300 THOMAS YOUNG. 
to produce colour in water, it suffices to reduce it to the 
state of a thin film; that “thin” is, so to speak, the syn- 
onym of “coloured ;” 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 propositions are only the 
necessary consequences deduced from the accidental ob- 
servation of the colours presented by soap bubbles, and 
even by extremely thin films of all sorts of substances. 
To comprehend how such phenomena have, during 
more than 2000 years, daily met the eyes of philoso- 
phers without exciting their attention, we have 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 colours. Hooke, his fellow-labourer, went fur- 
ther. He believed that he had discovered the cause of 
this kind of colours in the coincidences of the rays, or to 
speak in his own language, in the mutual action on each 
other of the waves reflected by the two surfaces of the 
thin film. This was, we may admit, a suggestion char- 
acteristic 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 colours of thin films a favourite 
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 
