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 colored bow, was more than atoned for by its weirdness and its novelty to both observers.
The white rainbow (l'arc en ciel 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 observed 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 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 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 observed by Ulloa.
The white rainbow has been explained in various ways. A learned Frenchman, M. Bravais, who has written much on the optical phenomena of the atmosphere, and who can claim the additional recommendation 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.[1] 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[2] were the very kind needed for the production of the phenomenon. But the observations 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."
- ↑ 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.
