PREFACE. vii 



these difficulties when I set about my task; but I 

 am consoled for my pains and anxiety by observing 

 that none of the multitudinous criticisms with 

 which I have been favoured and, often, instructed, 

 find fault with me on the score of having strayed 

 out of bounds. 



Among my critics there are not a few to whom 

 I feel deeply indebted for the careful attention 

 which they have given to the exposition thus 

 hampered; and further weakened, I am afraid, by 

 my forgetfulness of a maxim touching lectures of 

 a popular character, which has descended to me 

 from that prince of lecturers, Mr. Faraday. He 

 was once asked by a beginner, called upon to ad- 

 dress a highly select and cultivated audience, what 

 he might suppose his hearers to know already. 

 Whereupon the past master of the art of exposi- 

 tion emphatically replied " Nothing! " 



To my shame as a retired veteran, who has all 

 his life profited by this great precept of lecturing 

 strategy, I forgot all about it just when it would 

 have been most useful. I was fatuous enough to 

 imagine that a number of propositions, which I 

 thought established, and which, in fact, I had ad- 

 vanced without challenge on former occasions, 

 needed no repetition. 



I have endeavoured to repair my error by 

 prefacing the lecture with some matter — chiefly 

 elementary or recapitulatory — to which I have 

 given the title of " Prolegomena." I wish I could 



