G Rheinbei-g: Common Basis ofthe Theories ofMicroscopic Vision. XIX, 1. 



or pliotograplüc plate to separate thera from tlie former, and 

 because tliey are very faint as compared witli the liglit of tlie 

 disc itself. 



The action of the convex lens in forniing tlie diso and rings niay 

 be illnstrated diagramatically. 



P (fig. 2) is a Inminous point propagatiug 



— waves equally in all directions. It is therefore 

 ~ the ceutre of spherical snrfaces, represented 



in the diagram by circles, every point of which 

 are at any moment in the same State of Vibra- 

 tion or phase. We may therefore say that 

 the wave front, or outermost snrface of equal 

 phase , diverges spherically from the point P. 

 Part of this wave front reaches the lens AB^ 

 and owing to the velocity of light being slower 

 in the denser medium glass than in air , its 



-P' 



K 



shape undergoes alteration during its passage 

 through the lens, passing successively tlirongh 

 the forms indicated in the diagram, and finally 

 emerging as a spherical snrface JvY conver- 

 ging towards point P' . Consequently the light 

 from every point on the snrface of the lens 

 AB reaches P' in the same phase at the 

 same instant. It will be convenient to consider 

 the light at P' with reference to the wave 

 front ^ Y on its emergence from the lens, 

 and it then becomes apparent that an ether 

 particle at P' receives the maximnm amount of 

 amplitude which this wave front can give rise 

 to. For no greater Joint eftect can be produ- 

 ced, than when the nndulations all arrive simul- 

 taneously crest by crest and trongh by trongh, 

 It is trne that at any point K intermediate be- 

 tween the wave front Jt Y and P' the amplitude 

 arising from isolated poiuts on XY would be greater than at P', 

 since the amplitude varies inversely as the distance from the origin. 

 But this is much more than connterbalanced by the interference of 

 vibrations from the other points, arriving in a ditferent phase, com- 

 biued with the diminishiug effect of vibrations arriving obliquely to 

 the wave front. The truth of this is proved by analogous reasoning 



