CONTENTS. 



Part I. Historical Survey. 



(a) Experimental. . 



(b) Theoretical. 



Part II. Experimental Work. 

 Introduction. 



(a) The Visible Part of the Spectrum. 



(b) Investigations in the Ultra Violet 



(c) The Method of Crossed Prisms. 



(d) The Interferometer Method. . 

 Conclusion. 



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249 



258 



268 

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 287 



Bibliography.. 



288 



