, 

 got our new instrument into position, and the larger 

 of the two stars on the wire, all we have to do is to 

 rotate the apparatus from left to right till the wire 

 appears to bisect the other star, when the reading on 

 the circle will show the angle which the line joining 

 the two stars makes with the meridian. In practice 

 a few minutire, which are here omitted, must be 

 attended to first, in order to make the observation 

 correct. 



But their distance from each other, as well as their 

 relative positions, must be determined. To do this- 

 we must avail ourselves of another subsidiary ap- 

 pliance, the parallel-wire micrometer. This consists 

 essentially of an oblong box which, with its positive 

 eye-piece, can be placed in the eye-tube of the 

 telescope. On looking through it we see a fixed 

 horizontal wire and two others in a vertical position, 

 and, therefore, parallel to each other, which can be 

 caused mutually to approach or recede by means of a 

 screw at either end of the box. These screws have 

 large graduated,' heads, and when the value of one 

 revolution of the screws is determined, we can find 

 the distance of the two stars, the diameter of a 

 planet, &c, by first bringing the wires together, so 

 as apparently to touch one of the stars or one side of 

 the planet's disc, and then by means of its screw 

 causing one of the wires to touch the other star or 

 the other side of the disc in a direction indicated by 

 the fixed wire. The number of turns of the screw 

 will give the angular distance. The double and 

 multiple stars are so numerous that it is very difficult 

 to make a selection, but the amateur will be much 

 gratified by measuring such objects as Polaris, o and 

 e Lyne, 6 Orionis, and ( Cancri. 



Among the mysteries yet unsolved must be placed 

 the variable stars, the light emanating from which does 

 not remain constant. The first whose variability was 

 noticed was o Ceti, which in the course of three hundred 

 and thirty days varies in brightness from that of a star of 

 the second magnitude to invisibility. Some members 

 of this class accomplish their periods in a few days 

 only, while there are others, such as 77 Argus, which 

 take many years to go through their variations. One 

 of the theories that have been brought forward in 

 explanation of these phenomena, is that these stars are 

 accompanied by obscure bodies which revolve either 

 round them or round their common centre of gravity. 



We shall hardly escape being shown the Pleiades, 

 or Praesepe, or the glorious globular cluster in 

 Hercules, or the marvellous Milky Way, which 

 consists of an infinite number of stars, with which 

 our own system is supposed to have some connection. 



A question will naturally present itself to the 

 enquiring mind, At what distance are these stars 

 from us? — a question very difficult of exact solution. 

 If a person were to stand not far from the base of the 

 Westminster clock-tower at, say, a quarter-past the 

 hour, and another person were to ascend in a balloon 

 until he was at a level with the face of the clock, 

 the two, though watching the minute hand at the same 

 moment, would not agree as to its exact position. 

 When the quarter is struck, he who is on a level with 

 the clock will notice that the hand has reached the III., 

 but to an observer at the foot of the tower it will 

 appear to be about fourteen minutes past the hour. 

 This apparent difference is due to parallax — " the 

 apparent change of place which bodies undergo by 

 being viewed from different points." Astronomers 



