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kind that I have described ; I can only state that it is so received by all 

 competent persons; and that this being so, it must be regarded as a 

 great fundamental truth to which all the conclusions of science must of 

 necessity be subject. 



Hitherto I have been speaking, directly or indirectly, chiefly of 

 heat : it is the investigation of the nature of heat, principally due (it may 

 be said with satisfaction) to Mr Joule, a countryman of our own, that 

 has given the enormous advance in our physical knowledge, which I 

 have pointed at, rather than described. Let me turn now to /ighf. 

 Heat and light generally go together, and they have both been the fields 

 recently of great discoveries, though in very different ways. 



Light, as almost every one is supposed to know now-a-days, is due 

 to the vibrations of a fine elastic medium, very much in the same way 

 as sound is due to the vibrations of the air. The undulatory, or wave 

 theory, of Light, as it is called, has been for some time recognized as 

 established ; and a vast number of complicated phenomena have been 

 worked out mathematically upon the wave hypothesis in such a manner 

 as to leave no doubt upon the minds of those who can follow the 

 investigations, that the hypothesis must be sound. Of late, however, 

 the discoveries made concerning the nature of light have been diverted, 

 as it were, from light itself to the universe, in which light is so important 

 an agent, in a manner which cannot be passed over by any one who is 

 touching ever so slightly upon recent advances in Science. 



Some of those, to whom I am speaking, will probably be acquainted 

 with the nature and results of Spectrum Analysis ; it is a great question 

 whether it will be possible for me to make intelligible to others the 

 principles upon which the analysis is based. I should utterly have 

 despaired of doing so, had it not been for the lucid pages of Professor 

 Tait; whether his light, when further refracted through the medium of my 

 brain, will continue to be luminous may be gravely doubted. But with 

 your kind acquiescence the attempt shall be made. 



We will start with sound, the analogy of which with light is 

 frequently most helpful. Every one knows that if he whistles or sings 

 the note to which a tuning fork, or stretched string, or even a China 



