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 



