colours which accompany them in calcareous spar. 285 
proved by grinding off the angles EG, FH. Fig. 1, so as to 
make the interrupting stratum parallel to the two faces of the 
crystal. There is in this case no multiplication of images, 
and no production of colour. 
The complementary fringes described in Sect. I. are like- 
wise produced by the transmission of polarised light through 
the interrupting stratum. When the ray RS, Fig. 3, is inci- 
dent at various angles upon EB from the position rS, Fig. 4, 
to the position RS, Fig. 3, the refracted ray ST passes through 
the stratum AB at various thicknesses, and it therefore exhi- 
bits different colours corresponding to these thicknesses. 
When the ray has the position RS, the thickness of the film 
becomes so great, that the colours cease to be developed, and 
this limit of the production of colour is marked by parallel 
fringes gradually diminishing in breadth towards that limit. 
In like manner the ray rs being refracted in the direction s T, 
and falling upon AB at T, will pass through it at the same 
thickness as the ray RS does, and being reflected from the 
posterior surface of the stratum, will move in the direction 
TV, and emerge in the line XV. Fringes of the same charac- 
ter, but complementary to the former, will thus be produced by 
reflection, and from the equality of the angles STB, s TB and 
ATV, the fringes formed by transmission will be seen in the 
same direction XV as those seen by reflection. The reason is 
therefore manifest why the one set of fringes is seen by cover- 
ing the face EB, and the other set by covering the face BF, 
and why both sets are visible when only part of EB is covered. 
In order to show that these fringes are produced by the 
action of the crystallized film upon polarised light, I examined 
the phenomena in the following manner. As the double 
