124 



EULOGY ON THOMAS YOUNG. 



not exist a siugle point in space where a tbousancl rays of the same 

 origin do not proceed to cross one another after reflections more or less 

 oblique, and we shall perceive at a glance the whole extent of the unex- 

 plored region which interferences open to the investigations of experi- 

 menters. 



When Young published this theory, many phenomena of periodical 

 colors had been already ottered to the notice of observers, and, we should 

 add, had resisted all attem[)ts at explanation. Among the number we 

 might instance the colored rings which are formed by reflection, not oi 

 thin films, but on mirrors of thick glass slightly concave; the iridesceni 

 bands of different breadtl s, with which the shadows of bodies are bor- 

 dered on the outside, and in some instances covered within, which Gri- 

 maldi first noticed, and which afterward uselessly exercised the genius 

 of l^ewton, and of which the completion of the theory was reserved for 

 Fresnel ; the bows, colored red and green, which are perceived in greater 

 or less number immediately under the innermost of the prismatic bands 

 of tbe rainbow,* and which seemed so comi)letely inexplicable that the 

 writers of elementary books on physics had given up making mention of 

 them; and, lastly, the "coronas," or broad colored circles with varying 

 diameters, which often appear surrounding the sun and moon. 



round each edge as a new center, while part of the original set coutinned to pass on at 

 cacli side. On the principle just mentioned, these would inierfere with the new por- 

 tions on the outside ; and the two new portions would interfere with each other iu the 

 inside of the shadow, in either case giving stripes or bands. To complete the proof, 

 when an opaque screen was placed so as to intercept the rays on one side, though 



+ b 



alTundance of light was presented on the other, yet all the internal hands immediately 

 disappeared, demonstrating that the effect was due solely to the concurrence of the 

 light from hoth sides. The bands produced by light admitted through narrow apertures, 

 and numerous other phenoniena of the same kind, may receive a general and popular 

 explanation in the same way. — Translator. 



* This explanation has been recently controverted by Professor Potter.— Philosophical 

 Magazine, May, 1855. 



