122 SCIENCE PROGRESS 



have been obtained in the manner proposed by Kapteryn, the 

 photographic plate being exposed at the epoch when the parallax 

 factor is a maximum, then put away without being developed, 

 and re-exposed after an interval of six months when its 

 parallax factor is a minimum. This method has some advantages 

 over the "single plate" method used at Yerkes, in which 

 separate plates are exposed at the two epochs and developed 

 immediately. The change in the position of the parallax star 

 relatively to the much fainter neighbouring stars is measured 

 in each case. The rapid advance in our knowledge was summed 

 up by Sir F. W. Dyson thus : "We may expect that the number 

 of stars whose distances are fairly well known will soon amount 

 to thousands, as compared with 3 in 1838, about 20 in 1880, 

 about 60 in 1900, and now perhaps 200." 



The stars whose parallaxes have been sought have mostly 

 been either bright stars or stars with large proper-motion, the 

 probability of obtaining an appreciable parallax being largest for 

 such stars, which are likely, on the average, to be the nearest 

 to us. The Greenwich observing programme includes all stars 

 within 35 of the Pole known to have a proper-motion greater 

 than 20" per century. The determination of stellar distances 

 may be said now to have been placed upon so sure a footing 

 that it has been felt desirable to organise a scheme of co- 

 operation between the various observatories who are devoting 

 much time to this work (these include Yerkes, Swarthmore, 

 Dearborn, McCormick and Allegheny in America, Greenwich 

 and, later, others in Europe) so as to avoid unconscious duplica- 

 tion. A committee has been formed for this purpose with 

 F. Schlesinger, Director of the Allegheny Observatory, as 

 chairman. The large number of accurate results which will 

 soon be available will form very valuable material for many 

 astronomical investigations. 



PHYSICS. By James Rice, M.A., University, Liverpool. 



Recent Work on Atom-Structure. — At present two fairly definite 

 atomic models are being subjected to theoretical discussion 

 and experimental test by scientific workers. There is the 

 Thomson model, in which the atom is conceived as a 

 sphere of positive electricity in which negatively charged 

 electrons rotate in coplanar rings about the centre of the sphere. 

 There is also the Rutherford model, in which the electrically 



