304 Maxwell. 



to the rays of the sun concentrated by a mirror, and observed 

 a deflexion. He was not satisfied that the effect of the heating 

 of the air had been altogether excluded, but " there seems to 

 be no doubt," in Priestley's opinion, " but that the motion above 

 mentioned is to be ascribed to the impulse of the rays of light." 

 A similar experiment was made by A. Bennet,* who directed 

 the light from the focus of a large lens on writing-paper 

 delicately suspended in an exhausted receiver, but " could not 

 perceive any motion distinguishable from the effects of heat." 

 " Perhaps," he concluded, " sensible heat and light may not be 

 caused by the influx or rectilineal projections of fine particles, 

 but by the vibrations made in the universally diffused caloric 

 or matter of heat, or fluid of light." Thus Bennet, and after 

 him Young, f regarded the non-appearance of light- repulsion in 

 this experiment as an argument in favour of the undulatory 

 system of light. " For," wrote Young, " granting the utmost 

 imaginable subtility of the corpuscles of light, their effects 

 might naturally be expected to bear some proportion to the 

 effects of the much less rapid motions of the electrical fluid, 

 which are so very easily perceptible, even in their weakest 

 states." 



This attitude is all the more remarkable, because Euler 

 many years before had expressed the opinion that light-pressure 

 might be expected just as reasonably on the undulatory 'as on 

 the corpuscular hypothesis. "Just as," he wrote, J "a vehement 

 sound excites not only a vibratory motion in the particles of 

 the air, but there is also observed a real movement of the small 

 particles of dust which are suspended therein, it is not to be 

 doubted but that the vibratory motion set up by the light 

 causes a similar effect." Euler not only inferred the existence 

 of light-pressure, but even (adopting a suggestion of Kepler's) 

 accounted for the tails of comets by supposing that the solar 

 rays, impinging on the atmosphere of a comet, drive off from 

 it the more subtle of its particles. 



* Phil. Trans., 1792, p. 81. + Ibid., 1802, p. 46. 



J Histoire de /' Acad. de Berlin, ii (1748), p. 117. 



