SIX LECTURES ON LIGHT 



answer to my inquiries, however, I was given 

 to understand (by Professor Youmans princi- 

 pally) that a course of experimental lectures 

 would materially promote scientific education 

 in this country, and I at once resolved to 

 meet this desire, as far as my time allowed. 



Experiments have two uses a use in dis- 

 covery, and a use in tuition. They are the 

 investigator's language addressed to Nature, 

 to which she sends intelligible replies. These 

 replies, however, are, for the most part, at 

 first too feeble for the public ear ; for the in- 

 vestigator cares little for the loudness of Na- 

 ture's voice if he can only unravel its meaning. 

 But after the discoverer comes the teacher, 

 whose function it is so to exalt and modify the 

 resu ts of the discoverer as to render them fit 

 for public presentation. This secondary 

 function I shall endeavor, in the present in- 

 stance, to fulfil. 



I propose to take a single department of 

 natural philosophy, and illustrate, by means 

 of it, the growth of scientific knowledge under 

 the guidance of experiment. I wish, in this 

 ii st lecture, to make you acquainted with cer- 



single increment is made good by the indefi- 

 nite number of su-ch increments, summed up 

 in what may be regarded as practically infinite 

 time. 



We will not now go back to man's first 

 intellectual gropings ; much less shall we 

 enter upon the thorny discussion as to how 

 the groping man arose. We will take him at 

 a certain stage of his development, when, fay 

 evolution or sudden endowment, he became 

 possessed of the apparatus of thought and 

 the power of using it. For a time and that 

 historically a long one he was limited to 

 mere observation, accepting what Nature of- 

 fered, and confining intellectual action to it. 

 The apparent motions of sun and stars first 

 drew towards them the questionings of the in- 

 tellect, and accordingly astronomy was the 

 first science developed. Slowly, and with difE.- 

 culty, the notion of natural forces took root 

 in the mind, the seedling of this notion being 

 the actual observation of electric and mag- 

 netic attractions. Slowly, and with difficulty, 

 the science of mechanics had to grow out of 

 this notion ; and slowly at last came the full 



tain elementary phenomena ; then to point application of mechanical principles to the 

 out to you how those theoretic principles by motions of the heavenly bodies. We trace 

 which phenomena are explained, take rooi, I the progress of astronomy through Hip- 

 and flourish in the human mind, and after- J parchus and Ptolemy; and, after a long halt, 

 wards to apply these principles to the whole through Copernicus, Galileo, Tycho Bratue, 



and Kepler; w ile, from the high table-land 

 of thought raised by these men, Newton-. 



tx dy of knowledge covered by the lectures. 



The science of optics lends itself to this mode 



of reatment, and on it, therefore, I propose Lshoots upward like a peak, overlooking all 



to draw for the materials of the present course. I others from his dominant elevation. 



It will be best to begin with the few simple I But other objects than the motions of the 



facts regarding light which were known to the stars attracted the attention of the ancient 



ancients, and to pass from them in historic 

 gradation to the more abstruse discoveries of 

 modern times. 



All our notions of Nature, however exalted 



world. Light was a familiar phenomenon, 

 and from the earliest times we find men's 

 rr.inds busy with the attempt to render some 

 account of it. But, without experiment, 



or however grotesque, have some foundation ; which belongs to a later stage of scientific 



development, little progress could be madein 

 this subject. The ancients, accordingly, 



in experience. The notion of personal voli- 

 tion in Nature had this basis. In the fury 

 and the serenity of natural phenomena the 

 savage saw the transcript of his own varying 

 moods, and he accordingly ascribed these 

 phenomena to beings of like passions with 

 himself, but vastly transcending him in power. 

 Thus the notion of causality the assumption 

 that natural things did not com 2 of themselves, 



were far less successful in dealing with light 

 than in dealing with solar and stellar mo- 

 tions. Still, they did make some progress. 

 They satisfied themselves that light moved 

 in straight lines; they knew, also, that these 

 lines or rays of light were reflected from pol- 

 ished surfaces, and that the angle of inei- 



but had unseen antecedents lay at the root \ dence was equal to the angle of reflection, 

 of even the savage's interpretation of Nature. ! These two results of ancient scientific curios- 

 Out of this bias of the human mind to seek j ity constitute the starting-point of our pres- 

 for the antecedents of phenomena all science ' ent course of lectures. 



But, in the first place, it may be useful to 

 say a few words regarding the source of light' 

 to be employed in our experiments. The, 



has sprung. 



The development of man, indeed, is ulti- 

 mately due to his interaction with Nature. 

 Natural phenomena arrest his attention and 

 excite his questionings, the intellectual activity 



rusting of iron is, to all intents and purposes, 

 the slow burning of iron. It develops heat, 



thus provoked reacting on the intellect itself, j and, if the heat be preserved, a high temper- 

 and adding to its strength. The quantity of j ature may be thus attained. The destruc- 

 power added by any single effort of the in- tion of the first Atlantic cable was probably 

 tellect may be indefinitely small ; but the in- due to heat developed in this way. Other 

 tegration of innumerable increments of this J metals are still more combustible than iron, 

 kind has raised intellectual power from its j You may light strips of zinc in a candle- 

 rudiments to the magnitude it possesses to- flame, and cause them to burn almost like 

 day. In fact, the indefinite smallness of the strips of paper. But, besides combustion in 



