touching his Theory of Light and Colours. 193 



a much greater height than the surface of the stagnating wa- 

 ter into which they are dipped*. So I suppose aether, though 

 it pervades the pores of crystal, glass, water, and other na- 

 tural bodies, yet it stands at a greater degree of rarity in those 

 pores than in the free aetherial spaces, and at so much greater 

 a degree of rarity as the pores of the body are smaller. 

 Whence it may be that spirit of wine, for instance, though a 

 lighter body, yet having subtiler parts, and consequently 

 smaller pores than water, is the more strongly refracting 

 liquor. This also may be the principal cause of the cohesion 

 of the parts of solids and fluids, of the springiness of glass 

 and other bodies whose parts slide not one upon another in 

 bending, and of the standing of the mercury in the Torricel- 

 lian experiment, sometimes to the top of the glass, though a 

 much greater height than twenty-nine inches. For the denser 

 aether which surrounds these bodies must crowd and press 

 their parts together, much after the manner that air surround- 

 ing two marbles presses them together if there be little or no 

 air between them. Yea, and that puzzling problem, by what 

 means the muscles are contracted and dilated to cause animal 

 motion, may receive greater light from hence than from any other 

 means men have hitherto been thinking on. For if there be any 

 power in man to condense and dilate at will the aether that 

 pervades the muscle, that condensation or dilatation must vary 

 the compression of the muscle made by the ambient aether, 

 and cause it to swell, or shrink, accordingly ; for though 



* The imprinted paper of" Observations" which accompanied this, and 

 which was the first form of the "Optics," contained a passage on the pre- 

 sent subject, omitted in that publication, which deserves to be quoted for 

 the acknowledgment it contains of the merit of Hook. In the third prop, 

 of the second book, Newton remarks : — " To the increase of the opacity of 

 these bodies it conduces something, that by the 23rd observation, the re- 

 flexion of thin "transparent substances are considerably stronger than those 

 made by the same substances of a greater thickness." Here the paper sub- 

 joins, — "And to the reflexion of solid bodies it may be further added, that 

 the interstices of their parts are void of air. For that for the most part 

 they are so is reasonable to believe, considering the inaptitude which air 

 hath to pervade small cavities, as appears by the ascension of water in slen- 

 der glass pipes, paper, cloth, and other such like substances, whose pores 

 are found too small to be replenished with air, and yet large enough to 

 admit water, and by the difficulty wherewith air pervades the pores of a 

 bladder, through which water finds ready passage. And according to the 

 Uth observation, the cavities thus void of air will cause the same kind of 

 effect, as to reflexion, which those do that are replenished with it; but yet 

 something more manifestly, because the medium in relation to refractions 

 is rarest when most empty of air, as Mr. Hook hath proved in his Mico- 

 graphia, in which book he hath also largely discoursed of this and the pre- 

 cedent proposition, and delivered many other very excellent things con- 

 cerning the colour of thin plates, and other natural bodies, which I have 

 not scrupled to make use of as far as they were for my purpose." 



