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 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, 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 notions 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, "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.