DIFFRACTION. 



307 



Two rays do not always annihilate each other com- 

 pletely at their point of intersection. Sometimes we 



To exhibit these appearances ordinarily requires the sun's light. 

 But the translator has found a very simple method of exhibiting these 

 phenomena on a minute scale by candle light, by merely placing a 

 fine wire across one surface of a lens of short focus, and lookin^ 



O 



through it at light admitted through a narrow slit parallel to the wire, 

 or even the flame of a candle at a considerable distance. 



Next, as to the theoretical explanation, an inspection of the accom- 

 panying diagram will perhaps help to convey an idea of the manner 

 in which the several sets of waves are formed and interfere in the case 

 now supposed. 



Young conceived the beam of light as a series of waves propagated 

 onward, till, on reaching the card, they were broken up into two new 

 sets of waves spreading in circles round each edge as a new centre, 

 while part of the original set continued to pass on at each side. On 

 the principle just mentioned these would interfere with the new por- 

 tions on the outside; and the two new portions would interfere with 



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