Radiation of Light from Diffracting Apertures. 113 



of Foucault's test by Lord Rayleigh *. The present investi- 

 gation was, in fact, suggested by a perusal of Lord Rayleigh's 

 paper, and the photographs reproduced in Plate IV. were 

 taken with an arrangement analogous to that used in 

 Foucault's test, though I have also made visual obser- 

 vations in the cases in which the incident waves do not 

 converge to a focus in the plane of the second screen. 



The most remarkable result found in the course of the 

 experimental work (and which, so far as I know, has not 

 been noticed by any previous writer) is that in all cases in 

 which the apertures in the focal plane through which the 

 diffracted rays pass (whatever be their actual form) are 

 symmetrically disposed about the centre of the field, the 

 latter itself being excluded, the image of the boundary 

 of the diffracting surface appears as a perfectly black line 

 surrounded on either side by luminous bands. This is 

 irrespective of the actual form of the boundary itself, that 

 is, whether it is circular or of any other shape whatsoever. 

 A similar result is also found when the screen with the 

 apertures is placed symmetrically in any plane either in 

 advance of or behind the focus. When the apertures on 

 the screen are wide enough to admit a large portion of the 

 diffraction pattern formed at the screen into the field of view 

 of the observing telescope, this black line is characterized by 

 extreme fineness and is surrounded on either side by broad 

 luminous bands. Narrowing the apertures is, however, 

 attended by an increase in the width of this black line 

 and the appearance of a large number of well-defined fringes 

 on either side. This remarkable feature is explained in a 

 general way if we regard each element of the edge of a 

 diffracting aperture as sending out two streams of light 

 in directions more or less normal to itself (one on each side 

 of the wave-normal), and that these streams are in opposite 

 phases. This is distinctly suggested by Sommerfeld's well- 

 known investigation on the diffraction of plane waves of 

 light by a semi-infinite screen. The expression given by 

 him for the radiation emitted by the edge is 



& MS Wr4)+i}\/r[~— |t? — hw]' 



Lcos 9 cos J 

 where (r, <£) are the coordinates of a point in the medium 



* Lord Rayleigh, " On Methods for detecting: small Optical Retarda- 

 tions, and on the Theory of Foucault's Test," Phil. Mag. Feb. 1917. 

 Phil. Maq. S. 6. Vol. 37. No. 217. Jan. 1919. I 



