PHYSICS OF THE NINETEENTH CENTURY. LIGHT. 459 



ing the propagation of undulations. He showed that in an elastic 

 medium, uniform in all parts and directions, a disturbance set up at 

 any point will expand in concentric spherical waves. To put the 

 matter in another form and introduce a conception due to Huyghens, 

 let us suppose that light emanates from a point in such a medium as 

 that just spoken of. Taking any one point within that medium, the 

 particles at that point will pass through all the successive phases in a 

 certain very small portion of time. But let us consider any one instant 

 of time, or suppose that the motion of the particles of the medium is 

 arrested absolutely and instantaneously while we leisurely make in idea 

 the following construction : taking any point of the medium, the par- 

 ticles are in some one phase, and there are other immediately con- 

 tiguous particles also in that phase ; and again, others contiguous to 

 them in the same phase, and so on ; a continuous surface might be 

 drawn so as to contain all the series of these particles. In the case 

 supposed, these surfaces would be spherical, and in all cases such sur- 



faces are called wave fronts. If we consider the wave-fronts at in- 

 creasing distances from the centre of disturbance, it will be obvious 

 that each very small portion of the wave-front becomes more nearly a 

 plane. Each wave-front advances with the velocity of light, and every 

 point of it may be considered as a centre from which secondary dis- 

 turbances of the medium are being continually propagated. But these 

 secondary disturbances are relatively very small, and from the manner 

 of their production they tend to destroy each other's effects. Let us 

 see Huyghens' principles applied in the construction by which reflec- 

 tion and refraction may be explained. Let A B, Fig. 206, represent a 

 reflecting surface ; c D is a wave-front, which at the point c we will 

 suppose has at this instant reached A B.' While the wave-front at D is 

 advancing to G, a new wave-front has been advancing from c as a new 



