l he Colours of Mixed Plates. 



865 



of these phenomena will be obtained from the diagram given 



below. 



Fiar. 1. 



WATER 



AIR 



^CK*0 MA TIC 



INVISIBLE 



Colours 



VAtiYiva N\ 



WITH • 



OBUQOlTJT 



XNTENSE 

 Colour es 

 IF FILH 

 is thi/v 



VER/ 



FAINT 



The fluctuations of colour with obliquity are most striking 

 in the region on the left of the diagram where they com- 

 mence, and are continued over to the right, where they are 

 much less vivid, apart from the greatly decreased intensity 

 of the diffracted light on this side. If, instead of white 

 light, a pencil of monochromatic light be used to illuminate 

 a laminar boundary, the light scattered by it shows a similar 

 asymmetry, the intensity fluctuating as the obliquity is 

 varied, and vanishing in a series of directions which are 

 closer together or wider apart according to the thickness of 

 the film. 



4. Explanation of the Diffraction-Haloes . 



We are now in a position, on the basis of the observa- 

 tions described in the preceding section, to form a general 

 idea of the manner in which the diffraction-haloes due to a 

 mixed plate arise. To begin with, it may be assumed 

 that the mixed plate is of uniform thickness, and that the 

 light is incident normally upon it. Consider the light 

 diffracted by the film in a direction making an angle 6 with 

 the incident pencil of rays. To the aggregate scattering in 



