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a Cbaptcr on Xigbt anb Colour;^ 



By G. H. Bryan, Sc.D. 





^^\J*i^^ 1. -Elementary Phenomena of Light. 





^ 



o3Vfe 



IHE agent which affects the sense of sight is spoken 

 of as Light. It has been conceived of as small 

 particles emitted from visible bodies and striking 

 the eye, or as a wave motion propagated through a 

 medium (called the B^'her) and exciting the nerves 

 of the retina by this motion. The latter view has 

 now come to be accepted to the complete exclusion 

 of the former, though such eminent names as those 

 of Newton, Laplace, and Biot are connected with 

 the development of the Corpuscular Theory of Light. 



The most striking of simple light phenomena, the production 

 of shadows, and the sharpness of the shadows when the source of 

 light is nearly a point, was for a long time a great difficulty in the 

 way of the acceptance of the Undulatory Theory, but this diffi- 

 culty has been overcome, and the simplicity of the explanation, 

 afforded by this theory, of many somewhat complex phenomena, 

 has placed it in complete command of the field. The vibrations 

 which produce light are transverse to the direction of propagation. 

 2. — The explanation of many facts can be obtained by an 

 application of four simple laws of light propagation, 

 (i.) Light is transmitted in straight lines, 

 (ii.) When a ray of light is reflected, the reflected ray makes 

 with the reflecting surface an angle equal to that made by the 

 incident ray, and the two rays are in a plane perpendicular to the 

 surface. 



(iii.) When a ray of light is refracted at the surface of two dif- 

 ferent media, the angles of incidence and refraction have a deter- 

 minate relation to one another depending on the media, and the 

 plane of the two rays is perpendicular to the refracting surface. 

 The relation referred to is that the sines of the angles of incidence 

 and refraction are in a constant ratio. 



International Journal of Microscopy and Natural Science. 

 Third Series. Vol. VII. h 



