24 REPORT ON PHYSICAL OPTICS. 



obtained by Michell at a later period, and with the aid of a more 

 sensitive apparatus than any before employed, seemed to be de- 

 cisive in favour of the materiality of light.* The effects observed 

 in these experiments, however, have been with much probability 

 referred to aerial currents, produced by unequal temperature, or 

 even to a difference in the elastic force of the air in contact with 

 the opposite surfaces of the body acted on. f The subsequent ex- 

 periments of Mr. Bennett were made under circumstances far more 

 favourable ; and in particular, having been repeated in a vacuum, 

 they are 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 at- 

 tempted to remove the difficulty ; but his laborious arithmetical 

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

 terial, must be of extreme minuteness. It must at the same time 

 be confessed that objections of this nature are entitled to little 

 weight. It is easy to attribute to the molecules of light a minute- 

 ness sufficient to evade any means that we possess of detecting 

 their inertia by their effects upon other bodies; and in whatever 

 point of view we regard the phenomena of optics, we are forced to 

 contemplate quantities immeasurably smaller than any to which the 

 imagination has been accustomed. 



The aberration of the light of the fixed stars, resulting from the 

 motion of the earth and that of light, is an easy consequence of 

 the theory of emission, in which these motions are conceived to 

 subsist independently. In order to account for the phenomenon 

 in the theory of waves, it seems necessary to assume that the ether 

 which encompasses our globe does not participate in its motion ; so 

 that the ethereal current produced by this relative motion must be 

 supposed to 'have a free passage through the solid mass of the 

 earth; or that, in the words of Young, "the luminiferous ether 

 pervades the substance of all material bodies with little or no re- 

 sistance, as freely perhaps as the wind passes through a grove of 

 trees." || Fresnel has maintained the same opinion, and, startling 



* Priestley's History of Optics, p. 387. 



t Young " On the Theory of Light and Colours," Phil. Trans. 1801. 



t Phil. Tram. 1792. 



I Ibid. 1770. I. 



II " Experiments and Calculations relative to Physical Optics," Phil. Trans. 1803. 



