SIX LECTURES ON LIGHT. 



45 



astronomer, ' Bradley, who noticed that the 

 fixed stars did not really appear to be fixed, 

 but describe in the heavens every year a little 

 orbit resembling the earth's orbit. The re- 

 sult perplexed him, but Bradley had a mind 

 open to suggestion, and capable of seeing, in 

 the smallest fact, a picture of the largest. He 

 was one day upon ihe Thames in a boat, and 

 noticed that, as long as his course remained 

 unchanged, the vane upon his mast-head 

 showed the wind to be blowing constantly in 

 the same direction, but that the wind ap- 

 peared to vary with every change in the di- 

 rection of his boat. " Here," as Whewell 

 s.iy~, " was the image of his case. The boat 

 was the earth, moving in its orbit, and the 

 wind was the light of a star." 



We may ask in passing, what, without the 

 faculty which formed the ''image," would 

 Bradley's wind and vane have been to him ? 

 'A wind and vane, and nothing more. You 

 will immediately understand the meaning of 

 Bradley's discovery. Imagine yourself in a 

 motionless railway-train with a shower of rain 

 descending vertically downward. The mo- 

 ment the train begins to move, the rain-drops 

 begin to slant, and the quicker the train the 

 greater is the obliquity. In a precisely sim- 

 ilar manner the rays from a star vertically 

 overhead are caused to slant by the motion o'f 

 the earth through space. Knowing the 

 speed of the train, and the obliquity of the 

 falling rain, the velocity of the drops 

 may be calculated ; and knowing the speed 

 of the earth in her orbit, and the obliquity of 

 the rays due to this cause, we can calculate 

 just as easily the velocity of light. Bradley 

 did this, and the "aberration of light," as his 

 discovery is called, enabled him to assign to 

 it a velocity almost identical with that de- 

 duced by Roemer from a totally different 

 method of observation. Subsequently Fizeau, 

 employing not planetary or stellar distances, 

 but simply the breadth of the city of Paris, 

 determined the veloci y of light : while after 

 him Foucault a man of the rarest mechanical 

 genius solved the problem without quitting 

 his private room. 



Up to his demonstration of the composition 

 of white light, Newton had been everywhere 

 triumphant triumphant in the heavens, tri- 

 umphant on the earth, and his subsequent 

 experimental work is for the most part of 

 immortal value. But infallibility is not the 

 gift of man, and, soon after his discovery of 

 the nature of white light, Newton proved 

 himself human. He supposed that refraction 

 and dispersion went hand in hand, and that 

 you could not abolish the one without at the 

 same time abolishing the other. Here Dol- 

 land corrected him But Newton committed 

 a graver error than this. Science, as I 

 sought to make clear to you in our second 

 lecture, is only in part a thing of the senses. 

 The roots of phenomena are embedded in a j 

 region beyond the reach of the sen es, and \ 

 less than the root of the matter will never ' 



satisfy the scientific mind. We find, accord- 

 ingly, in this career of optics, the greatest 

 minds constantly yearning to pass from the 

 phenomena tD their causes to explore them 

 to their hidden roots. They thus entered 

 the region of theory, and here Newton, 

 though drawn from time to time towards the 

 truth, was drawn still more strongly towards 

 the error, and made it his substantial choice. 

 His experiments are imperishable, but his 

 theory has pa-sed away. For a century it 

 stood like a dam across the course of discov- 

 ery ; but, like all barriers that rest upon 

 authority, and no. upon truth, the pressure 

 from behind increased, and eventually swept 

 the barrier away. This, as you know, was 

 done mainly through the labors of Thomas 

 Young, and his illustrious French fellow- 

 worker Fresnel. 



In 1808, Malus, looking through Iceland 

 spar at the sun reflected from the window of 

 the Luxembourg Palace in Paris, discovered 

 the polarization of light by reliection. In 

 1811 Arago discovered the splendid chro- 

 matic phenomena which we have had illus- 

 trated by plates of gypsum i:: polarized ligtit ; 

 he also discovered the rotation of tne plane 

 of polarization by quartz-crystals. In 1813 

 Seebeck discovered the polarization of light 

 by tourmaline. That same year Brewscer 

 discovered those magnificent bane's of color 

 that surround the axes of biaxal crystals. In 

 1814 Wollaston discovered the rings of Ice- 

 land spar. All these effects, whkh, without 

 a theoretic clue, would leave the human 

 mind in a hopeless jungle of phenomena 

 without harmony or relation, were organically 

 connected by the theory of undulation. The 

 theory was applied and verified in all direc- 

 tions, Airy being especially conspicuous for 

 the severity and conclusiveness of his proofs. 

 The most remarkable verification fell to the 

 lot of the late Sir William Hamilton, of Dub- 

 lin, a profound mathematician, who, taking 

 up the theory where Fresnel had left it, ar- 

 rived at the conclusion that, at foar bpjjial 

 points at the surface of the ether-wave in 

 double-refracting crystals, the ray was divided 

 not into two parts, but into an infinite num- 

 ber of parts ; forming at these points a con- 

 tinuous conical envelope in3tead of two 

 images. No human eye had ever seen this 

 envelope when Sir William Hamilton inferred 

 its existence. Turning to nis fri nJ Dr. 

 Lloyd, he asked him to test experimentally 

 the truth of his theoretic conclusion. Lloyd, 

 taking a crystal of arragomtc, and following 

 with the most scrupulous exactness the indi- 

 cations of theory, cutting the crystal wheie 

 theory said it ought to be cut, observing it 

 ..here theory said it ought to be observed, 

 found the luminous envelope which had pre 

 viousiy been a mere idea in the mind of the; 

 mathematician. 



Nevertheless this great theory cf undula- 

 tion, like many another truth, which i i the 

 long-rua has proved a blessi ig ti humanity. 



