428 



Lord Bayleigh 



[Jan. 15, 



distance across whicli the shadow is thrown, must also be taken into 

 consideration. The optical shadow of a small object, thrown across 

 a considerable distance, partakes of the imperfections generally ob- 

 served in connection with sound shadows. 



It was calculated by the French mathematician, Poisson, that, 

 according to the wave theory of light, there should be a bright spot 

 in the middle of the shadow of a small circular disc — a result that 

 was thought to disprove the wave theory by a reductio ad ahsiudum. 

 Although unknown to Poisson, this very phenomenon had actually 

 been observed some years earlier, and was easily verified when a 

 suitably arranged experiment was made. 



Under suitable conditions a bright spot can be observed at the 

 centre of the shadow of a three-penny bit. The coin may be sup- 

 ported by three or four very fine wires, and its shadow thrown by 



Fig. 1. 



-Reproduction of a Photograph of the Shadow of a 

 Silver Penny Piece. 



sunlight admitted at a pin-hole aperture placed in the shutter of a 

 darkened room. The coin may be at a distance of about fifteen feet 

 from the aperture, and the screen at about fifteen feet beyond the coin. 

 To obtain a more convenient illumination, a larger aperture in the 

 shutter may be filled by a short focus lens, which forms a diminutive 

 image of the sun ; this image serves as a point source of light. A 

 smaller disc has some advantages. Fig. 1 is reproduced from a jihoto- 

 graph of the shadow of a silver ])enny piece, struck at the time of the 

 Coronation. The shadow, formed in the manner just described, was 

 allowed to fall directly on a photographic plate; after development a 

 negative was obtained, in which the dark parts of the shadow were 

 represented by transparent gelatine, while the bright parts were 

 represented by opaque deposits of silver. To obtain a correct repre- 

 Bentation, a contact print was formed from the negative in the usual 



