THE WAVE THEORY OF 

 LIGHT 



[A LECTURE DELIVERED AT THE ACADEMY 



OF MUSIC, PHILADELPHIA, UNDER THE 



AUSPICES OF THE FRANKLIN INSTI- 



TUTE, SEPTEMBER 2^ 1884] 



THE subject upon which I am to speak to you this even- 

 ing is happily for me not new in Philadelphia. The 

 beautiful lectures on light which were given several 

 years ago by President Morton, of the Stevens' Institute, and 

 the succession of lectures on the same subject so admirably 

 illustrated by Professor Tyndall, which many now pres- 

 ent have heard, have fully prepared you for anything I 

 can tell you this evening in respect to the wave theory 

 of light. 



It is indeed my humble part to bring before you only 

 some mathematical and dynamical details of this great theory. 

 I cannot have the pleasure of illustrating them to you by 

 anything comparable with the splendid and instructive ex- 

 periments which many of you have already seen. It is 

 satisfactory to me to know that so many of you, now 

 present, are so thoroughly prepared to understand anything 

 I can say, that those who have seen the experiments will not 

 feel their absence at this time. At the same time I wish to 

 make them intelligible to those who have not had the advan- 

 tages to be gained by a systematic course of lectures. I must 

 say, in the first place, without further preface, as time is 

 short and the subject is long, simply that sound and light 

 are both due to vibrations propagated in the manner of 

 waves; and I shall endeavour in the first place to define 

 the manner of propagation and the mode of motion that 



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