INTERFERENCES. 299 
bles so brilliantly coloured, so light, which 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 purity never shows any 
more than some very slight shade of colour, blue or green, 
hardly sensible, and that 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, without changing the proportions of its 
constituent 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 
