the Colours of Mixed Plates. 863 



act as centres or sources of diffracted radiation. Each 

 dement o£ a laminar boundary may be regarded as sending 

 oat tico streams of radiation — one on the more retarded, 

 and one on the less retarded side of the wave-front. In 

 directions nearly coincident with that of regular trans- 

 mission of the incident waves, these two streams are of 

 practically equal intensity and of opposite phases. In such 

 directions the phenomena observed are in agreement with 

 the indications of Lord Rayleigh's theory, according to whish 

 the colour of the laminar boundary, as seen in the Foucault 

 test, should be complementary to the colour of the central 

 fringe in the laminar diffraction pattern produced by it. 

 For very small angles of diffraction, therefore, the elementary 

 diffraction theory gives results which are substantially valid. 

 As we shall see presently, this ceases to be true when we 

 -consider larger angles of diffraction. 



3. The Unsijmmetrical Scattering of Light by Laminar 

 Boundaries : Normal Incidence. 



Very simple observation suffices to show that the scatter- 

 ing of light through larger angles by laminar boundaries 

 exhibits features not indicated by the elementary theory. 

 For this purpose, a thin film of liquid mixed with air 

 enclosed between two glass plates is placed normally in the 

 track of a strong pencil of light from a lantern, and viewed 

 obliquely by the eye with or without the aid of a magnifier. 

 It will be noticed at once that the edges of separation of 

 liquid and air diffract light in a strikingly un symmetrical 

 manner. Any given edge can easily be observed diffracting 

 light at all angles up to 90° when viewed on the side 

 passing through the liquid ; but viewed on the side passing- 

 through air, it can hardly be seen at all except in directions 

 making less than about 10° or 15° with the direction of the 

 incident beam, so small is the intensity of the diffracted 

 light in this region. If a closed curved boundary enclosing 

 air be viewed at a slight obliquity to the direction of the 

 incident light, the two limited portions of the boundary 

 visible appear differently coloured, one being much fainter 

 than the other. The fainter portion which is seen through 

 air vanishes altogether when viewed at greater obliquity, 

 while the part of the boundary seen through the liquid, 

 i. e. on the more retarded side of the wave-front, remains 

 visible throughout, its colour changing periodically and 

 becoming richer as the obliquity of observation is increased, 



3 L2 



