504 



NATURE 



[September 23, 1897 



British Central Africa, presented by Captain C. F. Beeching ; a 

 Raven {Corvus corax) British, presented by the Rev. F. C. A. 

 Barrett ; a European Pond Tortoise {Etnys orbicularis), 

 European, presented by Miss W. Fenwick ; a Common Marmo- 

 set {ffapale jacchus) from South-east Brazil, a Burchell's Zebra 

 (Eqtius burchelli), born in the Menagerie, deposited ; two Red 

 Foxes {Canis fulvus) from Canada, a Black Woodpecker (Picus 

 7nartius), z.'Roo'pot {Upupa efops), ioMT Little Ringed Plovers 

 {Aigialieis ciironica), European, purchased. 



4 ■934- Both these values are somewhat in excess of the average 

 of previous observations, but the divergence between them 

 renders further investigation desirable. 



The Diameters ok Jupiter and his Satellites. — Herr 

 Leo Brenner communicates to the Astronomische Nachrichten 

 (No. 3444) the results of recent measures, made by him at the 

 Manora Observatory, of the widths of the various bands and 

 belts on Jupiter, and the angular diameters of the planet and 

 its four large satellites. The following are the results of the 

 measurements of diameters, reduced to mean distance :— 



OUR ASTRONOMICAL COLUMN. 



The Cause of the Proper Motions of Stars.— When 

 the parallax of the star 1830 Groombridge is considered in con- 

 nection with the large proper motion of seven seconds of arc per 

 annum, the conclusion is arrived at that the star is moving 

 through space with a velocity which probably exceeds two 

 hundred miles per second. In his " Popular Astronomy," Prof 

 Simon Newcomb briefly discussed the problem of stellar 

 dynamics involved in this enormous velocity. He showed that 

 if the universe be considered of such an extent that light would 

 take 30,000 years to cross it, and if it contained one hundred 

 million stars, having, on the average, a mass five times the mass 

 of the sun, the gravitational attraction of a universe thus con- 

 stituted would only be sufficient to give a velocity of twenty-five 

 miles per second to a body drawn from infinity to the centre of the 

 system of masses. The calculated limit is thus only about j 

 one-eighth the velocity deduced from the observed proper 

 motion and parallax. Prof. Newcomb therefore concluded : 

 *' Either the bodies which compose our universe are vastly more 

 massive and numerous than telescopic examination seems to 

 indicate, or 1830 Groombridge is a runaway star, flying on a 

 boundless course through infinite space with such momentum that 

 the attraction of all the bodies of the universe can never stop it." 



A new contribution to this inquiry was read recently before 

 the American Philosophical Society by Mr. Luigi d'Auria. The 

 object of the investigation was to determine whether, assuming 

 the interstellar ether to possess the virtue of gravitational 

 attraction, the force exerted by it would be sufficient to account 

 for the proper motions of stars, and especially of the flying star 

 1830 Groombridge. In this paper it is shown that, " given the 

 ether the density as estimated by Maxwell, and the power of 

 attracting matter by gravity, a body placed within the sphere of 

 ether containing all the stars of the visible universe, and at a 

 distance from the centre of such sphere equal to that passed 

 over by light in 2200 years, would pass this centre with a 

 velocity equal to that of the star 1830 Groombridge, taking into 

 account the attraction of the ether alone ; and such body would 

 oscillate about the same centre, rectilinearly, with a period of a 

 little over seven viillion years, which would be also the period 

 of oscillation of every other star." Mr. d'Auria recognises that 

 some other, and unquestionable, cause may eventually prove to 

 be responsible for stellar proper motions, nevertheless he thinks 

 his results are worth putting on record. 



New Determination of the Solar Constant. — A 

 fresh contribution to our knowledge respecting the sun's heat 

 appears in the Metnorie della Societh degli Spettroscopisti 

 Italiani, vol. xxvi., 1897, where Dr. G. B. Rizzo describes a 

 series of observations for determining the solar constant, made 

 at the station " Regina Margherita" on Monte Rosa. The 

 apparatus used was a slight modification of Angstrom and 

 Chwolson's ; the sun's rays being received on two brass discs 

 attached to thermometers, which were alternately exposed and 

 protected by two aluminium screens so arranged that when one 

 disc was covered the other was exposed. To determine the 

 quantity of solar heat absorbed per unit area per unit time, the 

 formula of Chwolson was employed. Owing to the unsettled 

 weather in September last, when the observations were made, 

 the results were found at times to fluctuate considerably. In 

 determining the solar constant or quantity of heat (measured in 

 calories per minute) incident normally on a square centimetre at 

 the earth's distance from the sun, it is necessary to assume some 

 law for the effect of atmospheric absorption at the place of 

 observation. Dr. Rizzo finds that the formulae of Forbes and 

 Crova for this purpose, when applied to his present observations, 

 give for the solar constant the respective values 3 '133 and 



NO. 1456, VOL. 56] 



Jupiter 

 SatelUte I. .. 

 Satellite II. 

 Satellite IH. 

 Satellite IV. 



Equatorial 

 diameter. 



38-539 



1*063 

 1704 

 1-550 



Polar 

 diameter. 



36-134 

 I 060 

 0-958 

 1-504 

 1-345 



I : 16 024 



I : I0'I23 

 I : 8-52 

 i: 7-568 



Action of Jupiter and Saturn upon Encke's Comet. — 

 In a memoir which will shortly appear, M. A. Lebeuf gives 

 formulae for calculating secular inequalities when the mutual 

 inclinations of orbits, and the eccentricity of the orbit of the 

 disturbed body, are known. The formulae are applied by M. 

 Lebeuf, in the Bulletin Astronomique, to determine the secular 

 inequalities of the elements of the orbit of Encke's comet in 

 consequence of the action of Jupiter and Saturn. The values 

 obtained are tabulated below : — 



Elements of 

 orbit. 



Secular inequality 

 due to Jupiter. 



■ 1-38 

 256 

 35-9 



• 28-3 

 1336 



^=^=!:' 4SlSr' 



0-2 



0-45 

 0-85 

 0-66 

 304 



+ I "40 



- 26 •! 



- 36-8 

 + 290 



-136-6 



It is pointed out that the large eccentricity of Encke's comet, 

 and the small distance of the comet from Jupiter, makes the use 

 of the formulae difficult in the case of Jupiter ; but the results 

 seem to justify their application to the case of Saturn. 



PHASE-CHANGE OF LIGHT ON REFLECTION 

 AT A SILVER SURFACE. 

 A LIGHT wave, when reflected ^ at the surface of separation 

 ■^"^ of two media, may be altered in amplitude, or wave- 

 length, or phase. Whilst, however, a change of amplitude or 

 wave-length produces an obvious diffierence between the incident 

 and reflected light, the existence and nature of a change of 

 phase can only in general be inferred from the result of some 

 kind of interference experiment. Thus the fact that a very 

 thin transparent film is black when viewed by reflected light 

 leads to the conclusion that a light wave is altered in phase by 

 half a wave-length on reflection, either at a denser or at a 

 rarer medium. Mechanical analogies suggest that the change 

 probably takes place at the denser medium ; and an experiment 

 of Lloyd's, in which coloured fringes with a black centre were 

 obtained by the interference of two beams of light, one directly 

 transmitted, and the other reflected from a glass mirror, led to 

 the same conclusion. 



Jamin's experiments on metallic reflection showed that when 

 light is reflected from a silver surface a phase-change is pro- 

 duced, and, moreover, that this change is different according 

 as the light is polarised in, or perpendicular to, the plane of in- 

 cidence. His experiments led to the determination of the 



1 The term "reflection" is here used in its most general sense, to include 

 such phenomena as phosphorescence, &c. 



