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On Sal Nitnim ana Nitro- Aerial Spirit 143 



of finer matter compacted with greater continuity than 

 the medium by which the rays of Hght are transmitted. 

 For we must suppose a very subtle and ethereal matter 

 which, interwoven with the nitro-aerial particles, oc- 

 cupies and fills up all the interstices of the atmosphere, 

 and that by its impulse visible forms are propagated. 

 In fact, the structure of the eye is so tender and 

 deHcate that it is able to perceive those very gentle 

 blows of ethereal matter, imperceptible to the other 

 senses, and besides to distinguish the variation in its 

 pressure and rhythm on which vision and colour depend. 

 And indeed it seems to be owing to the extreme 

 tenuity of the medium of vision that the image of a 

 lamp is conveyed to such a great distance without any 

 delay. For the visible form of a luminous body is 

 transmitted far beyond the limit of its light, and 

 probably with a swifter motion than the rays of light 

 are transmitted through the air. 



That the image of an illuminated body is trans- 

 mitted by the impulse of this special medium, and by 

 a motion different from the action of light, I have en- 

 deavoured to show above. Let us therefore now 

 consider next, how it is that the medium of vision is 

 struck. On this point I was for some time in a 

 difficulty as to whether it is by the impulse of the 

 illuminated body or of the luminous particles that the 

 medium of vision is affected. For I think we must 

 maintain either that the luminous particles imping- 

 ing on the surface of the illuminated body, impress 

 such an impulse upon it as is fitted, when conveyed 

 through the medium of vision and by its instru- 

 mentality to the eye, for presenting an image of 

 the illuminated body, or (as seems to me more prob- 

 able) that the nitro-aerial and luminous particles 

 falling upon the illuminated body, acquire themselves 



