THE HISTORY OF A DOCTRINE. 385 



emerges from the icy regions of the Caucasus into the wooded 

 hills, gentle sloj^es, sunny meadows, and neatly fenced barley- 

 fields. Compared with the warrens or stone-heaps which serve 

 the Tartars of the northern valleys for dwellings, even the 

 towered villages have at a distance a false air of civilization. 

 Suanetia in June, in the flower-time of the rhododendrons and 

 azaleas, and again in October, when the azalea-leaves are red, and 

 the birches golden against fresh autumn snows, must be one of 

 the wonders of the world. Spaciousness, sunniness, variety, are the 

 constant qualities of Suanetian landscapes. The great basin of 

 the Ingur, forty miles long by ten to fifteen broad, is broken by 

 no ridges that approach the snow-line, and the long, undulating, 

 grassy spurs that divide the glens, in place of narrowing the hori- 

 zon, furnish in their soft lines the most effective contrast possible 

 to the icy peaks and rigid precipices of Shkara and Tetnuld, of 

 Ushbe and the Leila. From the varied beauty of forests and flow- 

 ers the eyes are carried at once to the pure glaciers, which hang 

 like silver stairs on the green lower slopes of the snowy chain. 

 The atmosphere has none of the harshness of that of Switzerland 

 in summer. The breezes from the Black Sea bring up showers 

 and moisture to soften the outlines and color the distances ; the 

 wind from the steppe suffuses the air with an impalpable haze, 

 through which the great peaks glimmer like golden pillars of the 

 dawn. 



THE HISTORY OF A DOCTRINE.* 



By Prof. S. P. LANGLEY. 

 II. 



THE first five years of this century are notable in the history 

 of radiant energy, not only for the work of Leslie, and for 

 the observation by Wollaston, Ritter, and others, of the so-called 

 "chemical" rays beyond the violet, but for the appearance of 

 Young's papers, re-establishing the undulatory theory, which he 

 indeed considered in regard to light, but which was obviously 

 destined to affect most powerfully the theory of radiant energy 

 in general. 



We are now in the year 1804, or over a century and a quarter 

 since the corpuscular theory was emitted, and during that time it 

 has gradually grown to be an article of faith in a sort of scientific 

 church, where Newton has come to be looked on as an infallible 

 head, and his views as dogmas, about which no doubt is to be tol- 

 erated ; but if we could go back to Cambridge in the year 1668, 



* President's address before the American Association for the Advancement of Science, 

 at Cleveland, Ohio, August 15, 1888. Reprinted from "Science." 

 TOL. xxxrv. — 25 



