RECENT ADVANCES IN SCIENCE 13 



van Wet. Amsterdam, 25, 963-71), J. de Vries (ibid. 1414-18) 

 G. Grimaldi (Rend, del Circ. mat. di Palermo, 191 7, 42, 80-84), 

 and G. Marietta (ibid. 1916, 41, 180-86); on complexes of 

 straight lines see C. Segre (ibid. 191 7, 42, 85-93) ', on algebraic 

 hypersurfaces of n dimensions see E. Brambilla (Ann. di Mat. 

 1 91 6, 25, 317-42) ; on infinitesimal geometry of space see P. 

 Tortorici (ibid. 205-28), C. H. Yeaton (ibid. 26, 1-33), H. Weyl 

 (Zurich Vjs. 191 6, 61, 40-72), G. Fubini (Palermo Rend. 191 6, 

 41, 135-62), Z. Geocze (Math, es term. ert. Budapest, 191 5, 33, 

 730-48 ; 1 91 6, 34, 337-54 ; Math, is phys. lapok, Budapest, 

 191 8, 25, 61-81), L. Bianchi (Ann. di Mat. 191 6, 25, 129-204), 

 A. V. Backlund (K. Sv. Vet. Handl. 191 6, 55, No. 2), and V. 

 Strazzeri (Palermo Rend. 191 7, 42, 1-45) ; and on invariant 

 transformations of curves on a surface, see G. Fubini (Ann. 

 di Mat. 1916, 25, 229-52). 



ASTRONOMY. By H. Spencer Jones, M.A., B.Sc, Royal Observatory, 

 Greenwich. 



Stellar Parallaxes. — The first of this series of notes on recent 

 progress in astronomy (Science Progress, 10, 120, 191 5) 

 was devoted entirely to an account of the modern develop- 

 ments in the determination of stellar parallaxes ; a rapid 

 expansion of our knowledge of accurately determined paral- 

 laxes, consequent upon the successful employment of photo- 

 graphic methods, was then foreshadowed. The developments 

 since have been even more rapid than seemed possible at that 

 time. 



One of the principal requirements for accurate parallax 

 work is a telescope with a long focal length, which gives a large 

 scale on the photographic plate. Good definition is not so 

 essential, as the exposures are short. European observatories 

 can therefore co-operate in this work effectively, whereas for 

 observations requiring good definition they are generally 

 handicapped through being in unsuitable sites. Most of the 

 parallax work at present, however, is being done in America. 

 The Committee of the American Astronomical Society which 

 was formed to co-ordinate the work and to prevent unneces- 

 sary duplication presented a report at the last meeting of 

 the Society, which gives a good idea of the rapidity with which 

 results are now being obtained. 



At the Allegheny Observatory in one year, 2,500 plates 



