SIX LECTURES ON LIGHT. 



upon black sealing-wax, we transfer the 

 grooves, and produce upon the wax the colors 

 of *nother-of-pearl. 



LECTURE III. 



Relation of Theories to Experience : Origin of the 

 Notion of the Attraction of Gravitation : Notion 

 of Polarity, how generated : Atomic Polarity : 

 Structural Arrangements due to Polarity : Archi- 

 tecture of Crystals considered as an 'Introduction to 

 their Action upon Light: Notion of Atomic Po- 

 larity applied to Crystalline Structure : Experi- 

 mental Illustrations: Crystallization of Water: 

 Expansion by Heat and by Cold : Deportment of 

 Water considered and explained: Molecular Ac- 

 tion illustrated by a Model: Force of Solidifica- 

 tion : Bearings of Crystallization on Optical Phe- 

 nomena : Refraction : Double Refraction : Po- 

 larization : Action of Tourmaline: Character of 

 the Beams emergent from Iceland Spar : Polariza- 

 tion by ordinary Refraction and Reflection : De- 

 polarization. 



IN our last lecture we sought to familiarize 

 Our minds with the characteristics of wave- 

 motion. We drew a clear distinction between 

 the motio i of the wave itself and the motion 

 of its constituent particles. Passing through 

 water-waves and air-waves, we prepared our 

 mi ds for the conception of light-waves prop- 

 agated through the luminiferous ether. The 

 analogy of sound will fix the whole mechan- 

 ism in your minds. Here we have a vibrat- 

 ing body which originates the wave motion, 

 we have, in the air, a vehicle which conveys 

 it, and we have the auditory nerve which re- 

 ceives the impressions of the sonorous waves. 

 In the case of light we have in the vibrating 

 atoms of the luminous body the originators of 

 the wave-motion, we have in the ether its 

 vehicle, while the optic nerve receives the im- 

 pression of the luminiferous waves. We 

 learned, also, that color is the analogue of 

 pitch, that the rapidity of atomic vibration 

 augmented, and the length of the ether-waves 

 decreased, in passing from the red to the blue 

 end of the spectrum. The fruitful principle 

 of interference we also found applicable to 

 the phenomena of light ; and we learned that, 

 in consequence of the different lengths of the 

 ether- waves, they were extinguished by dif- 

 ferent thicknesses of a transparent film, the 

 particular thickness which quenched one color 

 glowing, therefore, with the complementary 

 one. Thus the colors of thin plates were ac- 

 counted for. 



But one of the objects of our last lecture, 

 and that not the least important, was to illus- 

 trate the manner in which scientific theories 

 are iormed. They, in the first place, take 

 their rise in the desire of the mind to pene- 

 trate to the sources of phenomena. This de- 

 sire has long been a part of human nature. 

 It yrompted Caesar to say that he would ex- 

 change his victories for a rlirnpse of the 

 sources of the Nile ; it may be seen working 

 in Lucretius ; it impels Darwin to those dar- 

 ing speculations which of late years have so 

 agitated the public mind. We have learned ' 

 that in framing theories the imagination does 



not create, but that it expands, diminishes, 

 moulds, and refines, as the case may be, 

 mater als derived from the world of fact and 

 observation. 



This is more evidently the case in a theory 

 like that of light, where the motions of a sub- 

 sensible medium, the ether, are presented to 

 the mind. But no theory escapes the condi- 

 tion. Newton took care not to encumber 

 gravitation with unnecessary physical concep- 

 tions ; but we have reason to kno.v that he 

 indulged in them, though he did not connect 

 them with his theory. But even the theory 

 as it stands did not enter the mind ai' a reve- 

 lation dissevered from the world of experi- 

 ence. The germ of the conception that the 

 sun and planets are held together by a force 

 of attraction is to be found in the fact that a 

 magnet had been previously seen to attract 

 iron. The notion of matter attracting matter 

 came thus from without, not from within. In 

 our present lecture the magnetic force must 

 serve us till further ; but here we must master 

 its elementary phenomena. 



The general facts of magnetism are most 

 simply illustrated by a magnetized bar of 

 steel, commonly called a bar magnet. Placing 

 such a magnet uyrfurht upon a table, and 

 bringing a magne, ieedle near its bottom, 

 one end of the rtf"r*.>K promptly retreats from 

 the magnet, v"> the other as promptly 

 approaches. ' aeedle is held quivering 

 there by some ^risible influence exerted 

 upon it. Raising the needle along the mag- 

 net, but still avoiding contact the rapidity 

 of its oscillations decreases, because the force 

 acting upon it becomes weaker. At the 

 centre the oscillations cease. Above the 

 centre, the end of the needle which had been 

 previously drawn towards the magnet re- 

 treats, and the opposite end approaches. As 

 we ascend higher, the oscillations become 

 mere violent, because the force becomes 

 stronger. At the upper end of the magnet, 



FIG. 5. 



as at the lower, the force reaches a maximum, 

 t>ut all the lower half of the magnet, from 

 E to S (Fig. 5), attracts one end of the 



