SIX LECTURES ON LIGHT. 



both m the ranks of science and out of them, of elastic collision. The fact of optical re- 



uncomfortable. That men in the ranks of 

 science should feel thus is, I think, a proof 



flection certainly occurred as if light consist- 

 ed of elastic particles, and this was Newton's 



that they have suffered themselves to be mis- ( sole justification for introducing them, 

 led by the popular definition of a great ( But this is not all. In another important 

 faculty instead of observing its operation in | particular, also. Newton's conceptions re- 

 their own minds. Without imagination we garding the nature of light were influenced 



by his previous knowledge. He had been 

 working at the phenomena of eravitation. 

 and had made himself at home amid the oper- 

 ations of this universal power. Perhaps hrs 

 mind at this time was too freshly and too 

 deeply imbued with these notions to permit 

 of his forming an unfettered judgment re- 



imagmation 



cannot take a step beyond the bourne of the 

 mere animal world, perhaps not even to the 

 edge of this. But, in speaking thus of 

 imagination, I do not mean a riotous power 

 which deals capriciously with facts, but a 

 well-ordered and disciplined power, whose 

 sole function is to form conceptions which 

 the intellect imperatively demands. Imagina- 

 tion thus exercised never really severs itself 

 from the world of fact. This is the store- 

 house from which all its pictures are drawn ; 

 and the magic of its art consists, not in 

 creating things anew, but in so changing the 

 magnitude, position, and other relations of 

 sensible things, as to render them fit for the 

 Requirements of the intellect in the subsen- 

 uible world.* 



I will take, as an illustration of this sub- 

 ject, the case of Newton. Before he began 

 to deal with light, he was intimately ac- 

 quainted with the laws of elastic collision, 

 which all of you have seen more or less per- 

 fectly illustrated on a billiard-table. As re- 

 gards the collision of sens : ble masses, New- 

 ton knew the angle of incidence to be equal 

 to the angle of reflection, and he also knew 

 that experiment, as shown in our last lecture, 

 had established the same law with regard to 

 light. He thus found in his previous knowl- 

 edge the mateiial for theoretic images. He 

 had only to change the magnitude of concep- 

 tions already in his mind to arrive at the 

 Emission Theory of Light. He supposed 

 light to consist of elastic particles of incon- 

 ceivable minuteness shot out with inconceiv- 

 able rapidity by luminous bodies. Such par- 

 ticles impinging upon smooth surfaces were 

 reflected in accordance with the ordinary law 



* The following charming extract, bearing upon 

 this point, was discovered and written out for me by 

 my friend, Dr. Bence Jones, Hon. Secretary to the 

 Royal Institution 



" In every kind of magnitude there is a degree or 

 'sort to which our sense is proportioned, the percep- 

 tion and knowledge cf which is of the greatest use to 

 mankind. The same is the groundwork of philoso- 

 phy : for, though all sorts and degrees are equally 

 the object of philosophical speculation, yet it is from 

 those which are proportioned to sense that a philoso- 

 pher must set out in his inquiries, ascending or de- 

 scending afterwards, as his pursuits may require. He 



garding the nature of light. Be that 

 as it may, Newton saw in refraction 

 the action of an attractive force exerted 

 on the light-particles. He carried his 

 conception out with the most severe con- 

 sistency. Dropping vertically downwards 

 towards the earth's surface, the motion of a 

 body is accelerated as it approaches the earth. 

 Dropping in the same manner downwards on 

 a horizontal surface, say through air on glass 

 or water, the velocity of the light-particles, 

 when they come close to th2 surface, was, 

 according to Newton, also accelerated. Ap- 

 proaching such a surface obliquely, he sup- 

 posed the particles, when close to it, to be 

 drawn down upon it, as a projectile is 

 drawn by gravity to the surface of the earth. 

 This deflection was, according to Newton, 

 the refraction seen in our last lecture. Final- 

 ly, it was supposed that differences of color 

 might be due to differences in the sizes of the 

 particles. This was the physical theory of 

 light enunciated and defended by Newton; 

 and you will observe that it simply consists 

 in the transference of conceptions born in the 

 world of the senses to a substnsible world. 



But, though the region of physical theory 

 lies thus behind the world of senses, the veri- 

 fications of theory occur in that world. Lay- 

 ing the theoretic conception at the root of 

 matters, we determine by rigid deduction 

 what are the phenomena which must of neces- 

 sity grow out of this root. If the phenomena 

 thus deduced agree with those of the actual 

 world, it is a presumption in favor of the 

 theory. If as new classes of phenomena arise 

 they also are found to harmonize with theo- 

 retic deduction, the presumption becomes still 

 stronger. If, finally, the theory confers pro- 

 phetic vision upon the investigator, enabling 

 him to predict the existence of phenomena 

 which have never yet been seen, and if those 



does well indeed to take his views from many points | predictions be found on trial to be rigidly cor- 

 rect, the persuasion of the truth of the theory 

 becomes overpowering. Thus working back- 

 wards from a limited number of phenomena, 

 genius, by its own expansive force, reaches a 

 conception which covers all the phenomena. 

 There is no more wonderful performance of 

 the intellect than this. And we can render 

 no account of ii. Like the scriptural gift of 

 the Spirit, no man can tell whence it cometh. 

 The passage from fact to principle is some- 



of sight, and supply the defects of sense by a well- 

 regulated imagination ; nor is he to be confined by 

 any limit in space or time ; but, as his knowledge of 

 Nature is founded on the observation of sensible 

 things, he must begin with these, and must often re- 

 turn to them to examine his progress by them. 

 Here is his secure hold ; and as he sets out from 

 thence, so if he likewise trace not often his steps 

 backwards with caution, he will be m hazard of losing 

 ki way in the labyrinths of Nature." (Maclaurin : 

 /in Account of Sir I. Neivtoris Philosophical Dis- 

 coveries. Written 1728 ; sttcond edition, 1750 ; pp. 



