120 EULOGY ON THOMAS YOUNG. 



ceptiblo enrreiit of air. Before so enliglitened an audience it would, 

 without doubt, be su]>ortluous to remark that the difficulty of producing- 

 a pheuoinenou, its variety, its utility to the arts, are not the necessary 

 indications of its importance in a scientitic i)oint of view. 1 liave, there- 

 fore, to connect with a chihl's sport the discovery which I proceed to 

 analyze, with the certainty that its credit will not sufLer from its origin. 

 At any rate, I shall liave no need to recall the apple, which, dropping 

 from its stock and falling unexpectedly at the feet of Newton, develope- 

 the ideas of that great man respecting the simi)le and compreheusiv- 

 laws which regulate the celestial motions; nor the frog and the touch 

 of the bistoury, to which physical science has recently been indebted 

 Ibr the marvelous i)ile of Volta. Without referrin g in i)articular to soap- 

 bubbles, I v/ill sup})ose that a physicist has taken for the subject of ex- 

 periment some distilled water, that is to say, a liquid which in its state 

 of purity never shows any more than some very .slight shade of color, 

 blue or green, hardly sensible, and that only Avhen the light traverses 

 great thicknesses. 1 would next ask what we should think of his vera- 

 city if he were to announce to us, without further explanation, that to 

 this water, so limpid, he could at pleasure communicate the most 

 resplendent colors ; that he knew how to make it violet, blue, green ; 

 then yellow like the ])eel of citron, or red of a scarlet tint, without 

 affecting its i»urity, without mixing with it any foreign substance, with- 

 out changing the proportions of its constituent gaseous elements. 

 Would not the public regard our physicist as unworthy of all belief, 

 especially when, after such strange assertions, he should add, that to 

 produce color in water, itsufiices to reduce it to the state of a thin film ; 

 that "thin" is, so to speak, the synonym of "colored;" that the passage 

 of each tint into one the most different from it is the necessary conse- 

 quence of a simple variation of the thickness of the liquid film ; that 

 this variation, for instance, in passing from red to green, is not the 

 thousandth part of the thickness of a hair ? Yet these incredible propo- 

 sitions are only the iiecessary consequences deduced from the accidental 

 observation of the colors presented by soap-bubbles, and even by ex- 

 tremely thin films of all sorts of substances. 



To comprehend how such phenomena have, during more than two 

 thousand years, daily met the eyes of philosophers without exciting 

 their attention, we Imve need to recollect to how few persons nature 

 imparts the valuable faculty of being astonished to any purpose. Boyle 

 was the first to penetrate into this rich mine. He confined himself, 

 however, to the minute description of the varied circumstances which 

 gave rise to these iridescent colors. Hooke, his fellow-laborer, went 

 further. He believed that he had discovered the cause of this kind of 

 colors in the coincidences of the rays, or, to speak in his own language, 

 ill the mutual action on each other of the «y( res reflected by the two 

 surfaces of the thin film. This was, we may admit, a suggestion charac- 

 teristic of genius ; but it could not be made use of at an epoch when 

 the compound nature of white light was not as yet understood. 



Newton made the colors of thin films a favorite object of study. He 

 devoted to them an entire book of his celebrated treatise, the "Optics." 

 He established the laws of their formation by an admirably connected 

 chain of experiments, which no one has since surpassed in excellence. 

 In illuminating with homogeneous light the very regularly-formed 

 series of bands of which Hooke had already made mention, and which 

 (U-iginated round the point of contact of two lenses pressed closely 

 together, he proved that for each species of simple color there exists, in 

 thin film's of every substance, a series of thicknesses gradually increasing, 



