SIX LECTURES ON LIGHT. 



when 1 reached this city. They have been 

 begun, continued, and ended in New York, 

 and bear bniy too evident marks of the rapid- 

 ity of their production. I thought it, how- 

 ever, due. bo;h to those who heard them with 

 such marked attention, and to those who wish- 

 ed to hear them, but were unable to do so, to 

 leave t.:em behind me in an authentic form. 

 The execution of this work has cut me off 

 from many social pleasures ; it has also pre- 

 vented me from makingmyself acquainted with 

 institutions in the working of which I feel a 

 deep interest. But human power is finite, 

 and mine has been expended in the way which 

 I deemed most agreeable, not to my more 

 intimate friends, but to the people of the 

 United States. 



In the opening lecture are mentioned the 

 names of gentlemen to whom I am under 

 lasting obligations for their friendly and often 

 laborious aid. The list might readily be ex- 

 tended, for in every city I have visited willing 



helpers were at hand. I must not, however, 

 omit the name of Mr. Rhets, Professor 

 Plenry's private secretary, who, not only in 

 Washington, but in Boston, gave me most 

 important assistance. To the trustees of the 

 Cooper Institute my acknowledgments are 

 due ; also to the directors of the Mercantile 

 Library at Brooklyn. I would add to these a 

 brief but grateful reference to my high-miuded 

 friend and kinsman, General Hector Tyn- 

 dale, for his long-continued care of me, and for 

 the thoughtful tenderness by which he and his 

 family softened, both to me and to the parents 

 of the>vuth, the pain occasioned by the death 

 of my junior assistant in Philadelphia. 



Finally, I have to mention with warm com- 

 mendation the integrity, ability, and devo- 

 tion, with which, from first to last, I have 

 been aided by my principal assistant, Mr. 

 John Cottrell. 



NEW YORK, February, 1873. 



LECTURE I. 



INTRODUCTORY : Uses of Experiment : Early Scien- 

 tific Notions: Sciences of Observation: Knowl- 

 edge of the Ancients Regarding Light : Nature 

 judged from Theory defective: Detects of the 

 Eye: Our Instruments: Rectilineal Propagation 

 of Light : Law of Incidence and Reflection : 

 Sterility of the Middle Ages: Refraction: Dis- 

 covery of Snell : Descartes and the Rainbow : 

 Newton r s Experiments on the Composition of 

 Solar Light : His Mistake as regards Achroma- 

 tism : Synthesis ot White Li<ht : Yellow and 

 lUut: Lights proved to produce White by their 

 Mixture: Colors of Natural Bodies: Absorption: 

 Mixture of Pigments contrasted with Mixture of 

 LLhts. 



SOME twelve years ago I published, in 

 England, a little book entitled the " Glaciers 

 of the Alps," and, a couple of years subse- 

 quently, a second volume, entitled " Heat as 

 a Mode of Motion." These volumes were 

 followed by others, written with equal plain- 

 ness, and with a similar aim, that aim being 

 to develop and deepen sympathy between 

 science and the world outside of science. I 

 agreed with thoughtful men* who deemed 

 it good for neither world to be isolated from 

 the other, or unsympathetic towards the 

 other, and, to lessen this isolation, at least in 

 one department of science, I swerved aside 

 from those original researches which had pre- 

 viously been the pursuit and pleasure of my 

 life. 



These books were, for the most part, re- 

 published by the Messrs. Appleton, under 



*Among whom may be mentioned, specially, the 

 l:e Sir Edmund Head, Bart. 



the auspices of a man who is untiring in his 

 efforts to diffuse sound scientific knowledge 

 among the people of this country; whose 

 energy, ability, and single-mindedness, in the 

 prosecution of an arduous task, have won for 

 him the sympathy and support of many of us 

 in "the old country." 1 allude to Professor 

 Youmans, of this city. Quite as rapidly as 

 in England, the aim of these works was un- 

 derstood and appreciated in the United 

 States, and they brought me from this side 

 of the Atlantic innumerable evidences of 

 good-will. Year after year, invitations 

 reached me * to visit America, and last year 

 I was honored with a request so cordial, and 

 signed by five-and-twenty names so distin- 

 guished in science, in literature, and in ad- 

 ministrative position, that I at once resolved 

 to respond to it by braving, not only the dis- 

 quieting oscillations of the Atlantic, but the 

 far more disquieting ordeal of appearing in 

 person before the people of the United 

 States. , 



This request, conveyed to me by my ac- 

 complished friend, Professor Lesley, of Phil- 

 adelphia, and preceded by a letter of the 

 same purport from your scientific Nestor, 

 Professor Joseph Henry, of Washington, de- 

 sired that I would lecture in some of the 

 principal cities of the Union. This I agreed 

 to do, though much in the dark as to what 

 foim such lectures ought to to take. In 



* One of the earliest came from Mr. John Amory 

 Lowell, of Boston. 



