February 6, 1919] 



NATURE 



445 



totality, but the sun's altitude is only about i.s°, 

 and the journey is difficult. 



There is no information to hand at presetH as 

 to expeditions from other countries. American 

 astronomers have taken a prominent part in the 

 observation of receni eclipses, but, apparently, 

 thc\ are satisfied with their successful observa- 

 tions in their own country last June, and do not 

 contemplate making observations next May; it is 

 hoped, however, that the South American observa- 

 tories may lake part. 



Besides the long totality, this eclipse is also 

 noteworthy for the rich field of stars round the 

 sun; the Astronomer Royal save a diagram of 

 their configuration in the Monthly Notices for 

 March, 1917, and directed attention to the very 

 favourable opportunity that would be presented 



addition to our knowledge of physic*. Should 

 the decision be in favour of the Einstein shift, it 

 would, in combit h the success of the 



l.nter In explaining the motion of the perihelion 

 ol Mercury, suffio to its acceptance as 



the actual system of the universe.' Its definite 

 disproof would also be , since it would 



avoid the dissipation ol fu ther energy in its 

 elaboration, though it would still deserve our ad- 

 miration as an ingenious system of ideal geometry. 

 Consequently, the British observers will leave 

 questions of solar or coronal physics altogether 

 alone on this occasion, and will concentrate on 

 the effort to obtain accurate photographs of the 

 Star-field round the sun for comparison with 

 photographs thai have already been obtained of 

 the same region in the night sky. There are 



TOTAL SOLAR ECLIPSE, May 29, 1919. 



for testing Einstein's theory of relativity, 

 according to which a ray tangential to the sun 

 from a star would be deflected through 174", the 



'ion for other stars being inversely propor- 

 tional to their angular distance from the sun's 

 centre. Prof. Eddington has directed attention 

 to the deduction that, since a ray of light carries 

 energy, even apart from Einstein's theory, 

 should expect the same shift as would be 

 produced by the sun's gravitation on a particle 

 passing close to its surface with the speed of 

 light; it is easy to show that this shift would be 

 exactly half that predicted by Einstein, or 0*87 

 at the sun's limb. There are thus three possi- 

 bilities: no shift, the half shift, or the full Ian- 

 stein shift. The definite establishment of any one 

 of the three as the truth would be -in important 



NO. 2571. VOL. I02] 



thirteen stars in the region down to magnitude 

 70 within the field of an astrographic plate, 

 which is a square slightly more than 2° in the 

 side; nine of them are as bright as or brighter than 

 o'o mag. It is not proposed to give exposures ex- 

 ceeding ios., and it is hoped that, with restrained 

 development, ill the thirteen stars may be re- 



led without being overpowered by the diffu :d 



light of the corona. The object-glasses of the 

 Greenwich and Oxford astrographic equatorials 

 will both be employed, also some smaller lenses 

 of longer focus. The driving clocks of the ccelo- 

 siats have given some trouble in former eclipses, 

 but they have been carefull) erhauled by Mr. 

 Cottingham, and a notable improvement is ex- 

 1. In any case, exposures limited to ios. do 

 not require very accurate driving. 



