198 



Re/lection and Refraction of Light. 



oscillate, but tlie length of the vibration, and that of the wave are distinct 

 things. 



The direction of a ray is a line perpendicular to the surface of a wave, 

 that is to its front, the surface in which all the phases are the same 5 hence, 

 in uniform media, light will appear to advance in a rectilinear course, from 

 the centre of the spherical wave. The velocity with which the waves are 

 propagated through space, that is, the velocity of light, depends on the 

 elasticity of the medium, and not on the velocity, or on the amplitude of 

 excursion of the vibrating particles; it is greater or less according as the 

 elasticity is more or less perfect ; hence in a given medium the velocity 

 of light is constant, which is in accordance with the first fact, stated art. 

 12. The intensity of light, or the quantity or degree of illumination on a 

 given surface, is, in this theory, measured by the square of the amplitude 

 of excursion of the vibrating molecule ; the sense of vision being supposed 

 to be affected by the vibrations, more or less, according to the magnitude 

 of this square. 



16. To explain reflection, and refraction of light at the surface of a new 

 medium, the vibrating particles are supposed to impinge on those of the 

 medium, in a manner analogous to that of an elastic ball striking another 

 on which it falls : when the media have the same degree of elasticity, the 

 vibrations are propagated forward without any reflection or deviation : but 

 when the media have different degrees of elasticity, every point on the 

 surface becomes a new centre of vibrations, propagating spherical waves, 

 and thus producing at once both reflection and transmission. 



Fig. 1. 



/ 



Let 8, fig. 2, be the source of light traversing the medium Y, and meet- 

 ing the surface of the new medium Z in the line A B ; if the elasticity of 

 the ether in Y and Z be the same, the spherical wave A P a, touching 

 the surface at A, will advance uniformly in its course, passing X Qo', 

 DCB, &c. and no reflection, it is said, will occur. AVhen the elasticities 



