THE PRESSURE DUE TO RADIATION." 



By E. F. Nichols and G. F. Hull. 



As early as 1619 Kepler'' announced his belief that the solar repul- 

 sion of the finely divided matter of comets' tails was due to the out- 

 ward pressure of light. On the corpuscular theor}^ of light, Newton^ 

 considered Kepler's idea as plausible enough, but he was of the opinion 

 that the phenomenon was analogous to the rising of smoke in our own 

 atmosphere. In the first half of the eighteenth century De Mairan and 

 Du Fay '^ contrived elaborate experiments to test this pressure-of -light 

 theory in the laboratory, but, because of the disturbing action of the 

 gases surrounding the illuminated bodies employed in the measure- 

 ments, they obtained wholl}' confusing and contradictory results. 

 Later in the same century Rev. A. Bennef performed further exper- 

 iments, but could find no repulsive force not traceable to convection 

 currents in the gas surrounding the body upon which the light was 

 projected, due, in his opinion, to the heating efl'ect of the ra3's. Find- 

 ing no pressure due to radiation, he made the following unique sug- 

 gestion in support of the wave theory of light: 



Perhaps sensible heat and light may not be caused by the influx or rectilinear 

 projection of fine particles, but by the vibrations made in the universally diffused 

 caloric or matter of heat or fluid of light. 1 think modern discoveries, especially 

 those of electricity, favor the latter hypothesis. 



In the meantime Euler,-^' accepting Kepler's theory attributing the 

 phenomenon of comets' tails to light pressure, had hastened to the 



a Presented to the American Academy of Arts and Sciences, December, 1902. 

 Reprinted from the Astrophysical Journal, Vol. XVII, No. 5, June, 1903, omitting 

 some of the tabulated results of experiments. 



''De Mairan, Traite physique et historique de I'Aurore boreale (2d ed.), pp. 357, 

 358. Paris, 1754. 



<'Isaaci Newtoni Opera quse Existant Omnia. Samuel Horsley, LL.D., R. S. S., 

 Tom. Ill, pag. 156. Londinium, 1782. 



<i De Mairan, loc. cit., p. 371. This treatise contains also the accounts of still earlier 

 experiments by Hartsoeker, p. 368, and Homberg, p. 369. The later experiments 

 are of more historic than intrinsic interest. 



''A. Bennet, Phil. Trans., p. 81, 1792. 



/L. Euler, Histoire de I'Academie royale de Berlin (2), p. 121, 1746. 



115 



