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 XLV. Intelligence and Miscellaneous Articles. 



TREATISE ON OPTICAL PHYSICS. 



To the Editors of the Philosophical Magazine and Journal. 



United University Club, 

 GENTLEMEN, August 4th, 1892. 



T T is to be regretted that the anonymous author of the review of 

 -*- my treatise on Physical Optics has not adopted the manly and 

 straightforward course of appending his name to the review. No 

 reasonable person can complain of legitimate criticism ; but surely 

 those principles of fair play, which happily prevail amongst English 

 gentlemen, require that an assailant who has made rather a severe 

 attack upon my book should disclose his identity. 



It would be hazardous to speculate whether the anonymous 

 reviewer belongs to a school of thought which has on more than 

 one occasion come forward as the satirist of Cambridge Mathe- 

 matics, and which, if I mistake not, has its headquarters in the 

 capital of the northern portion of this Island ; or whether he hails 

 from a part of the United Kingdom whose political constitution 

 will shortly be the subject of much discussion. If, however, he 

 will lay aside the bias which he evidently entertains against Cam- 

 bridge text-books, and will read more carefully the work of one 

 who still looks back with gratitude upon the admirable system of 

 instruction provided by the University of Cambridge, I think he 

 will find that the " skeleton " is by no means devoid of flesh, nor 

 is it in a state of nudity. 



The ancient school of investigators postulated the existence of a 

 medium, or aether ; but their ideas were not sufficiently clear to 

 enable them to construct a satisfactory dynamical theory. They 

 succeeded, however, in explaining a variety of phenomena relating 

 to isotropic media; they discovered the form of the wave-surface 

 in biaxal crystals ; and they accounted for the production of 

 coloured rings by thin crystalline plates. But I should be very 

 much surprised to find any modern physicist who would attempt 

 to justify the dynamical reasoning upon which Eresnel attempted 

 to base his theory ; and there are grounds for thinking that he 

 originally discovered his wave- surface by an inductive method, 

 founded upon the construction of Huygens' for uniaxal crystals. 



The modern school of investigators commenced with Green, who 

 did the one thing I have so strongly insisted upon, and which the 

 anonymous reviewer describes as the foolish process of writing 

 down equations. Green formulated a definite hypothesis concern- 

 ing the physical constitution of the aether; he thereby obtained 

 the equations of motion of the medium ; and he solved them both 

 in the case of an isotropic medium and a doubly refracting one. 

 Further research has undoubtedly shown that Green's theory is 

 defective, since it furnishes several results which are inconsistent 

 with experiment ; but the fact that Green did write down his 



