
Cases of Interference and Diffraction. 307 
sight no interference appears to have taken place, we may, 
by employing polarized monochromatic light, obtain fringes 
of a very curious nature, which are the result of the ee 
ference between the elliptical vibration coming from the 
metal surface and the plane-polarized vibration reflected from 
the surface of the transparent film. We will go back now to 
the earlier experiments. 
Preparation of Films for the exhibition of Newton’s Colows. 
k 7 t 
In the case of a thin transparent lamina, such as a soap- 
film, the amplitudes of the disturbances reflected from the 
two surfaces are equal, and consequently completely destroy 
each other when the phase-difference is 180°. Inasmuch, 
however, as only a smail percentage of light is reflected from 
each surface, the colours, though saturated, are not as intense 
as is desirable. In the course of some experiments with 
selenium I found, if a plate of mica is pressed against a pool 
of the molten substance on a glass plate, and the whole 
allowed to cool under pressure, that on stripping off the mica, 
films of mica of variable thickness were left upon the Seine 
of the selenium, which showed Newton’s colours of great 
beauty, arranged in mosaics. The patches of equal thickness 
being sharply bounded by straight lines, present an appearance 
similar to that of selenite-films under the polariscope. The 
selenium has a much higher refractive index than the mica, 
consequently the reflexion at each surface is the radeon 
of rays incident from a rare to a denser medium, and the 
difference of phase is given by the difference of path alone ; 
i.e., we do not have the loss of half a wave-length due to 
reflexion under opposite conditions, as would be the case if 
the mica films were in air. 
If the mica is cemented to the plate with sealing-wax or 
any of the common resinous cements, very little trace of the 
colours is to be seen, owing to the fact that the refractive 
indices of the two media being so nearly the same, practically 
no energy is reflected from the boundary. The use of 
selenium can be avoided by very lightly silvering the surface 
of the mica, which may then be cemented to the glass with 
any good laboratory cement, the metallic layer taking the 
place of the medium of high refractive index. This latter 
method is the best for the preparation of large mosaics 
suitable for lantern-projection. In the patches w which show 
no colour by reflected light, the interference may be detected 
with a small spectroscope, the spectrum appearing crossed by 
black bands, corresponding in position to the wave-lengths 
absent in the reflected light. Still more brilliant films can 
