THEORY OF ETHER 159 



reaches the skin with any intensity, we have the sensation of 

 warmth. When it falls upon a body, it may make it luminous 

 or raise its temperature. 



Whatever may be the state of the ether which is conveying 

 radiation, we may be quite certain of one thing that there are 

 two phases of that state. At one time the movement is posi- 

 tive, at another negative. At one instant the displacement is 

 in one direction, at the next it is equal in amount but opposite 

 in direction. The distance between two points which are suc- 

 cessively moving in the same manner is called the length of a 

 wave or undulation. In other words, the length of a wave is 

 the space through which the vibratory movement passes during 

 the time occupied in the complete vibration of any single por- 

 tion of the ether through which the wave is transmitted. The 

 total displacement of each portion of the ether from its original 

 position is called the amplitude of the wave. Upon this 

 depends the intensity of the light (and likewise of the thermal 

 effects produced). 



When a given point is in the paths of two ether waves, 

 their joint effect may be either an increased or diminished dis- 

 placement that is, the amplitude of the joint wave at the 

 point of coincidence may be larger or smaller than that of 

 either of its constituent waves. The movement communicated 

 by one travelling wave may be, at that given point, in the same 

 direction as that communicated by the other wave. In this 

 case the total displacement is equal to the sum of the sepa- 

 rate displacements, and the intensity of light is greater than 

 that from either source alone. If the displacements happen 

 to be, however, in opposite directions, on account of the posi- 

 tion of the point with regard to the concurring waves, then 

 the resultant displacement will be the sum of a positive and a 

 negative quantity, i.e. the difference between these quantities, 

 and the direction will be that of the larger quantity. Hence 

 there will be a diminution of light. If the two displacements 

 are exactly equal and in opposite directions at the given point, 

 then there is evidently no resultant motion at this point, and 

 hence no light. It is obvious that displacements which neu- 

 tralise one another must be in the same plane, and in the case of 



