iiopal institution of ®reat 13ntanu 



WEEKLY EVENING MEETING, 



Friday, January 18, 1884. 



George Busk, Esq. F.R.S. Treasurer and Vice-President, in the 



Chair. 



Professor Tyndall, D.C.L. F.R.S. M.B.I. 



On Bainhows, 



The 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 conceptions of the theologian 

 succeeded the desire for exact knowledge characteristic of the man of 

 science. Whatever its ultimate cause might have been, the proximate 

 cause of the rainbow w^as physical, and the aim of science 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 refrac- 

 tion 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, wh.o lived at the beginning of the twelfth century, 

 the views entertained regarding refraction were utterly vague and 

 incorrect. After Alhazan came Roger Bacon and Vitellio,* who made 

 and recorded many observations and measurements on the subject of 

 refraction. To them succeeded Kepler, who, taking the results 

 tabulated by his predecessors, applied his amazing industry to extract 

 from them their meaning — that is to say, to discover the physical 

 principles which lay at their root. In this attempt he was less suc- 

 cessful than in his astronomical labours. In 1604, Kepler published 

 his ' Supplement to Vitellio,' in which he virtually acknowledged his 

 defeat, by enunciating an approximate rule, instead of an all-satisfying 

 natural law. The discovery of such a law, which constitutes one 

 of the chief corner-stones of optical science, was made by Willebrord 

 Snell, about 1621.t 



♦ Whewell (' History of the Inductive Sciences,' vol, i. p. 345) describes Vitellio 

 as a Pole. His mother was a Pole; but Poggendorflf (' Handwbrterbuch d. 

 exacten Wissenschaften ') claims Vitellio himself as a German, born in Thuriiigen. 

 " Vitellio " is described as a corruption of Witolo. 



t Born at Leyden 1591 ; died 1626. 



Vol. X. (No. 77.) 2 h 



