616 



ON THE THEORY OF LIGHT AND COLOURS. 



swiftness. Now, these vibrations, beside their 

 use in reflection and refraction, may be sup- 

 posed the chief means by which the parts of 

 fermenting or putrifying substances, fluid 

 liquors, or mehed, burning, or other hot 

 bodies, continue in motion." (Birch, III. 

 G51. Dec. 1675.) 



" When a ray of hght falls upon the sur- 

 face of any pellucid body, and is there re- 

 fracted or reflected, may not waves of vibra- 

 tions, or tremors, be thereby excited in the 

 refracting or reflecting mediuui ? — And are 

 not these vibrations propagated from the 

 point of incidence to great distances? And 

 do they not overtake the rays of light, and 

 by overtaking them successively, do they not 

 put thein into the fits of easy reflection and 

 easy transmission described above ? (0|)tics, 

 Qu. 17.) 



" Light is in fits of easy reflection and 

 easy transmission, before its incidence on 

 transparent bodies. And probably it is put 

 into such fits at its first emission from lu- 

 minous bodies, and continues in them dur- 

 ing all its progress." (Optics, Second Book, 

 Part iii. Prop. 13.) 



Hypothesis iii. The Sensation of differ- 

 ent Colours depends on the dijferent frequency 

 of Vibrations, excited bj/ Light in the Re- 

 tina. 



passages from NEWTON. 



" The objector's hypothesis, as to the fun- 

 damental part of it, is not against me. That 

 fundamental supposition is, that the parts of 

 bodies, when briskly agitated, do excite vi- 

 brations in the ether, which are propagated 

 every way from those bodies in straight lines, 

 and cause a sensation of ligiit,by beating and 

 dashing against the bottom of the eye, some- 

 thing after the manner that vibrations in the 



air cause a sensation of sound, by beating 

 against the organs of hearing. Now, the 

 most free and natural application of this hy- 

 pothesis to the solution of phenomena, I take 

 to be this : that the agitated parts of bodies, 

 according to their several sizes, figures, and 

 motions, do excite vibrations in the ether of 

 various depths or bignesses, which, being pro- 

 miscuously 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 bignesses be separated from one 

 another, the largest beget a sensation of a 

 red colour, the least or shortest of a deep 

 violet, and the intermediate ones of interme- 

 diate 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 a 

 sound : that the largest vibrations are best 

 able to overcome the resistance of a refract- 

 ing superficies, and so breakthrough it with, 

 least refraction ; whence the vibrations of se- 

 veral bignesses, that is, the rays of several co- 

 lours, which are blended together in light, 

 must be parted from one another by refrac- 

 tion, and so cause the phenomena of prisms, 

 and other refracting substances; and that it 

 depends on the thickness of a thin transpa- 

 rent plate or bubble, whether a vibration , 

 shall be reflected at its further superficies, or 

 transmitted ; so that, according to the num- 

 ber of vibrations, interceding the two super- 

 ficies, they may be reflected or transmitted 

 for nianv successive thicknesses. And, since 

 the vibrations which make blue and violet, 

 are supposed shorter than those which make 

 red and yellow, they must be reflected at a 

 less thickness of the plate: which is sufficient 

 to explicate all tlie ordinary phenomena of 



