REORT ON PHYSICAL OPTICS. 299 



medium were directly excited by those of the luminous body, 

 his theory would have resolved itself into that of Huygens and 

 of Hooke. It may be observed, in connexion with this subject, 

 that Newton seems actually to have admitted the wave-theory 

 with respect to radiant heat; and that he supposed it to be pro- 

 pagated, not by the translation of material particles, but by the 

 vibrations of an ethereal medium *. 



The peculiar part of the theory of emission — the supposition 

 that the rays of light are bodies projected with a great velocity — 

 would seem to offer an easy criterion of its truth. If the weight 

 of a molecule of light amounted to one grain, its momentum 

 would equal that of a cannon ball 150 pounds in weight, and 

 moving with the velocity of 1000 feet in a second. The weight of 

 a single molecule maybe supposed many millions of times less than 

 this ; but, on the other hand, millions of such molecules maybe 

 made to act together, by concentrating them in the foci of lenses 

 or mirrors, and the effects of their impulse might, it was ex- 

 pected, be thus rendered sensible. This easy test of the materi- 

 ality of light was long since appealed to. The experiments of 

 Homberg seemed to have established the existence of a sensible 

 impulsive effect ; but when these experiments were repeated 

 with more caution by Mairan and Dufay, they conducted to the 

 opposite conclusion. The results obtained by Michell at a later 

 period, and with the aid of a more sensible apparatus than any 

 before employed, seemed to be decisive in favour of the materi- 

 ality of light f -. The effects observed in these experiments, 

 however^ have been with much probability referred to aerial 

 currents, produced by unequal temperature, or even to a differ- 

 ence in the elastic force of the air in contact with the opposite 

 surfaces of the body acted on J, The subsequent experiments 

 of Mr. Bennet were made under circumstances far more favour- 

 able ; and in particular, having been repeated in vacuum, they 

 ax*e independent of the sources of error now alluded to. Their 

 result was conclusive as to the non-existence of a sensible 

 effect §. 



The objection to the materiality of light, arising from its want 

 of sensible momentum, was first urged by Franklin. Horsley 

 attempted to remove the difficulty || ; but his laborious arithme- 

 tical calculations only go to prove that the particles of light, if 

 material, must be of extreme minuteness. It must at the same 

 time be confessed that objections of this nature are entitled to 



* Optics, Query 18. 



t ¥riest\ey'» Hisiuri/ of Oplics, p. 387. 



X Young " On the Theory of Light aad Colours, " Phil. Trans. ISOL 



§ Phil. Tram. 1792. ' || lb. 1770. L 



