touching his Theory of Light and Colours. 211 



transmitted light, the glasses transmitting light of one colour 

 at the same place, where they reflect that of another. Nor 

 need I add anything further of the colours of other thinly 

 plated mediums, as of water between the aforesaid glasses, or 

 formed into bubbles and so encompassed with air, or of glass 

 blown into very thin bubbles at a lamp furnace, &c. ; the case 

 being the same in all these, excepting that where the thickness 

 of the plates is not regular, the rings will not be so, that in 

 plates of denser transparent bodies the rings are made at a less 

 thickness of the plate, (the vibrations, I suppose, being shorter 

 in rarer aether than in denser), and that in a denser plate sur- 

 rounded with a rarer body, the colours are more vivid than in 

 the rarer surrounded with the denser; as for instance, more 

 vivid in a plate of glass surrounded with air, than in a plate of 

 air surrounded with glass ; of which the reason is, that the 

 reflexion of the second superficies, which causes the colours, 

 is, as was said above, stronger in the former case than in the 

 latter; for which reason also the colours are most vivid when 

 the difference of the density of the medium is greatest. 



Of the colours of natural bodies also I have said enough in 

 those papers, showing how the various sizes of the transparent 

 particles of which they consist is sufficient to produce them all, 

 those particles reflecting or transmitting this or that sort of 

 rays, according to their thickness, like the aforesaid plates, as 

 if they were fragments thereof. For, I suppose, if a plate of 

 an even thickness, and consequently of an uniform colour, 

 were broken into fragments of the same thickness with the 

 plate, a heap of those fragments would be a powder much of 

 the same colour with the plates. And so, if the parts be of 

 the thickness of the water in the black spot at the top of a 

 bubble described in the seventeenth of the observations I send 

 you, I suppose the body must be black. Tn the production 

 of which blackness, I suppose, that the particles of that size 

 being disposed to reflect almost no light outward, but to re- 

 fract it continually in its passage from every part to the next, 

 by this multitude of refractions the rays are kept so long 

 straggling to and fro within the body, till at last almost all 

 impinge on the solid parts of the body, and so are stopped 

 and stifled ; those parts having no sufficient elasticity, or other 

 disposition to return nimbly enough the smart shock of the 

 ray back upon it. 



I should here conclude; but that there is another strange 

 phenomenon of colours, which may deserve to be taken no- 

 tice of. Mr. Hook, you may remember, was speaking of an 

 odd straying of light, caused in its passage near the edge of a 

 razor, knife, or other opake body in a dark room ; the rays, 



