386 Prof. R. W. Wood: Some new 
be classed with the phenomena of mixed plates, their bril- 
lianey and saturation reminding one of the appearances 
produced by laminary retardation. The films, however, show 
no colour by transmitted light when deposited on olass, and 
the effective doubling of the retardation, by the reflexion 
back through the film by the metal surface, ean hardly 
account for the observed effects. Moreover, the energy 
stream reflected from the surface of the collodion appears to 
be essential, for if we employ light polarized perpendicular 
to the plane of incidence, and set the plate at the polarizing 
angle of collodion, so that no reflexion occurs except at the 
metal surface, all trace of colour disappears. If the angle 
of incidence is larger than the polarizing angle, the colour of 
the reflected light changes to its complementary tint when 
the plane of polarization i is made parallel to the plane of in- 
cidence. As I have shown in the preceding part of this 
paper, the effects at large angles of incidence involve the 
interference of two streams of light polarized in planes in- 
clined at 90° to each other, w hich are complicated enough 
with monochromatic light and structureless films. For the 
present only normal incidence will be considered. Though 
L know of no direct way of proving that, in this case, the 
light reflected from the collodion surface is an essential 
factor, there is strong indirect evidence. 
If the film is wedge-shaped and sodium light is employed, 
the dark fringes seen at normal incidence move towards the 
thick edge of the wedge as the angle of incidence is increased, 
exactly as they do with thin films of the ordinary type. If 
the incident light is polarized perpendicularly to the plane 
of incidence, the fringes gradually fade out, disappearing at 
the polarizing angle. This indicates that they are produced 
in the same way at normal incidence as at the polarizing 
angle, namely, through the agency of light reflected from 
the surface of the collodion. 
If we consider some value of A, for which the path-dif- 
ference between the rays reflected from the collodion and 
metal surfaces amounts to an odd number of half-waves, the 
colour corresponding to this wave-length will be weakened 
in the reflected beam owing to interference. In the case of 
transparent thin films the absent colour appears im excess in 
the transmitted light, while in the present case it is thrown 
back through the film by the metal surface. It is thus clear 
that the colours which are weakened in the reflected light 
are made to traverse the frilled film a greater number of 
times than the colours for which the path-difference is an 
even number of half-waves. 
