44. 



SIX LECTURES ON LIGHT. 



the sea-level bf the intellect to his amazing 

 elevation. At the time that he appeared, the 

 table-land of knowledge was already high. 

 He juts, it is true, above the table-land, as a 

 massive peak; still he is supported by it, and 

 a great part of his absolute height was the 

 height of humanity in his time. It is thus 



'rn thediscowies of Kirchhoff. Much had 

 beta previously accomplished; this he mas- 

 tered, and the*: by the force of individual 

 genius went beyond it. He replaced uncer- 

 tainty by certaiuty, vagueness by definite- 

 ness, confusion Ly oxler; and I do not think 

 that Newton has a s:i er claim to the discov- 

 ciies that have made h';s name immortal than 

 Kirchhoff has to the credit of gathering up the 

 fragmentary knowledge of his time, of vastly 

 extendir it, and of infusing into it the life 

 of great principles. Splendid results have 

 since been obtained with whk ii the names of 

 Janssen, Muggins, I,ocl:>?r, Respighi, 

 Young, and others, are honorably associated, 

 but, splendid as they are, they are but the 

 sequel and application of the principles es- 

 tablished in his Heidelberg laboratory by the 

 celebrated German investigator. 



SUMMARY AND CONCLUSION 



My desire in these lectures hz?s bt a .n to 

 show you, with as little breacn of continuity 

 as possible, the past growth and p/esent as- 

 pect of a department cf science, ;'n which 

 have labored some of th^ gre; test i itellccts 

 the world has ever seen. My f "lend Profes- 

 sor Henry, in introducing me at Washington, 

 spoke of me as an apostle; but theonh apos- 

 tolate that I intended to fulfil was to place, 



in plain words, my subject before you, and to 

 permit its own intrinsic attrsctions to act up- 

 on your minds. In the \vayof experiment, I 

 have tried to give you the best which, under 

 the circumstances, could be provided; but I 

 have sought to confer on each experiment a 

 distinct intellectual vaLie, for experiments 

 ought to be the representatives and exposi- 

 tors of thought a language addressed to the 

 eye as spoken words are to the ear. In as- 

 sociation with its context, nothing is more 

 impressive or instructive than a fit experi- 

 ment; but, apart from its context, it rather 

 suits the conjuror's purpose of surprise than 

 that purpose of education which ought to be 

 the ruling motive of the scientific man. 



And now a brief summary of our work 

 will not be out of place. Our present mas- 

 tery over the laws and phenomena of light 

 has its origin in the desire of man to know. 

 We have seen the ancients busy whh this 

 problem, but, like a child who uses his aims 

 aimlessly for want of the necessarv muscular 

 exercise, so these early men speculated vagut- 

 ly and confusedly regarding light, not having 

 as yet the discipline needed to give clearness 

 to their insight, and firmness to their grasp 

 of principles. They assured themselves of 

 th~ rectilineal propogation of light, and that 

 the angle of incidence was equal to the angle 

 of reflection. For more than a thousand 

 years I might say, indeed, for more than 

 ritteen hundred years subsequently the 

 scientific intellect appears as if smitten with 

 paralysis, the fact being that, during this 

 time, the mental force, which might have run 

 in the direction of science, was diverted in- 

 ;o other directions. 



The course of investigation as regards 

 light was resumed in 1100 by an Arabian 

 philosopher named Alhazan. Then it was 

 taken up in succession by Roger Bacon, Vi- 

 tellio, and Kepler. These men, though fail- 

 ing to detect the principle which ru.ed the 

 facts, kept the fire of investigation constantly 

 burning. Then came the fundamental dis- 

 covery of Snell, that corner-stone of optics, 

 as I have already called it, and immediately 

 afterward we have the application by Des- 

 cartes of Snell's discovery to the expianaticn 

 of the rainbow. Then came Newton's 

 crowning experiments on the analysis and 

 i synthesis of white light, by which it was 

 proved to be compounded of various kinds 

 of light of different degrees of refrangibiiity. 



In 1676 an impulse was given to optics by 

 astronomy. In that year Olaf Roemer, a 

 learned Dane, was engaged at the Observa- 

 tory of Paris in observing the eclipses of Ju- 

 piter's moons. He converted them into so 

 many signal-lamps, quenched when they 

 plunged into the shadow of the planet, and 

 relighted when they emerged from the shadow. 

 They enabled him to prove that light 

 requires time to pass through space, and to 

 assign to it the astounding velocity of 190,- 

 ooo miles a second. Then came the English 



