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 looking 
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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