662 PROCEEDINGS OF THE AMERICAN ACADEMY. 



for the aurora borealis. Swartzschild * computed from radiation pressure 

 ou small spherical conductors the size of bodies of unit density for which 

 the ratio of radiation pressure to gravitational attraction would be a 

 maximum. 



Before the Congres International de Physique in 1900, Professor Lebe- 

 dew t of the University of Moscow described an arrangement of apparatus 

 which he was using at that time for the measurement of light pressure. 

 He summarizes the results already obtained as follows: " Les resultats 

 des mesures que j'ai faites jusqu'ici peuvent se resumer ainsi : L'experi- 

 ence montre qu'un faisceau lumineux incident exerce sur les surfaces 

 planes absorbantes et reflechissantes des pressions qui, aux erreurs pres 

 d'observation, sont egales aux valeurs calculees par Maxwell et Bartoli." 

 No estimate of the " errors of observation " was given in the paper nor 

 other numerical data. Unfortunately the proceedings of the Paris 

 Congress did not reach the writers nor any intimation of the methods 

 or results of Professor Lebedew's work until after the publication of 

 their own preliminary experiments. 



The writers | presented the results they had obtained by measure- 

 ments of radiation pressure at eight different gas pressures, in a prelim- 

 inary communication to the American Physical Society, meeting with 

 Section B of the American Association at Denver, August 29, 1901. 

 A condensed abstract of this paper follows. 



In the experiments of earlier investigators every approach to the 

 experimental solution of the problem of radiation pressure had been 

 balked by the disturbing action of gases which it is impossible to remove 

 entirely from the space surrounding the body upon which the radiation 

 falls. The forces of attraction or repulsion, due to the action of gas 

 molecules, are functions, first, of the temperature difference between the 

 body and its surroundings, caused by the absorption by the body of a 

 portion of the rays which fall upon it; and second, of the pressure of 

 the gas surrounding the illuminated body. In the particular form of 

 apparatus used in the present study the latter function ap^iears very com- 

 plicated, and certain peculiarities of the gas action remain inexplicable 

 upon the basis of any simple group of assumptions which the writers 

 have so far been able to make. 



* K. Swartschild, Kgl. Bayer. Akademie d' Wissenschaften, XXXI. 293 (1901). 



t P. Lebedew, Rapports presentes au Congres International de Pliysique (2), 

 p. 133. Paris, 1900. 



t E. F. Nicliols and G. F. Hull, Science, XIV. 588 (Oct. 18, 1901), and Phys. 

 Rev., XIII. 293 (Nov., 1901). 



