im*. in.] BISSERTATION SECOND. 81 



stance enters into the composition of that fluid, — a conclu- 

 sion which has heen confirmed by one of the most certain but 

 most unexpected results of chemical analysis. The views 

 thus suggested by Newton have been successfully pursued by 

 future inquirers, and the action of bodies on light is now 

 regarded as one of the means of examining into their in- 

 ternal constitution. 



I should have before remarked, that the alternate dispo- 

 sition to be easily reflected and easily transmitted, serves to 

 explain the fact, that all transparent substances reflect a por- 

 tion of the incident light. The reflection of light from the 

 surfaces of opaque bodies, and from the anterior surfaces of 

 transparent bodies, appears to be produced by a repulsive 

 force exerted by those surfaces at a determinate but very 

 small distance, in consequence of which there is stretched 

 out over them an elastic web, through which the particles 

 of light, notwithstanding their incredible velocity, are not 

 always able to penetrate. 1 " In the case of a transparent 

 body, the light which, when it arrives at this outwork, as 

 it may be called, is in a Jit of easy reflection, obeys of 

 course the repulsive force, and is reflected back again. 

 The particles, on the other hand, which are in the state 

 which disposes them to be transmitted, overcome the repul- 

 sive force, and, entering into the. interior of the transpa- 

 rent body, are subjected to the action of its attractive 

 force, and obey the law of refraction already explained. 

 If these rays, however, reach the second surface of the trans- 

 parent body (that body being suppposed denser than the 

 medium surrounding it), in a direction having a certain obli- 

 quity to that surface, the attraction will not suffer the rays 



1 A velocity that enables light to pass from the sun to the 

 earth in 8' 13", as is deduced from the eclipses of Jupiter's 

 satellites. 



11 



