664 THE POPULAR SCIENCE MONTHLY. 



distances the same. The consequence is that when the rays emerge 

 from the drop they are in a condition either to support or to destroy 

 each other. By such alternate re-enforcement and destruction, the 

 colored zones are produced within the primary bow. They are called 

 " supernumerary bows," and are seen not only within the primary but 

 sometimes also outside the secondary bow. The condition requisite 

 for their production is, that the drops which constitute the shower shall 

 all be of nearly the same size. When the drops are of different sizes, 

 we have a confused superposition of the different colors, an approxi- 

 mation to white light being the consequence. This second step in the 

 explanation of the rainbow was taken by a man the quality of whose 

 genius resembled that of Descartes or Newton, and who eighty-two 

 years ago was appointed Professor of Natural Philosophy in the Royal 

 Institution of Great Britain. I refer, of course, to the illustrious 

 Thomas Young.* 



But our task is not, even now, complete. The finishing touch to 

 the explanation of the rainbow was given by our last, eminent, As- 

 tronomer Royal, Sir George Airy. Bringing the knowledge possessed 

 by the founders of the undulatory theory, and that gained by subse- 

 quent workers to bear upon the question, Sir George Airy showed 

 that, though Young's general principles were unassailable, his calcula- 

 tions were sometimes wide of the mark. It was proved by Airy that 

 the curve of maximum illumination in the rainbow does not quite co- 

 incide with the geometric curve of Descartes and Newton. He also 

 extended our knowledge of the supernumerary bows, and corrected 

 the positions which Young had assigned to them. Finally, Professor 

 Miller, of Cambridge, and Dr. Galle, of Berlin, illustrated by careful 

 measurements with the theodolite the agreement which exists between 

 the theory of Airy and the facts of observation. Thus, from Des- 

 cartes to Airy, the intellectual force expended in the elucidation of the 

 rainbow, though broken up into distinct personalities, might be re- 

 garded as that of an individual artist engaged throughout this time 

 in lovingly contemplating, revising, and perfecting his work. 



We have thus cleared the ground for the series of experiments 

 which constitute the subject of this discourse. During our brief resi- 

 dence in the Alps this year, we were favored with some weather of 

 matchless perfection ; but we had also our share of foggy and drizzly 

 weather. On the night of the 22d of September, the atmosphere was 

 especially dark and thick. At 9 p. m. I opened a door at the end of a 

 passage and looked out into the gloom. Behind me hung a small 

 lamp, by which the shadow of my body was cast upon the fog. Such 

 a shadow I had often seen, but in the present case it was accompanied 

 by an appearance which I had not previously seen. Swept through 

 the darkness round the shadow, and far beyond, not only its boundary, 



* Young's works, edited by Peacock, voL i, pp. 185, 293, 357. 



