492 



NATURE 



[February 21, 19 18 



OVR ASTRONOMICAL COLUMN. 

 Relativity and Gravitation. — A pamphlet has just 

 reached us entitled " La spostamento del perielio di 

 mercurio, e la deviazione del raggi luminosi, secondo 

 la teoria di Einstein," by Attilio Palatini (from Nuovo 

 Cimento, July, 1917; Pisa: Stabilimento Tipografico 

 Toscano). The pamphlet, like the article by Prof, 

 Eddington in Nature of December -28, 19 16 (vol. 

 xcviii.,.-p. 328), aims at making the outlines of Ein- 

 stein's relativity theory clear to those who have not 

 access to his original works. The points in which the 

 new theory differs from our earlier conceptions of 

 Euclidean space and Newtonian dynamics are clearly 

 brought out. As the title indicates, particular stress 

 is laid upon the manner in which it- completely accounts 

 for the excess of 43" per century in the motion of the 

 perihelion of Mercury's orbit, which had been recog- 

 nised as a difficulty in the Newtonian theory. It is 

 especially noteworthy that the Einstein theory was laid 

 down quite independently of this result, which is there- 

 fore in the nature of an undesigned coincidence. It 

 differs in this respectrfrom some other reljitivity theories, 

 whkh have assumed arbitrary values for certain co- 

 efficients, in order to -satisfy the observed facts. Ein- 

 stein's result involves no afbitrary constant, but simply 

 depends "on the ratio df Mercury's velocity to that of 

 light. The pamphlet emp^loys two different methods of 

 dCYelopment, each leading to >the result that ' the peri- 

 tielion advances o-i" in one revolution of 'Mercury. 



The other itest ":proposed by Einstein for ' his theory 

 isthat amyof light from a ^tar just grazing the sun's 

 -surface and :passing on to thie eafth would be deflected 

 '^through-afi-angle of r-75". It is shown in the pamphlet 

 how this result is deducible from Einstein's princip-les, 

 and allusion is made to total solaf eclipses as affording 

 Oipportunittes -for a ^practical test. The Astronomer 

 RoyaHhas alfeady urged thsit advantage be taken of 

 'tte very 'favourable 'total edlipse of May, 19 19, for 

 i^xperhttertts of^thiskind. Prof. Eddington has pointed 

 OUt'that the doctrine'that light has inertia would lead 

 •us to ejfpeet a d<^flection of'O'88" at the sun's limb in 

 -any case; so the Einstein test depends on the difference 

 between 'this value and 1-75". 



The System of k Pegasi.— The star k -Pegasi -is 

 a visual binary having the unusually short .period of 

 11-35 years, and one of the components, as found by 

 Campbell 'in 1900, is a spectroscopic binary. An in- 

 vestigation of thi-s interesting triple system has been 

 made by Dr. F. Henroteau, utilising spectrograms pre- 

 viously taken at the Lick Observatory, and rtutrferous 

 others recently obtained by himsfelf (Lick Observatory 

 Bulletin, No. 304). Elements of the orbit of the spec- 

 troscopic pair, computed for the epochs 1900, 1912, and 

 1917, clearly show the changes to be expected from the 

 revolution round 'the centre of mass of the' visual sys- 

 tem, and they 'dlso indicate a revolution of the line 

 of apsides, probably due to perturbations occurring in 

 the spectroscopic binary orbit urtder'the influence of the 

 third body, Corribining the ddtaobtained by telescopic 

 and spectroscopic observations, it is shown that the 

 semi-major axis of the orbit of the spectroscopic binary 

 is 511,100,000 km., vJ-hile that of the visual pair is 

 1,826,000,000 km. Since the apparent semi-maior axis 

 is 0-29", it follows that the parallax is 0025". The 

 total mass of the spectroscopic pair is '10-33 times, ^""^ 

 the mass of the other visual component 400 times, that 

 of the sun. There are curious variations in the appear- 

 ance of the spectrum, which seem to be satisfactorily 

 explained by the superposition of an F class spectrum, 

 oscillating in a period of 5-9715 days, upon a spectrum 

 of possibly the same class oscillating by a smaller 

 amount in a period of 11-35 years. 

 NO. 2521, VOL. 100] 



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