10 PROPAGATION OF- LIGHT. 



It is only within a period comparatively recent, however, 

 that either of them has been stated formally, or supported 

 by any show of reasoning. Descartes put forward, very 

 distinctly, the hypothesis that light consisted of 4 small par- 

 ticles emitted by the luminous body ; and he even endea- 

 voured to explain the laws of reflexion and refraction on that 

 supposition. But as Newton was the first to deduce the 

 mathematical consequences of the theory of emission, he has 

 been usually regarded as its author. The wave-theory was 

 propounded by Hooke, in the year 1664 ; and was developed 

 into several of its consequences, a few years later, by Huy- 

 gens. Let us examine each of these theories by the only test 

 to which a physical theory can be subjected, namely, the 

 accordance of its consequences with phenomena. 



(13) The fundamental assumption of the theory of emis- 

 sion the hypothesis that light consists of bodies moving with 

 an immense velocity would appear to be easily submitted 

 to the test of experiment. If the weight of a molecule of 

 light amounted to but one grain, its momentum would equal 

 that of a cannon-ball, 150 pounds in weight, moving with 

 the velocity of 1000 feet in a second. The weight of a 

 single molecule may be assumed to be many millions of times 

 less than what has been here supposed ; but, on the other 

 hand, many millions of such molecules may be made to act 

 together, by concentrating them in the foci of lenses or mir- 

 rors, and the effects of their impulse might be expected in this 

 manner to be rendered evident. 



This apparently easy test of the materiality of light was 

 appealed to by many experimental philosophers of the last 

 century, and with various results. The effects observed have 

 been traced, with much probability, to extraneous causes, such 

 as aerial currents produced by unequal temperature ; and it is 

 now universally conceded that no sensible effect of the impulse 



