PROPAGATION OF LIGHT PRINCIPLE OF INTERFERENCE.' 23 



tions 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 pro- 

 duce waves.* On the other hand, the molecules themselves are 

 supposed to be emitted by a vibratory motion of the parts of the 

 luminous body f 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 " Causas rerum naturalium 

 -non plut'cs admitti debere" &c. Had he simply left out the mole- 

 cular part of his hypothesis, and supposed that the vibrations of 

 his ethereal 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 sub- 

 ject, 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 may be supposed many millions of times less than this ; 

 but, on the other hand, millions of such molecules may be made to 

 act together, by concentrating them in the foci of lenses or mirrors, 

 and the effects of their impulse might, it was expected, be thus 

 rendered sensible. This easy test of the materiality 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 

 l)ufay, they conducted to the opposite conclusion. The results 



* " Were I to assume an hypothesis, it should be this, if propounded more gene-- 

 rallyso 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 compre- 

 hensive of other hypotheses, as to leave little room for new ones to be invented." 

 Hirch's II i*tr>/ <>f the I{ ;/1 .SV /<///, vol. Hi. p. 249. 



f Opt ',<*, Query S. ; Oy,//V.s, (Jui-ry IS. 



