INTERFERENCES. 299 



bles so brilliantly coloured, so light, wliicli when just 

 blown out of a pipe become the sport of every impercep- 

 tible current of air. Before so enlightened an audience, 

 it would without doubt be superfluous to remark that the 

 difficulty of producing a phenomenon, its variety, its util- 

 ity to the arts, are not the necessary indications of its 

 importance in a scientific point of view. I have, there- 

 fore, to connect with a child's sport the discovery which 

 I proceed to analyze, with the certainty that its credit 

 will not suffer from its origin. At any rate I shall have 

 no need to recall the apple, which, dropping from its stalk 

 and falling unexpectedly at the feet of Newton, developed 

 the ideas of that great man respecting the simple and 

 comprehensive laws which regulate the celestial motions ; 

 nor the frog and the touch of the bistoury, to which phys- 

 ical science has recently been indebted for the marvellous 

 pile of Volta. Without referring in particular to soap 

 bubbles, I will suppose that a physicist has taken for the 

 subject of experiment some distilled water, that is to say, 

 a liquid, which in its state of pui'ity never shows any 

 more than some very slight shade of colour, blue or green, 

 hardly sensible, and thaj; only when the light traverses 

 great thicknesses. I would next ask what we should 

 think of his veracity if he were to announce to us, with- 

 out further explanation, that to this water, so limpid, he 

 could at pleasure communicate the most resplendent col- 

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

 then yellow like the peel of citron, or red of a scarlet tint, 

 without affecting its purity, without mixing with it any 

 foreign substance, Avithout changing the proportions of its 

 constituq^it gaseous elements. Would not the public re- 

 gard our physicist as unworthy of all belief, especially 

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



