78 



* KNOWLEDGE * 



[Jan. 30, 13e5. 



drop of oil of cloves fits it to receive tlie balsam. Another 

 wav is to immerse the object for some minutes in a drop of 

 saturated solution of carbolic acid ; drain that off, and then 

 apply the balsam, which agrees very well with the acid. 



Wben the slides are quite hardened, the superfluous 

 balsam may be removed easily with a rag wetted with either 

 benzine or wood-naphtha. If there is much to take away, 

 scraping with a penknife may be first resorted to. Chemists 

 sometimes sell, under the name of benzine, an impure article 

 which does pretty well to remove grease spots — though not 

 very well — and is useless for thinning balsam. The best is 

 highly volatile. Some objects require softening, which can 

 be done by treating them with a solution of caustic potash, 

 which roust be washed out before mounting, and the water 

 got rid of, as ju;t explained. Besides the little spring-clips 

 for pressing the co^■ers down, stationers now keep a stout, 



spring, paper-clip, with a wooden base, as in the annexed 

 sketch. This is useful when more pressure is wanted than 

 the light, brass, wire ones can give. 



THE EARTH'S SHAPE AXD MOTIONS. 



Br EicHARD A. Pkoctor. 



CHAPTER VI.— THE EARTH'S REVOLUTION. 



(Continued from p. 25.) 



BRADLEY'S discovery of the aberration of the fixed 

 stars forms one of the most interesting narratives in 

 the whole range of the history of astronomy. Yet, here I 

 must perforce leave it untouched, and deal only with the 

 phenomenon itself, sines properly to relate the steps by 

 which he was guided to that most important discovery, 

 would occupy all the rest of the space now available to me. 

 The great and general law, then, to which the appai-ent 

 annual motions of the stars are subjected — a law which 

 must be accounted for by any theory which pretends to 

 exhibit the real relations of the celestial bodies — is this : — 

 Every star in the heavens, whether obvious to the naked 

 eye or visible only liy the aid of powerful telescope.?, 

 whether shining in solitary splendour or lost, so to speak, 

 in the profundities of some rich star-cluster, travels once a 

 3'ear in a minute ellipse, whose major axis is somewhat 

 more than two-thirds of an arc minute in length, while its 

 minor axis depends on the position of the star with 

 reference to that great circle on the heavens in which the 

 sun seems annually to travel. A star close by the pole of 

 this circle — the ecliptic — has an almcst circular aberration- 

 ellipse ; one near the ecliptic itself has an aberration-ellipse 

 so eccentric as to be almost a straight line. But every star 

 has an aberration-ellipse of the same major axis. And that 

 major axis, though, as I have said, minute, belongs to the 

 order of magnitudes which are obvious to the ttlescopist — 

 palpable, unmistakable, clear as the sun at noon to the 

 worker in a well-appointed observatory.* 



* The mere fact that Bi-adlev. when telescopic appliances were 

 so imperfect, at once recognised the aberrations of the first fixed 

 star he watched with care, though lie had no reason to look for or 

 expect such a motion, is enough to show how very palpable the 

 phenomenon is. But it may be ivell to add that astronomers can 

 now recognise and feel certain about stellar displacements which 

 are only about one-hvmJinJth of the aberration motion. 



Fifr. 1. 



Now let us inquire what the particular law is according 



to whicli these remarkable 

 ellipses are described by 

 ihe star,^, for much depends 

 on tliis point. If merely 

 a vague notion is given of 

 the character of this in- 

 structive phenomenon, then 

 some vague and general 

 explanations will imme- 

 diately suggest themselves 

 to the supporters of para- 

 dox. When the exact 

 nature of alierration is 

 described, the proof of the earth revolution will be found 

 absolutely irrefragable. 



The best way of describing the nature of aberration is by 

 a reference to the accepted theory of the earth's motion. 

 Let A B C D, Fig. 1 represent the earth's path (perspec- 

 tively presented) around the sun, and suppose a star so 

 placed on the heavens as to occupy the pole of the ecliptic ; 

 then the star being at a distance practically infinite, we 

 should expect the lines of sight from all the points in 

 A B C D, to the star, to be directed towards this point — 

 the ecliptic pole. In other words, the lines A S, B S, C S, 

 D S, would all be at right angles to the plane A B C D. 

 Instead of this, when the earth is at A, the star is eeen 

 towards a, the plane n AH passing through the tangent 

 line to A B C D, at A. So when the earth is at B, the star 

 is seen towards h, the plane i B S passing through the 

 tangent to the circle A B C D at B. And similarly when 

 the earth is at C or D, the star is apparently displaced as 

 shown, towards c or d respectively, the displacement being 

 always an angle of one-third of a minute (approximately), 

 and always in the direction of the earth's motion. 



Next suppose the star placed on the ecliptic. Then the 

 earth's path being again represented by A, B, 0, D, and 

 the lines AS, B S, C S, D S (Fig. 2) being drawn in that 



plane parallel to each 

 other to indicate the 

 direction in which 

 one would expect to 

 see the star, it is 

 found that, whereas 

 when the earth is at 

 A and 0, where she 

 is moving in a lina towards or from the star, there is no 

 displacement fwhen she is at B and D, where she is moving 

 at right angles to the line of sight to the star, there is the 

 same displacement as in the former case. Also, in this 

 second case, the star seems more and more displaced as the 

 earth travels from A to B, then less and less till the earth is 

 at C, when there is no displacement ; then more and more, 

 but on the contrary side, till the eai'th is at D, and so 

 finally less and less till the earth is again at A. 



It will be easily seen that in the first case the star 

 appears to describe a circle, A B C D, Fig. 3, about its 

 mean position S, while in the latter it describes a straight 



Fi° 



Q, i 



e 



Fig. 3. 



Fin 



line A B C D, Fig. 4, through its mean position S, D B, in 

 Fig. 1, being equal to the diameter of the circle A B D, 

 in Fig. 3. 



