1884. 

 THE RAINBOW AND ITS CONGENERS* 



oldest historic reference to the rainbow is known 

 -*- to all : " I do set my bow in the cloud, and it shall 

 be for a token of a covenant between me and the earth. 

 . . . And the bow shall be in the cloud; and I shall 

 look upon it, that I may remember the everlasting 

 covenant between God and every living creature of all 

 flesh that is upon the earth." To the sublime concep- 

 tions of the theologian succeeded the desire for exact 

 knowledge characteristic of the man of science. What- 

 ever its ultimate cause might have been, the proximate 

 cause of the rainbow was physical, and the aim of sci- 

 ence was to account for the bow on physical principles. 

 Progress towards this consummation was very slow. 

 Slowly the ancients mastered the principles of reflection. 

 Still more slowly were the laws of refraction dug from 

 the quarries in which Nature had embedded them. I 

 use this language, because the laws were incorporate in 

 Nature before they were discovered by man. Until the 

 time of Alhazan, an Arabian mathematician, who lived 

 at the beginning of the twelfth century, the views enter- 

 tained regarding refraction were utterly vague and in- 

 correct. After Alhazan came Roger Bacon and Vitellio,f 



* A Friday evening discourse at the Royal Institution. 



f Whewell (History of the Inductive Sciences, vol. i. p. 345) 

 describes Vitellio as a Pole. His mother was a Pole ; but Poggen- 

 dorff (Handu-orterbuch d. exacten WissenscJiaften} claims Vitellio 



199 



