COLOURS IN METAL GLASSES AND IN METALLIC FILMS. 389 



the incident pencil of light ; in figs. 4 and 5 the plane of polarisation of the incident 

 light was inclined at 45 to the plane of incidence; while in fig. 6 the two planes 

 were perpendicular. 



In the figs. 3-6 the upper diagram represents the second focal plane of the 

 microscope when the diameters of the particles of metal in the glass are less than 

 O-l/i, the small lines being parallel to the planes of polarisation of the emergent 

 light in various parts of the field, the " emergent light " here meaning the light sent 

 up the microscope tube by the metal particles in the glass under observation. The 



Fig. 3. 



Fie. -I, 



Fig. 5. 



Fig. 6. 



lower diagrams in the same figures represent the appearances of a diffraction disc for 

 the same respective positions of the plane of polarisation of the incident light. 



It is to be noticed that the light emitted in any particular direction comes to a 

 focus at a corresponding point in the second focal plane of the microscope. Conse- 

 quently a black spot in that plane means that no light is emitted in the corresponding 

 direction. 



If all the particles are spheres sending up no light in some particular direction, 

 there will thus be a black spot in the second focal plane, as well as in each diffraction 

 disc, at the point corresponding to that direction. 



Suppose now, as in 2, that the incident light travels in the direction Oz and is 

 polarised in plane yQz, fig. 1. Instead of conceiving this plane to alter as we consider 

 the various cases of figs. 3-6, we shall imagine the microscope tube to move in the 

 plane xOy. 



