188 Newton's Letters, Hypothesis and Experiments 



occasion to say something of hypotheses where I gave a 

 reason why all allowable hypotheses in their genuine constitu- 

 tion should be conformable to my theories, and said of Mr. 

 Hook's hypothesis, that I took the most free and natural ap- 

 plication of it tophaenomena to be this: — "That the agitated 

 parts of bodies, according to their several sizes, figure and 

 motions, do excite vibrations in the aether of various depths 

 or bignesses, which being promiscuously propagated through 

 that medium to our eyes, effect in us a sensation of light of 

 a white colour; but if by any means those of unequal big- 

 nesses be separated from one another, the largest beget a sen- 

 sation of a red colour, the least or shortest of a deep violet, 

 and the intermediate ones of intermediate colours, much after 

 the manner that bodies, according to their several sizes, shapes, 

 and motions, excite vibrations in the air of various bignesses, 

 which according to those bignesses make several tones in 

 sound, &c.*" I was glad to understand, as I apprehended 

 from Mr. Hook's discourse at my last being at one of your 

 assemblies, that he had changed his former notion of all co- 

 lours being compounded of only two original ones, made by 

 the two sides of an oblique pulse, and accommodated his 

 hypothesis to this my suggestion of colours, like sounds, 

 being various, according to the various bigness of the pulses. 

 For this I take to be a more plausible hypothesis than any 

 other described by former authors ; because I see not how the 

 colours of thin transparent plates, or skins, can be hand- 

 somely explained without having recourse to aetherial pulses. 

 But yet I like another hypothesis better, which I had occasion 

 to hint something of in the same letter in these words : — " The 

 hypothesis of light's being a body, had I propounded it, has a 

 much greater affinity with the objector's own hypothesis than 

 he seems to be aware of, the vibrations of the aether being as 

 useful and necessary in this as in his. For assuming the rays 

 of light to be small bodies emitted every way from shining 

 substances, those, when they impinge on any refracting or re- 

 flecting superficies, must as necessarily excite vibrations in 

 the aether as stones do in water when thrown into it. And 

 supposing these vibrations to be of several depths or thick- 

 nesses, accordingly as they are excited by the said corpuscular 

 rays of various sizes and velocities, of what use they will be 

 for explicating the manner of reflexion and refraction, the 

 production of heat by the sun beams, the emission of light 

 from burning, putrifying, or other substances whose parts are 

 vehemently agitated, the phaenomena of thin transparent 

 plates and bubbles, and of all natural bodies, the manner of 

 * Birch's History of the Royal Society, vol. iii. p. 248. 



