1884.] on Hainhows, 4.63 



Between the plienomenon observed by us on Cliristmas Day, and that 

 described by Ulloa, there are some points of difference. In his case 

 fog of sufficient density existed to enable the shadows of him and his 

 six companions to be seen, each, however, only by the person whose 

 body cast the shadow, while around the head of each were observed 

 those zones of colour which characterise the " spectre of the Brocken." 

 In our case no shadows w^ere to be seen, for there was no fog-screen 

 on which they could be cast. This implies also the absence of the 

 zones of colour observed by Ulloa. 



The white rainbow has been explained in various ways. A learned 

 Frenchman, M. Bravais, who has written much on the optical plieno- 

 mena of the atmosphere, and who can claim the additional recom- 

 mendation of being a distinguished mountaineer, has sought to 

 connect the bow with the vesicular theory to which I have just 

 referred. This theory, however, is more than doubtful, and it is not 

 necessary.* The genius of Thomas Young throws light upon this 

 subject as upon so many others. He showed that the whiteness of 

 the bow was a direct consequence of the smallness of the drops which 

 produce it. In fact, the wafted water-specks seen by us upon Hind 

 Head f were the very kind needed for the production of the pheno- 

 menon. But the observations of Ulloa place his white bow distinctly 

 loithinJ.h.Q arc that would be occupied by the ordinary rainbow — that 

 is to say, in the region of supernumeraries ; and by the action of the 

 supernumeraries ujDon each other Ulloa's bow was accounted for by 

 Thomas Young. The smaller the drops the broader are the zones of 

 the supernumerary bows, and Young proved by calculation that when 

 the drops have a diameter of ^^^th or ^QL-g-th of an inch, the bands 

 overlap each other, and produce white light by their mixture. Unlike 

 the geometric bow, the radius of the white bow varies within certain 

 limits, which M. Bravais shows to be 38° 30' and 41° 46' respectively. 

 In the latter case the white bow is the ordinary bow deprived of its 

 colour by the smallness of the drops. In all the other cases it is 

 produced by the action of the supernumeraries. 



The physical investigator desires not only to observe natural 

 phenomena but to re-create them — to bring them, that is, under the 

 dominion of experiment. From observation we learn what nature is 

 willing to reveal. In experimenting we place her in the witness-box, 

 cross-examine her, and extract from her knowledge in excess of that 

 which would, or could, be spontaneously given. Accordingly, on my 

 return from Switzerland last October, I sought to reproduce in the 

 laboratory the effects observed among the mountains. My first object, 



* The vesicular theory was combated very ably in France by the Abbe Rail- 

 lard, who has also given an interesting analysis of the rainbow at the end of his 

 translation of my ' Notes on Light.' 



t Had our refuge in the Alps been built on the southern side of the valley of 

 the Rhone, so as to enable us to lock with the sun behind us into the valley and 

 across it, we should, I think, have frequently seen the white bow ; whereas on the 

 opposite mountain slope, which faces the sun, we have never seen it. 



