
Cases of Interference and Diffraction. 385 
under which they are obtained are’ very simple, and I con- 
sider their study far more instructive than the mere inspec- 
tion of a collection of crystal sections in a_polariscope 
arranged for convergent light. 
Colours of Frilled Transparent Films on 
Metallic Surfaces. 
Wewill now consider a remarkable case of interference 
which appears to be essentially different from any of the 
eases which have been previously studied. The theory of 
or films shows, as Lord Rayleigh points out in his article 
1 “ Wave-theory of Light,” that a transparent film on a 
Bees tly reflecting surface shows no interference-colours. 
As I have already pointed out, a thin film of collodion de- 
posited on a bright surface of silver shows brilliant colours 
in reflected light. It moreover scatters light of a colour 
complementary to the colour of the directly reflected light. 
This I find is due to the fact that the collodion film ‘ frills, i 
the mesh, however, being so small that it can only be detected 
with the highest powers of the microscope. Commercial ether 
and collodion should be used. If chemically pure ether ob- 
tained by distillation is used, the film does not frill, and no 
trace of colour is exhibited. 
In the cases of the transparent films with the first surface 
lightly silvered, the second heavily coated, the waves absent 
in the reflected light are absorbed by the metal, as I have 
already shown. In the present case these waves are scattered 
by the granular surface. If a spot on the film which appears 
purple by reflected light is illuminated with sunlight, it will 
be found that green light is scattered, not in all ‘directions, 
but through a range corresponding to the size of the granu- 
lation, as in the case of mixed plates. 
If the light is incident normally, the scattered light comes 
off through an angular range included between ten and thirty 
degrees, and again at an angle of nearly 90°, the latter being 
strongly polarized. Conversely, if the sunlight be incident 
at nearly 90°, strongly polarized light is scattered normally. 
Considerable difficulty has been found in explaining these 
colours satisfactorily. They appear to be saturated, 7. e. 
certain wave-lengths are completely absent in the reflected 
light, and until the granulation was detected with the 
microscope it was impossible to make even a satisfactory 
hypothesis. Even now the polarization effects are difficult 
to account for. 
At first sight it may seem as if the colours could 
