212 THE RAINBOW 



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 con- 

 siderably less than the angle subtended by the radius 

 of the ordinary bow. Between the phenomenon ob- 

 served 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 how- 

 ever only by the person whose body cast the shadow. 

 Around the head of each were observed those zones of 

 colour which characterise the ' spectre 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 im- 

 plies 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 phenomena of the atmo- 

 sphere, and who can claim the additional recommenda- 

 tion 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- 



1 The vesicular theory was combated very ably in France by the 

 Abb6 Kaillard, who has also given an interesting analysis of the 

 rainbow at the end of his translation of my Notes on Light. 



