
Cases of Interference and Diffraction. doe 
This accounts for the fact that these colours are more 
strongly scattered by the granulations of the films. 
A collodion surface only reflects about 5 per cent. of the 
incident energy; and it was found impossible to account for 
the strong tinge seen in the reflected light, by compounding 
the feeble stream of light from the collodion with the 
o) 
powerful stream coming from the metal. 
It appeared, however, that the observed effects could be 
accounted for, if the somewhat arbitrary assumption were 
made that the granulated surface reflected more strongly 
than a smooth surface. As I have already said, the granu- 
lations are too small to interfere with the regular reflexion 
of light, the scattering being selective, so to speak, 2. e. 
ined to the waves which, owing Ss interference, are 
compelled to traverse the film a number of times. 
The assumption above referred to appeared to be too 
arbitrary to make without some experimental evidence, and 
experiments were therefore made to determine the effect of 
the “frilling” of the film on its reflecting power. One 
of the faces of a 60° prism of crown glass was flowed with 
collodion of the same dilution as that used in the preparation 
of the coloured films. It showed in reflected light inter- 
ference-colours, which, however, were very much diluted 
with white light, owing to the small difference between the 
refractive indices of the two media. In working with the 
film on silver it was found that, if the colours did not appear 
at once, as soon as the film dried, they could be brought out 
by breathing on the film, the deposit of moisture being 
advantageous to the formation of the granulations. It was 
always -possible to intensify the colours in this way. The 
film deposited on the surface of the prism was treated in this 
way, one half of it being screened from the deposit of 
moisture by a plate of glass. As soon as the moisture had 
evaporated, it was found that the reflecting power of the 
surface had been greatly increased, the film appearing almost 
as bright as a half-silvered surtace. 
The increase in the brilliancy of the reflected light was 
about three-fold, as was shown by covering the unfrilled 
portion with a sheet of thin glass, which about equalized the 
intensities. In other words, the frilled collodion-surface 
regularly reflects white light, of an intensity very nearly 
equal to that of light reflected from thin glass surfaces. 
On examining the granular surface with polarized light, it 
was found that the at of maximum polarization was in the 
neighbourhood of 63°, which would make its refractive index 
about 1°96. The polarizing angle of the smooth collodion 
