506 



SCIENCE. 



[N. S. Vol. VIII. No. 193. 



"If at any time," he says, "I speak of 

 Light and Eays as coloured or endued with 

 Colours, I would be understood to speak not 

 philosophically and properly, but grossly, 

 and according to such Conceptions as vulgar 

 People in seeing all these Experiments 

 would be apt to frame. For the Rays to 

 speak properly are not coloured. In them 

 is nothing else than a certain Power and 

 Disposition to stir up a Sensation of this or 

 that Colour." 



" For as Sound in a Bell or musical 

 String or other sounding Body is nothing 

 but a trembling motion, and in the Air 

 nothing but that Motion propagated from 

 the Object, and in the Sensorium "tis a 

 Sense of that Motion under the form of 

 Sound ; so Colours in the Object are noth- 

 ing but a disposition to reflect this or that 

 sort of Rays more copiously than the rest ; 

 in the Rays they are nothing but their dis- 

 positions to propagate this or that motion 

 into the Sensorium, and in the Sensorium 

 they are Sensations of those Motions under 

 the forms of colours.'"* 



Again, with greater definiten ess, Newton 

 writes: "To explain colours, I suppose 

 that as bodies of various sizes, densities, or 

 sensations, do by percussion or other action 

 excite sounds of various tones, and conse- 

 quently vibrations in the air of different 

 bigness, so the rays of light, by impinging 

 on the stiff refracting superficies, excite vi- 

 brations in the ether * * * of various big- 

 ness ; the biggest, strongest, or most potent 

 rays, the largest vibrations ; and others 

 shorter, according to their bigness, strength, 

 or power ; and therefore the ends of the 

 capillamenta of the optic nerve, which pave 

 or face the retina, being such refracting su- 

 perficies, when the rays impinge upon them, 

 they must there excite these vibrations, 

 which vibrations (like those of sound in a 

 trunk or trumpet) will run along the aque- 

 ous pores or crystalline pith of the capil- 



*Opticks : 3d edition, 1721, p. 108. 



lamenta, through the optic nerve into the 

 sensorium ; and there, I suppose, affect the 

 sense with various colours, according to 

 their bigness and mixture ; the biggest with 

 the strongest colours, reds and yellows, the 

 least with the weakest, blues and violets ; 

 the middle with green ; and a confusion of 

 all with white, much after the manner that 

 in the sense of hearing, Nature makes use 

 of the aerial vibrations of several bignesses, 

 to generate sounds of divers tones ; for the 

 analogy of Nature is to be observed."* 



These passages are quoted— and several 

 others might be added — to show that New- 

 ton ascribes no peculiar function or activity 

 to the terminals of the optic nerve. They 

 are set in vibration by the rays of light ; 

 their vibrations are transmitted to the 

 higher regions of the sensorium, where 

 they become sensations of light and color. 

 As to the physical nature of the rays them- 

 selves, or the reason why they should ex- 

 cite in the nerve-fibres vibrations of differ- 

 ent length, Newton makes no guess. 



This is a definite theory of color-per- 

 ception, and, as Rutherford has pointed 

 out, one of great value, but presenting ob- 

 vious difliculties. 



Some of these dilBculties led Thomas 

 Young to that modified form of Newton's 

 views, which in the famous Bakerian 

 lecture, he describes as follows : " Since 

 * * * , it is probable that the motion of the 

 retina is rather of a vibratory than an undu- 

 latory nature, the frequency of the vibrations 

 must be dependent on the constitution of 

 this substance. Now, as it is almost im- 

 possible to conceive each sensitive point of 

 the retina to contain an infinite number of 

 particles, each capable of vibrating in per- 

 fect unison with every possible undulation, 

 it becomes necessary to suppose the number 

 limited, for instance, to the three principal 

 colours, red, yellow and blue * * * and that 

 each of the particles is capable of being put 



*Quoted by Yonng, Phil. Trans., 1802, p. 19. 



