DIFFRACTION. 101 



whose summit coincided with the edge of the obstacle, and 

 whose centre was the middle point of the line connecting that 

 edge with the luminous origin. 



If we consider these hyperbolas as coincident with their 

 asymptots (which may be done without sensible error, unless 

 near the edge of the obstacle), and if we then determine the 

 angles which they make with one another, and with the edge 

 of the geometric shadow, we shall find that these angles in- 

 crease rapidly as the distance of the obstacle from the lumi- 

 nous point diminishes. When this distance is about 40 inches, 

 the ^fringes are very close together, the fringes of the first 

 and second order making an angle with one another of less 

 than 2' in red light. At the distance of 4 inches this angle 

 is increased to more than 5 7 ; and at T % of an inch it exceeds 

 16'. Thus the fringes dilate, as the edge of the obstacle ap- 

 proaches the luminous origin. 



(116) In this experiment the incident light is supposed to 

 diverge from a luminous point. If the dimensions of the 

 luminous origin had been considerable, it will be easily 

 understood that each line in it, parallel to the edge of the 

 obstacle, would give rise to a different system of fringes ; and, 

 as the dark bands of some of these systems must coincide with 

 the bright bands of others, every trace of the phenomenon 

 would be obliterated. 



(117) The preceding experiments exhibit the effect of a 

 single edge. "When the light diverging from the luminous 

 point is suffered to pass by two neqr edges, the phenomena will 

 be varied in a very interesting manner. 



Let a fine wire be placed in the pencil of light diverging 

 from a luminous point, and let its shadow be received on a 

 screen, or plate of roughened glass, as before. We then ob- 

 serve, outside the geometric shadow, a set of parallel bands, 



