SIX LECTURES ON LIGHT. 



state of things likely to last. In America 

 there is a willingness ou the part of individu- 

 als to devote their fortunes, in the matter of 

 education, to the service of the common- 

 wealth, which is without a parallel elsewhere; 

 and this willingness requires but wise di- 

 rection to enable you effectually to wipe away 

 the reproach of De Tocqu^ville. 



Your most difficult problem will be no 1 " to 

 build institutions, but to make men ; not to 

 form the body, but to find tne spiritual em- 

 bers which, shall kindle within that body 

 a living soul. You have scientific genius 

 among you ; not sown broadcast, believe 

 me, but still ' cattered here and there. Take 

 a'l r.nnecessary impediments out of its way. 

 Drawn by your kindness I have come here to 

 give these lectures, and, now that my visit to 

 America has become almost a thing of the 

 past, I look back upon it ns a memory with- 

 out a stain. No lecturer was ever rewarded 



as I have been. From this vantage-ground, 

 howtver, let me remind you that the work of 

 the lecturer is not the highest work ; that in 

 science, the lecturer is usually the distributor 

 of intellectual wealth amassed by better men. 

 It is not solely, or even chiefly, as lecturers, 

 but as investigators, that your men of genius 

 ought to be employed. Keep your sympa- 

 thetic eye upon the originator of knowledge. 

 Give him the freedom necessary for his re- 

 searches, not overloading him either with the 

 duties of tu tion or of administration, not de- 

 manding from him so-called practical results 

 above all things, avoiding that question 

 which ignorance so often addresses to genius, 

 " What is the use of your work ? " Let him 

 make truth his object, however unpractical 

 for the time being that truth may appear. 

 If you cast your bread thus upon the waters, 

 then be assured it will return to you, though 

 it may be after many days. 



CONTENTS. 



Xscr. I. INTRODUCTORY 2 



II. Origin of Physical Theories ... 8 



III Relation of Theories to Experience . 18 

 IV. Chromatic Phenomena produced by 



Crystals 27 



LECT. V. Range of Vision and Range of Ra- 

 diation 34 



VI. Spectrum Analysis. ...... 45 



