300 PouuTii REPOur — 1834. 



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

 minuteness sufficient to evade any means that we possess of 

 detecting their inertia by their effects upon otiier bodies', and 

 in whatever point of view we regard tlie 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 tlie earth and that of light, is an easy consequence 

 of the theory of emission, in which these motions are con- 

 ceived 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 ; s-o that the ethereal current produced by this re- 

 lative 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 resistance, as freely perhaps as the wind passes 

 through a gi-ove of trees*. Fresnel has maintained the same opi- 

 nion, and, startling as tlie position seems at first, he has very 

 clearly shown that no fair argument can be advanced against it, 

 founded on the opacity of the mass which the ether is supposed 

 to permeate f. 



The discoveries of Bradley and Roemer, when compared to- 

 gether, have led to a further and most important conclusion re- 

 specting light, — namely, that its velocity is one and the same, 

 whatever be the luminous origin ; the light of the sun, the fixed 

 stars, the planets and their satellites, being all propagated with 

 the same swiftness. This conclusion must be allowed to present 

 a formidable difficulty in the theory of emission. Laplace has 

 shown that if the diameter of a fixed star were 250 times as great 

 as that of our sun, its density being the same, its attraction 

 would be sufficient to desti'oy the whole momentum of the 

 emitted molecules, and the star would be invisible at great di- 

 stances %. With a smaller mass there will be a corresponding 

 retardation ; so that the final velocities will be different, what- 

 ever be the initial. The suggestion of M. Arago seems to 

 offer the only means of avoiding this difficvilty. It may be sup- 

 posed that the molecules of light are originally projected with 

 very diflerent velocities ; but that among these velocities there 

 is but one which is adapted to our organs of vision, and which 



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

 J 803. 



t " Sur rinfluence du Mouvument terrestrc dans quelques Phenomenes 

 d'Optique," Annales de C/iimie, torn. ix. 



t Zach, Ephem., iv. 1. 



