XIX, 1. Rlieinberg: Common Basis ofthe Theories ofMicroscopic Vision. 3 



The uecessity of this proceeding is guided by tlie fact, tliat 

 eacli poiut of tlie luniinous source acts as an independent primary 

 centre of disturbaiice, 



Tbis is evident wlien we reflect that the li^lit at each separate 

 point of tlie himinous sonrce is diie to the jostling and colliding 

 against each other of the particles of matter there. Each collision 

 canses the particles to vibrate with great veloeity in the ether in 

 Avhich they are bathed and thereby communicates to this ether the 

 undulations which it propagates in all directions. Each collision of 

 particles sets up a so called „train" of millions of waves before 

 another jolt sets np another train or series. 



Auy nndulations emanating from one point of the Inminoiis 

 sonice which may meet at another point after travelling there by 

 ditferent paths , will have arrived there with some fixed and cal- 

 cnlable dilFerence of wave lengths , provided the undulations have 

 their origin in the same train of waves. It follows that if the 

 ditference in the paths is only a small number of wave lengths, 

 compared with the number of waves in a train, that the undulations 

 will arrive at the point with the same difference of wave lengths 

 during the greater part of any given period of time. Now this 

 means that the undulations arrive at the point at some fixed ditfe- 

 rence of phase, which is the measure of the State of Vibration, or 

 movement to and fro, of any portion of llie ether at any point on 

 the whole wave lengtli (fig. 1). But when undulations continously 

 arrive at a point with a fixed difference of phase , they are in a 

 condition to interfere, and the total amplitude (AMüg. 1) or greatest 

 distance from its position of rest over which the oscillating ether 

 particle moves to or tro , will be increased or diminished according 

 to this difference. The intensity of the light which varies as the 

 Square of the amplitude will alter correspondingly. Considering the 

 undulations coming from two points only of equal brightness, if there 

 is no difference of phase, the intensity will just be doubled, whilst 

 if the difference of phase is always 4, any movement measured 

 upwards from the median line WL (fig. 1) will be met by exactly 

 the same amount of movement measured downwards from it , and 

 the resulting amplitude is nil, and darkness results. Obviously any 

 other constant phase difference will result in light intensities between 

 the limits nil and double. 



Though the jolting of the particles of matter at any point of 

 the luminous source take place in numbers of directions, giving rise 



1* 



