298 FOURTH RKl'ORT — 1834. 



of space to another in time, with a velocity of about 195,000 

 miles in a second. The inquiry concerning the mode of this 

 propagation involves that respecting the nature of light itself. 



There are tMO distinct and intelligible ways of conceiving such 

 a motion. Either it is the self-same body which is found at 

 different times in distant points of space ; or there are a mul- 

 titude of moving bodies, occupying the entire interval, each of 

 which vibrates continually within certain limits, while the vi- 

 bratoiy motion is communicated from one to another, and so 

 advances uniformly. Nature affords numerous examples of each 

 of these modes of propagated movement ; and in adopting one or 

 other to account for the phenomena of light, we fall upon one 

 or other of the two rival systems, — the theories of Newton and of 

 Huygens. 



The Newtonian theory, in the shape in which it is usually 

 presented, is undoubtedly simpler in conception than its rival ; 

 but this simplicity is only apparent. Newton himself was 

 far too clear-sighted to suppose that the forces of attraction 

 and repulsion, by which the molecules of light were supposed to 

 be refracted and reflected, were adequate to account for all the 

 phenomena ; and it is remarkable that, Avhen he proceeds to 

 speculate on the physical theory of light, he has fomid it neces- 

 sary to admit all the apparatus required in the theory of waves. 

 In fact, Newton felt, and distinctly stated, that the vibrations 

 of an ethereal medium were necessary in his hypothesis *, al- 

 though he denied that these vibrations constituted light. He 

 has even gone further, and asserted that they were the chief 

 and essential parts of that hypothesis, the molecules emitted 

 from luminous bodies only performing the office of exciting these 

 vibrations, as stones flung into water produce waves f. On the 

 other hand, the molecules themselves are supposed to be emitted 

 by a vibratory motion of the parts of the luminous body J ; — the 

 same vibratory movement, though acting with a different energy, 

 in which he supposes heat to consist. It would appear, then, 

 that Newton assumed too much, and that he erred against his 

 own valuable rule: " Caiisas rerumnaturaliumnonjilures ad- 

 mitti dehere," &c. Had he simply left out the molecular part of 

 his hypothesis, and supposed that the vibrations of his ethereal 



• Phil. Trans. 1672. 



f " Were I to assume an hypothesis, it should he this, if propounded more 

 generally, — so as not to determine what light is, further than that it is something 

 or other capable of exciting vibrations in the ether ; for thus it will become so 

 general, and comprehensive of other hypotheses, as to leave little room for new 

 ones to be invented." — Birch's History of the Royal Society, vol. iii. p. 249. 



J Optics, Query 8. 



