LE CTURES 



UNDULATORY THEORY OF LiaHT, 



By F. a. p. BARNARD, S. T. D. , LL. D., 



LATE CHA^•CELLOR OF THE UNIVERSITY OF MISSISSIPPI. 



[In prepcaring the material of these lectures for publication, some transpositions have 

 been made in the original order of topics, and the lecture form has been generally aban- 

 doned. Mathematical illustrations have also been occasionally introduced, which would 

 not have been quite in place before a popular audience.] 



PAMT I. 



INTRODUCTORY. 



OUTLINE OF OPTICAL DISCOVERY. 



The ivnowledge which we possess of the material objects surrounding us in 

 the universe is principally received through the sense of vision. From the 

 other senses we derive a much more limited range of impressions. The touch 

 furnishes us with a valuable means of confirming or correcting the information 

 we receive from sight; but its usefulness extends only to objects in our own 

 immediate vicinity. The hearing, though through it, by the aid of spoken lan- 

 guage, we are supplied with a vast multitude of ideas which have had their 

 origin in impressions previously made upon other senses, contributes of itself, 

 in any other form, but very slightly to the great stock of our knowledge. 



Such therefore being the pre-eminence of vision among the senses, light, 

 which is its medium, is, and has ever been, the most important of physical 

 instrumentalities in promoting the intellectual development of the human race, 

 and making progress a possibility. But, while occupying this peculiar relation 

 to the history of our advancement in the knowledge of nature, while so fertile 

 in the revelations it has unfolded to us of the properties and qualities of other 

 things, it is remarkable that light has itself furnished, in its own nature, one of 

 the most difficult and perplexing of all the subjects of physical inquiry; so 

 that, even down to an advanced period of the present century, the world of 

 science may be said to have been upon no other subject more widely at vari- 

 ance than upon the elementary and fundamental question. What is light? 



Nor is it possible to explain tliis want of harmony by supposing the inquiry 

 to have but recently originated. Since, in the physical world, light has 

 been the ever present and ever most efficient handmaid of the human un- 

 derstanding, its phenomena must, to some extent at least, have attracted the 

 attention of the first intelligent inhabitants of our planet. The first man who 

 breathed could not have failed to notice the images of visible objects, formed 

 by reflection in the bosom of every quiet pool; and the first rude navigator 



