touching his Theory of Light and Colours. 



205 



which the colours written there took up, every colour being 

 most briskly specific in the middle of those spaces. Now lor 



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the cause of these and such like colours made by refraction, 

 the biggest or strongest rays must penetrate the refracting 

 superficies more freely and easily than the weaker, and so be 

 less turned awry by it, that is less refracted ; which is as much 

 as to say, the rays which make red are least refrangible, those 

 which make blue, or violet, most refrangible, and others other- 

 wise refrangible according to their colour. Whence if the 

 rays which come promiscuously from the sun be refracted by 

 a prism, as in the aforesaid experiment, those of several sorts 

 being variously refracted, must go to several places on an op- 

 posite paper or wall, and so parted, exhibit every one their 

 own colours, which they could not do while blended together. 

 And because refraction only severs them, and changes not the 

 bigness or strength of the ray, thence it is, that after they are 

 once well-severed, refraction cannot make any further changes 

 in their colour. On this ground may all the phaenomena of 

 refractions be understood. But to explain the colours made 

 by reflexions, I must further suppose, that though light be 

 unimaginably swift, yet the aetherial vibrations excited by a 

 ray move faster than the ray itself, and so overtake and outrun 

 it, one after another. And this I suppose they will think an 

 allowable supposition, who have been inclined to suspect that 

 these vibrations themselves might be light. But to make it 

 the more allowable, it's possible light itself may not be so 

 swift as some are apt to think; for notwithstanding any argu- 

 ment that I know yet to the contrary, it may be an hour or 

 two, if not more, in moving from the sun to us. This celerity 

 of the vibrations therefore supposed, if light be incident on a 

 thin skin or plate of any transparent body, the waves excited 

 by its passage through the first superficies, and taking it one 

 after another till it arrive at the second superficies, will cause 

 it to be there reflected or refracted, accordingly as the con- 

 densed or expanded part of the wave overtakes it there. If 

 the plate be of such a thickness that the condensed part of the 

 first wave overtake the ray at the second superficies, it must 



