The Cause of Life and Motion 33 



and it is instantly impelled in a new direction by 

 any set of vibrations which coincide with its form, 

 and at the same time balance it at the instant of 

 change. As it enters a new medium, the particle 

 changes its form by uniting with the elements of 

 the medium. 



Of a number of particles, impinging at or 

 near the surface of a new medium, a percentage 

 depending upon the angle of impact and upon 

 their coincidence with certain vibrations, will be 

 refracted, while the remainder will be reflected. 



A considerable reduction of light at the back 

 of a plate of glass will cause an apparent reflec- 

 tion of an object, not so much because the glass 

 reflects better with a darkened space behind it, 

 but that the image of reflection is lighter than 

 the darkened background, and is therefore made 

 visible by contrast. 



Light may be conveyed and in a great meas-" 

 ure maintained within a stream of water. The 

 phenomenon can only be made obvious in a dark 

 place and the effect then is that of a brilliant 

 stream of light. It is supposed that the light is 

 thus held captive by repeated reflections from the 

 surrounding darkness. But darkness does not 

 reflect light, otherwise we should have no nigh^ 

 on the earth. And besides, if the effect was 

 caused by the surrounding darkness, a stationary 

 cylinder of glass would produce the effect equally 



