Application of Undulatory Theory. By A. E. Conrady. 411 



combination illustrated in fig. 75 and again here, the two waves will, 

 therefore, invariably produce a node in the same p>osition as light 

 from the centre of the slit. In fig. 78, b, there results a small com- 

 bined wave still in the same sense as that from the centre. But 

 proceed to fig. 78, c, where the difference of phase of either wave is 

 more than { wave-length as compared with the wave from the 

 centre of the slit. We still get the same position of the nodes, 

 but these two waves produce a resultant wave of the opposite character 

 to that of the wave from the, centre of the slit, and this shows 

 graphically what I proved mathematically in my paper. 



If we apply this process to all the successive pairs and then 

 combine the resultants of these, we shall get the complete result ; 

 without going into the details, it may be pointed out that if the 

 differences of phase between the extreme edges of the slit and its 

 centre do not exceed \ wave-length, all the resultants are in the 

 same sense, and reinforce each other ; for a wider slit, the pairs 

 further removed from the centre combine to the opposite effect — 

 hence the total light is weakened, and eventually becomes zero 

 when the edges of the slit are £ wave-length out of phase compared 

 with the centre. With still wider slits the light reappears, but in 

 the opposite phase, in the manner described in my paper. 



