226 Biographical Memoir ofDr TJtomas Young 



tention, we have only to remember to how small a number of 

 persons Nature has imparted the valuable faculty of being as- 

 tonished upon fitting occasions. 



Boyle was the first to open up this rich mine. He neverthe- 

 less confined himself to a minute description of the various cir- 

 cumstances which produce the rainbow. His associate Hooke 

 went farther. He believed that he had discovered the cayse 

 of this kind of colouring in the mutual crossing of the rays, or, 

 to adopt his own language, in the mutual crossings of the waves 

 reflected by the two surfaces of the thin plate. This, as we 

 shall see, was a mark of his genius ; but it could not be appre- 

 ciated at a time when the complex nature of white light was not 

 yet known. 



Newton made the colours of thin plates the object of his fa- 

 vourite study. He devoted a whole book of his celebrated 

 treatise on optics to them ; and established the laws of their for- 

 mation by an admirable suite of experiments which has hitherto 

 been excelled by no one. By illuminating with homogeneous 

 light the regular small coloured rings which Hooke had already 

 mentioned, and which were produced around the point of con- 

 tact of two superimposed glass-lenses, he proved that, for each 

 kind of simple colour, there exists, for thin laminae of every sort, 

 certain thicknesses at which no light is reflected. This result was 

 of fundamental importance, as it contained the key of all these 

 phenomena. 



Newton was less happy in the theoretic views which this re» 

 markable observation suggested to him. To say, with him, 

 of a luminous ray which is reflected, that it is in a Jit of easy 

 reflection ; and to say of a ray which entirely traverses the plate 

 that it is in a Jit of easy transmission, is nothing more than to 

 announce in obscure terms that which the experiment of the 

 two lenses had previously shewn. 



The theory of Thomas Young was not liable to this objec- 

 tion. In it he did not admit^^* of any kind, as an original pro- 

 perty of rays ; and the thin plate was moreover assimilated, in all 

 respects, to a thick mirror composed of the same materials. If in 

 some of these points^ no light was seen. Young did not thence 

 conclude that its reflection had ceased in these points, but sup- 

 posed that, in the peculiar directions of these points, the rays 



