560 PROCEEDINGS OF THE AMERICAN ACADEMY. 



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

 gases surrounding the illuminated bodies employed in the measurements, 

 they obtained wholly confusing and contradictory results. Later in the 

 same century the Rev. A. Bennet* performed further experiments, 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 effect of the rays. Finding no pressure 

 due to radiation, he made the following unique suggestion 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. I think modern discoveries, especially those of elec- 

 tricity, favor the latter hypothesis." In the meantime Euler, t accept- 

 ing Kepler's theory attributing the phenomenon of comets' tails to light 

 pressure, had hastened to the support of the wave theory by showing 

 theoretically that a longitudinal wave motion might produce a pressure 

 in the direction of its propagation upon a body which checked its 

 progress. In 1825 Fresnel t made a series of experiments, but arrived 

 at no more definite conclusion than that the repulsive and attractive 

 forces observed were not of magnetic nor electric origin. 



Crookes§ believed in 1873 that he had found the true radiation pres- 

 sure in his newly invented radiometer and cautiously suggested that his 

 experiments might have some bearing on the prevailing theory of the 

 nature of light. Crookes' later experiments and Zollner's || measure- 

 ments of radiometric repulsions showed that the radiometric forces were 

 in some cases 100,000 times greater than the light pressure forces with 

 which they had been temporarily confused. Zollner's experiments are 

 anions the most ingenious ever tried in this field of work, and he missed 

 the discovery of the true radiation pressure by only the narrowest 

 margin. An excellent bibliography of the whole radiometric literature 

 is given by Graetz,H and an account of some of the older experiments not 

 mentioned above is given by Crookes.** 



* A. Bennet, Phil. Trans., p. 81 (1792). 



t L. Euler, Histoire de l'Acadenrie Royale de Berlin (2), p. 121 (1746). 

 t A. Fresnel, Ann. Chem. et Phys., XXIX. 57, 107 (1825). 

 § W. Crookes, Phil. Trans, p. 501 (1873). 

 II F. Zullner, Pogg. Ann, CLX. 156, 296, 459 (1877). 



If L. Graetz, Winckelmann's Handbuch der Physik, 2 b, p. 262. Breslau, 

 1896. 

 ** W. Crookes, 1. c, p. 501. 



