538 Dr. R. T. Glazebrook : 



It is, perhaps, the more necessary, for I think it is not always 

 recognized how much of our knowledge of Optical Science 

 is due to Stokes. It was he who first verified with any 

 degree of exactness Huyghens' construction for the refraction 

 of light at a uniaxial crystal ; the interpretation of KirehhofFs 

 discovery of the coincidence between the dark lines of the 

 solar spectrum and the bright lines of certain incandescent 

 solids and gases is due to him, and on this the whole of spec- 

 trum analysis rests ; he explained the phenomena of fluor- 

 escence, and as an old man, some years ago, expounded in his 

 own unrivalled manner the origin of the Rontgen rays and 

 their connexion with the kathode rays. The analysis of a 

 plane wave of light into its constituent parts, and the first 

 dynamical account of diffraction, are due to him; and his 

 experiments, if we accept any modification of the elastic- 

 solid theory of light as true, settled that FresneFs explanation 

 of the cause of refraction, rather than that of Neumann and 

 MacCullagh, is the right one. 



In his brilliant Rede Lecture, Oornu writes : — 



" The study of the properties of waves, looked at from 

 every aspect, is then at present the really fruitful path. 



" It is that which Stokes, in his double capacity of mathe- 

 matician and physicist, has followed.... All bis beautiful 

 investigations, whether in hydrodynamics or in theoretical 

 or experimental optics, relate to the transformations which 

 waves undergo in the diverse media through which they pass. 



" In the varied phenomena which he has discovered or 

 analysed, movement of fluids, diffraction, interference, 

 fluorescence, Rontgen rays, this guiding idea that I have 

 pointed out is ever visible, and it is this which has made the 

 scientific life of Sir George Stokes one harmonious whole.' 5 



Let us consider then very briefly the progress of theoretical 

 optics since the days of Stokes' first paper on the subject : 

 " On the Theories of the Internal Friction of Fluids in Motion, 

 and of the Equilibrium and Motion of Elastic Solids"*. The 

 advance in the early part of the century had been most 

 marked. The discovery of the principle of interference by 

 Young, and the brilliant work of Augustin Fresnel, who 

 had covered the ground with giaut strides, had placed the 

 undulatory theory on a firm footing, but there was no con- 

 sistent view of the subject which would account even for the 

 facts then known on a rational basis. 



Fresnel's theory of double refraction was not dynamical; 

 he arrived at it in the first place by purely geometrical reason- 

 ing, based on Huyghens' construction, and only attempted at 

 * Cainb. Phil. Trans, viii. (1845). 



