340 Dr. G. J. Stoney on Microscopic Vision* 



and its replicas emitted portions of light which at the instant 

 of starting were exactly alike, and that the undulations which 

 result from this state of things may be thrown into groups 

 of undulations, each of which is the same as would have been 

 emitted by one or other of the uniform rulings of equidistant 

 lines represented in the accompanying figure, as well as the 





^— — A, ^4 





^>J\^/f 





^^ 











//v></\\ 



\ ^i/^*^// 



^\\^ f\^/rf 





//^\r^ 



m' 



vast number of others that would arise from sufficiently ex- 

 tending the figure ; when we further bear in mind that every 

 equal element of any one of the lines in each such ruling 

 emits the same amount of light, which is in the same state as 

 that emitted by p except as regards intensity : when all these 

 things are taken into account we find that the entire of the 

 standard image may be regarded as built up of such luminous 

 rulings superposed upon and interfering with one another — 

 each of these rulings being due to the convergence and 

 mutual interference of two or more undulations of the uniform 

 plane waves which (since the reversal) have been travelling 

 inwards, and each ruling accordingly being uniform and 

 extending across and even beyond the whole range of the 

 objective field. 



This is our second theorem. It may be enunciated as 

 follows : — 



Proposition 2, 



The standard image may be regarded as resulting from the 



superposition and mutual interference of uniform luminous 



rulings of equidistant parallel bright lines extending over the 



whole field of view; each ruling being produced by the 



