ON RAINBOWS. 66 7 



During our walk the bow was broken and reformed several times, and, 

 had it not been for our previous experience, both in the Alps and at 

 Hind Head, it might well have escaped attention. What this white 

 bow lost in beauty and intensity, as compared with the ordinary col- 

 ored bow, was more than atoned for by its weirdness and its novelty 

 to both observers. 



The white rainbow {pare en del blanc) was first described by the 

 Spaniard, Don Antonio de Ulloa, Lieutenant of the Company of 

 Gentlemen Guards of the Marine. By order of the King of Spain, 

 Don Jorge Juan and Ulloa made an expedition to South America, an 

 account of which is given in two amply-illustrated quarto volumes to 

 be found in the library of the Royal Institution. The bow was ob- 

 served from the summit of the mountain Pambamarca, in Peru. The 

 angle subtended by its radius was 33 30', which is considerably less 

 than the angle subtended by the radius of the ordinary bow. Between 

 the phenomenon observed by us on Christmas-day, and that described 

 by Ulloa, there are some points of difference. In his case fog of suffi- 

 cient density existed to enable the shadows of him and his six com- 

 panions 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 color which characterize the " specter of the Brocken." In our case 

 no shadows were to be seen, for there was no fog-screen on which they 

 could be cast. This implies also the absence of the zones of color ob- 

 served by Ulloa. 



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

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

 nomena 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 ref erred. 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 phenomenon. But the observa- 

 tions of Ulloa place his white bow distinctly within the 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 upon each 

 other Ulloa's bow was accounted for by Thomas Young. The smaller 



* The vesicular theory was combated very ably in France by the Abbe Raillard, who 

 has also given an interesting analysis of the rainbow at the end of his translation of my 

 "Notes on Light." 



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

 Rhone, so as to enable us to look 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. 



