378 Prof. R. W. Wood: Some new 
be prepared by first thickly silvering the mica, cementing the 
silvered side to the glass plate, and then stripping the mica 
off. The colours are scarcely visible, owing to the dispro- 
portionality between the amplitudes of the two interfering 
streams of light, but appear as soon as the upper surface of 
the mica is half-silvered, which can be done by immersing 
the plate in a silvering solution until the colours reach their 
maximum brilliancy. To obtain films which show the 
maximum brilliancy, it is clear that the amplitude of the 
stream reflected from the first surface must have the largest 
possible value consistent with the fulfilling of the condition 
that it be completely destroyed by the disturbance coming 
from the second surface when the phase-difference is 180°. 
In the case of transparent films, the wave-lengths absent 
in the reflected light appear in excess in the transmitted light, 
there being no destruction of energy. If the second surface 
is a perfect reflector, the energy thrown down upon it 
by interference at the first surface will all be returned. 
If we consider the upper metallic surface as nonabsorbing, 
and work out the case by the method of multiple reflexions, 
we find that no colcur will be produced, light of all wave- 
lengths being reflected with equal facility. The fact that 
brilliant colours appear, means that light is absorbed at one 
or both of the silvered surfaces, since this is the only way in 
which the energy of the absent wave-lengths can disappear. 
If we assume a certain percentage reflected and a certain 
percentage absorbed by the thin silver film, we find that the 
waves for which the phase-difference is 180° are compelled 
to make more transits through the film than those for which 
the phase-difference is 360°. It is possible to obtain experi- 
mentally a condition in which the former are almost com- 
pletely absent in the reflected light, while the latter are 
reflected with scarcely any loss of intensity. ‘To calculate 
the most favourable conditions, we should require data 
regarding the percentages reflected and transmitted by films 
of various thicknesses. 
It next occurred to me to substitute a thin film of collodion 
for the mica, half-silvering the film as before. 
A sheet of glass can be silvered chemically, or procured 
by removing the varnish from the back of a piece of modern 
mirror-glass with alcohol. The silver film is then flowed with 
collodion diluted with three or four parts of ether. As soon 
as the film dries colours appear, contrary to theory. These 
colours may be quite brilliant, and are due to diffraction, as 
T shall show presently. Jf the plate be now immersed in 
Brashear’s silvering-bath, the colours will instantly disappear, 
>! 
