AND ITS CONGENERS. 207 



genius resembled that of Descartes or Newton, and 

 who eighty-two years ago was appointed Professor of 

 Natural Philosophy in the Royal Institution of Great 

 Britain. I refer, of course, to the illustrious Thomas 

 Young. 1 



But our task is not, even now, complete. The 

 finishing touch to the explanation of the rainbow 

 was given by our eminent Astronomer Royal, Sir 

 George Airy. Bringing the knowledge possessed by 

 the founders of the undulatory theory, and that gained 

 by subsequent workers, to bear upon the question, Sir 

 George Airy showed that, though Young's general 

 principles were unassailable, his calculations were some- 

 times wide of the maik. It was proved by Airy that 

 the curve of maximum illumination in the rainbow 

 does not quite coincide with the geometric curve of 

 Descartes and Newton. He also extended our know- 

 ledge of the supernumerary bows, and corrected the 

 positions which Young had assigned to them. Finally, 

 Professor Miller, of Cambridge, and Dr. Galle, of 

 Berlin, illustrated by careful measurements with the 

 theodolite the agreement which exists between the 

 theory of Airy and the facts of observation. Thus, 

 from Descartes to Airy, the intellectual force expended 

 in the elucidation of the rainbow, though broken up 

 into distinct personalities, might be regarded as that of 

 an individual artist, engaged throughout this time in 

 lovingly contemplating, revising, and perfecting his 

 work. 



We have thus cleared the ground for the series of 

 experiments which constitute the subject of this dis- 

 course. During our brief residence in the Alps this 

 year, we were favoured with some weather of matchless 



1 Young's Works, edited by Peacock, vol. i. pp. 185, 293, 357. 



