ON THE NATURE OF LIGHT AfrD COLOURS. 3G1 



A very striking circumstance, respecting the propagation of light, is the 

 uniformity of its velocity in the same medium. According to the projec- 

 tile hypothesis, the force employed in the free emission of light must be 

 about a million million times as great as the force of gravity at the earth's 

 surface ; and it must either act with equal intensity on all the particles of 

 light, or must impel some of them through a greater space than others, 

 if its action be less powerful, since the velocity is the same in all cases ; 

 for example, if the projectile force is weaker with respect to red light than 

 with respect to violet light, it must continue its action on the red rays to a 

 greater distance than on the violet rays. There is no instance in nature 

 besides of a simple projectile moving with a velocity uniform in all cases, 

 whateve^iiir 1 ^be its cause, and it is extremely difficult to imagine that so 

 immense a force of repulsion can reside in all substances capable of 

 becoming luminous, so that the light of decaying wood, or of two pebbles 

 rubbed together, may be projected precisely with the same velocity as the 

 light emitted by iron burning in oxygen gas, or by the reservoir of liquid 

 fire on the surface of the sun. Another cause would also naturally inter- 

 fere with the uniformity of the velocity of light, if it consisted merely in 

 the motion of projected corpuscles of matter ; Mr. Laplace has calculated,* 

 that if any of the stars were 250 times as great in diameter as the sun, 

 its attraction would be so strong as to destroy the whole momentum of the 

 corpuscles of light proceeding from it, and to render the star invisible at a 

 great distance ; and although there is no reason to imagine that any of the 

 stars are actually of this magnitude, yet some of them are probably many 

 times greater than our sun, and therefore large enough to produce such a 

 retardation in the motion of their light as would materially alter its effects. 

 It is almost unnecessary to observe that the uniformity of the velocity of 

 light, in those spaces which are free from all material substances, is a 

 necessary consequence of the Huygenian hypothesis, since the undulations 

 of every homogeneous elastic medium are always propagated, like those 

 of sound, with the same velocity, as long as the medium remains un- 

 altered. 



On either supposition, there is no difficulty in explaining the equality of 

 the angles of incidence and reflection ; for these angles are equal as well 

 in the collision of common elastic bodies with others incomparably larger, 

 as in the reflections of the waves of water and of the undulations of sound. 

 And it is equally easy to demonstrate, that the sines of the angles of inci- 

 dence and refraction must be always in the same proportion at the same 

 surface, whether it be supposed to possess an attractive force, capable of 

 acting on the particles of light, or to be the limit of a medium through 

 which the undulations are propagated with a diminished velocity. There 

 are, however, some casfe of the production of colours, which lead iis to 

 suppose that the velocity of light must be smaller in a denser than in a 

 rarer medium ; and supposing this fact to be fully established, the exist- 

 ence of such an attractive force could no longer be allowed, nor could the 

 System of emanation be maintained by any one.f 



* Zachs Geographische Ephemeriden, iv. 1. 



f Arago put this remark to the test, Annales de Chimie, Ixxi. 49. 



