On Sal Nitrtim and Nitro- Aerial Spirit 141 



some rays of light, in so far as it is polished in some 

 parts and fitted to reflect the rays of light. But it is 

 scarcely credible that the rays of light are reflected 

 from a white surface in such abundance as is requisite 

 for producing a white colour all round. But the reason 

 why the said plane cannot be seen if it is highly 

 polished is to be looked for in what is said below. • 



Further, it appears that the luminous body itself is 

 not seen by the rays of its own light, whether direct or 

 reflected. For a small lamp burning in a high place 

 can be seen clearly enough several miles off, far be- 

 yond the bounds of its illumination, by an eye in a 

 dark place. But this would not be if an image of it 

 were produced only by rays of its own light. 



But to submit my own view on this question, I think 

 it is to be held as at least a probable conjecture that 

 the image of a small lamp, seen beyond the limits of 

 its illumination, is propagated by a peculiar impulse 

 which is quite different from the impulse of light. 

 Indeed igneo-nitrous particles when violently agitated 

 in a luminous body, in so far as they strike and move 

 other nitro-aerial particles kindred with themselves 

 and these in their turn strike and move others, con- 

 stitute the action of light, as I tried to show above ; 

 but in so far as these igneous particles impel a peculiar 

 medium, distinct from the luminous medium, and im- 

 press upon it a sort of special undulation or impulse, 

 they seem to diff'use in all directions a visible image 

 of themselves. For it is probable that particles, igneous 

 and luminous per se^ and with their own motion, 

 do not affect so much the sense of sight as of touch. 

 For fiery particles cause very great pain, and the eye 

 when exposed to too fierce a light is injured as if it 

 were struck by some blow. Further, the solar rays 

 strike not only the eye but also the nostrils exposed 



