
Cases of Interference and Diffraction. 383 
viewing the reflected light through a nicol, the curious double 
fringes can be found easily by slowly turning the nicol. 
The light will be found to be plane-polarized along the lines 
1, 3, 5, 7, &e. of fig. 4, though in general the planes of 
polarization along one set of lines is inclined to the plane of 
polarization along the alternate lines, as is indicated below 
the figure, the arrows representing the direction of the 
vibration (electric vector). Between the lines of plane- 
polarized light, which appear as dark fringes when the nicol 
is so oriented as to extinguish the light, we have either 
elliptivally or circularly polarized light, as can at once be 
shown by the introduction of a quarter-wave plate, which 
enables us to extinguish the light along the lines 2, 4, 6, and 
8 by suitable adjustment of the mica plate and analyser. 
The direction of revolution of the vibration along lines 2 and 
4 is opposite to that along lines 6 and 8. 
I at first attempted to work out the problem by com- 
pounding a plane-polarized vibration, coming from the air- 
elass surface, with a nearly circular vibration coming from 
the metal. This was a failure, but it at once occurred to me 
that owing to the refraction by the glass or gelatine, the 
incidence-angle of the light falling upon the metal would be 
too small to produce cir cular polarization. 
It seemed best, upon the whole, to determine experimentally 
the exact nature of the vibration coming from each surface, 
und then to try whether, by compounding them with 
various phase-differences, the observed results could be 
accounted for. 
The incident light vibrated in a plane indicated by the 
arrow at the top of fig. 4. The light reflected from the 
glass surface is of course plane-polarized, vibrating parallel 
to the surface when the angle of incidence is equal to the 
polarizing angle. For larger angles of incidence, the plane 
of the reflected vibration makes an angle with the surface 
depending on the magnitude of the reflected component, 
which lies in the plane of incidence. To determine the 
nature of the vibration coming from the glass-metal surface, 
it was necessary to get rid of the light. reflected from the 
upper surface of the glass. This was done by laying a small 
piece of rather thick plate-glass on a plate of speculum 
metal with a film of benzole between, and allowing a narrow 
,beam of light to fall upon the surface of the glass. The 
images reflected from the two surfaces appeared separated, 
and could be independently examined with a nicol. The 
benzole film practically brought the metal surface into optical 
ZD2 
