322 Memoir of Dr. Thomas Young. 



he could, at pleasure, communicate the most resplendent 

 colours? That he could render it violet, blue and green, 

 that he could make it yellow like the skin of the lemon, 

 red like scarlet without affecting* its purity, without mixing 

 it with any foreign substance, without changing the pro- 

 portions of its principal gaseous constituents. Would not 

 the public consider our natural philosopher as unworthy of 

 all credit, if, after these remarkable results, he added, that 

 in order to produce colour in water, it is sufficient to give 

 it the form of a pellicle, that thinness is in truth synony- 

 mous with colour ; that the transition from each shade to 

 the most different tint is the necessary consequence of a 

 simple variation in the thickness of the liquid layer ; that 

 this variation in the transition from red to green, for ex- 

 ample, is not the thousandth part of the thickness of a 

 hair. Yet these incredible results are the inevitable con- 

 sequences of the chance colours presented by liquid bubbles 

 blown out, and even by thin plates of all kinds of bodies. 



To understand why such phenomena have daily been 

 presented to philosophers for 20 centuries without exciting 

 attention, we have only to consider to how few persons 

 nature has imparted the valuable faculty of being astonished 

 at the proper time. 



Boyle first penetrated into this fruitful mine. He con- 

 fined himself, however, to the minute description of the 

 varied circumstances which give birth to the iris. Hooke 

 went farther. He thought that these kinds of colours were 

 derived from the intersection of the rays, or to use his own 

 words, " in the intersection of waves reflected by the two 

 surfaces of the thin plate." This was a proof of genius, it 

 must be admitted; but it could not be properly taken 

 advantage of at a time when the complex nature of white 

 light was still unknown. 



Newton made the colours of thin plates the object of his 

 favourite study. He devoted to it a whole book of his cele- 

 brated treatise on optics ; he established the laws of their 

 formation by an admirable chain of experiments, which no 

 one has since surpassed. In explaining with homogeneous 

 light the regular evidence of which Hooke made mention, 

 and which is produced round the point of contact of two 

 lenticular glasses placed above each other, he proved that 



