I 



On Sal Nitriim and Nitro- Aerial Spirit 145 



with the swiftest motion, and in densest array, strike 

 with vivid action the very subtle medium of vision in 

 individual points, with no interruption, and by this, 

 transmitted to the eye, the perception of light is 

 caused. Moreover the variation of this glittering 

 whiteness results from the interruption of the impulse 

 of the very subtle medium at certain points by opaque 

 bodies interspersed in the medium. 



The white colour, which most nearly approaches 

 glittering white in brightness, does not seem to 

 depend, as is commonly supposed, on an extremely 

 dense reflection of the rays of light. For, as I have 

 endeavoured to show, all colours are different from 

 the action of light. 



It seems, therefore, that we should maintain that 

 the white colour arises from this, that the luminous 

 particles impinging on the illuminated surface, en- 

 counter in its very many points such a resistance that 

 these particles are, in consequence, excited to some 

 new tremulous motion with very frequent vibrations, 

 as we have just shown. And by these very 

 numerous particles, made to vibrate in this manner, 

 the medium of vision is struck very frequently and at 

 very many points with an impulse different from 

 light ; and in this way the white colour seems to be 

 produced. And hence, since a white surface has 

 usually many very minute excrescences distributed 

 thickly over it — not that these reflect the rays of 

 light (for rays of light are scarcely reflected at all 

 from a white surface such as paper, but on the 

 contrary the impulse of light is quite destroyed by it, 

 as was shown above) but so far as the very numerous 

 luminous particles falling on these molecules acquire a 

 certain new vibration by which the very subtle medium 

 of vision is struck in very many places — the white 



K 



