NEWTON'S THEORY OP COLOURS. . 137 



losopher finds subjects for his contemplation ; and from 

 the unrivalled play of colours which he discovers in 

 that attenuated film, he learns that the varying thick- 

 nesses of surfaces influence, in a most remarkable 

 manner, the colours of the sunbeam. Films of oil 

 floating upon water present similar appearances ; and 

 the colours developed in tempering steel are due en- 

 tirely to the thickness of the oxidized surface produced 

 by heat. There have lately been introduced some beautiful 

 specimens of paper rendered richly iridescent by the fol- 

 lowing process : A solution of a gum resin in chloroform 

 is floated upon water, where it forms a film giving all the 

 colours of Newton's rings. A sheet of paper which 

 has been previously sunk in the water is carefully lifted, 

 and the film thus removed adheres with great firmness 

 to the paper, and produces this rich and curious play of 

 colour. The rich tints upon mother-of-pearl, in the 

 feathers of many birds, the rings seen in the cracks of 

 rock-crystal, or between the unequal faces of two pieces 

 of glass, and produced by many chemical and indeed 

 mechanical operations are all owing to the same cause ; 

 the refraction of the luminous pencil by the condition 

 of the film or surface. If we take one of those steel 

 ornaments which are formed by being covered with an 

 immense number of fine lines, it will be evident that 

 these striae present many different angles of reflection, 

 and that, consequently, the rays thrown back will, at 

 some point or another, have a tendency to cross each 

 other. The result of this is, that the quantity of light 

 is augmented at some points of intersection, and annihi- 

 lated at others.* Out of the investigation of the phe- 

 nomena of diffraction, of the effects of thin and thick 

 plates upon light, and the results of interference, has 



* On the Nature of Light and Colours : Lecture 39, in Young's 

 Lectures on Natural Philosophy, Kelland's Edition, p. 373, and the 

 authorities there quoted. 



