460 Professor Tyndall [Jan. 18j 



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

 Thomas Young.* 



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

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

 Astronomer Eoyal, Sir George Airy. Bringing the knowledge 

 possessed by the founders of the undulatory theory, and that gained 

 by subsequent workers, to bear upon the question. Sir George Airy 

 showed that, though Young's general principles were unassailable, his 

 calculations were sometimes wide of the mark. It was proved by 

 Airy that the curve of maximum illumination in the rainbow docs 

 not quite coincide 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. Gallo, of Berlin, illustrated 

 by careful measurements with the theodolite the agreement which 

 exists between tho theory of Airy and the facts of observation. 

 Thus, from Descartes to Airy, the intellectual force expended in the 

 elucidation of the rainbow, though broken up into distinct person- 

 alties, might be regarded as that of an individual artist, engaged 

 throughout this time in lovingly contemplating, revising, and perfect- 

 ing his work. 



We have thus cleared the ground for tho series of exi^oriments 

 which constitute the subject of this discourse. During our brief 

 residence in the Alps this year, we were favoured with some weather 

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

 weather. On the night of the 22nd 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, but also beyond that of the illuminated fog, was a 

 pale, white, luminous circle, complete except at the point where it 

 was cut through by the shadow. As I walked out into the fog, this 

 curious halo went in advance of me. Had not my demerits been so 

 well known to me, I might have accepted the phenomenon as an evi- 

 dence of canonisation. Benvenuto Cellini saw something of the kind 

 surrounding his shadow, and ascrbied it forthwith to supernatural 

 favour. I varied the position and intensity of the lamp, and found 

 even a candle sufficient to render the luminous band visible. With 

 two crossed laths I roughly measured the angle subtended by the 

 radius of the circle, and found it to be practically the angle which 

 had riveted the attention of Descartes — namely, 41°. This and other 

 facts led me to suspect that the halo was a circular rainbow. A week 



* Yuung's ' Works,' editt-d by Peacock, vol. i. pp. 185, 293, 357. 



