﻿Observations on Diffraction, 11 



if the difference of AF and BF = ^, there will be darkness. 



Now if CH and DH are more nearly parallel than AF and 



BF are, then, in order to have CH— DH= 5, tne point H 



must be farther from G than F was from E, so the smaller 

 obstacle C D gives the larger bright centre to its shadow. 

 This principle has many consequences: the thinner needle has 

 the broader bright central line, or as a needle is narrowing 

 towards its point the central line will be opening out : at the 

 corner of a square or triangle there will be wedges of light 

 pointing inwards. The enlargement of the image of a star 

 at the focus of a telescope through diffraction is less in a 

 telescope of large aperture, because the waves from the 

 borders of the lens reach the focus at a greater inclination to 

 one another. Again, the length of a wave of yellow light is 

 about 24 millionths of an inch ; if the waves from A and B 

 move straight towards one another at K, then the diameter of 

 the bright spot here will be something less than '000024 inch, 

 too small to be seen, from which it appears that the condition 

 for making visible these phenomena which are connected with 

 such minute quantities is that AF, BF must be nearly- 

 parallel : the object must be narrow, or the screen for receiving 

 the shadow must be distant. 



The following notes describe the original figures from which 

 the photographs are the shadows : — 



14. A circle with four triangular areas. 



15. A circular area with four inscribed dark circles. 



16. Four circles touching one another ; the lines of the 

 circles are gradually made thinner towards the inner part, 

 but they are complete circles. 



19. Two ellipses, each through the focus of the other : 

 Arago's spot at the common part. 



20. Three pentagons with a small pentagonal area at the 

 common centre. 



24 and 29. A square with four external squares at the 

 corners : the latter is the shadow taken at a greater distance 

 from the object ; it is covered with fine interference detail. 



25 and 28. A square with inscribed square and diagonals. 

 23, 26, 27, 30. Right-angled triangles put together in sets 



of four at an acute angle : the detail contains a number of 

 round spots. 



31, 39, 40. A chessboard in various states : here again 

 systems of spots are developed. 



33. Rows of equilateral triangles, set alternately in suc- 

 cessive rows : spots again. 



