colours which accompany them in calcareous spar. 283 
that of a and B. These colours are therefore produced by 
' the transmission of polarised light through the crystallized 
film AB, Fig. 4. The light is first polarised by the prism 
ABE : it is then separated into its complementary colours by 
the crystallized film AB, and this compound beam is analyzed 
by the second prism ABF. This arrangement, indeed, is the 
very same as that which I have described in a former paper,* 
as necessary for the exhibition of the complementary colours, 
the light being polarised by double refraction instead of by 
reflection, and being analyzed by a prism of calcareous spar, 
instead of a plate of agate. 
In order to put this explanation to the test of direct expe- 
riment, I cut a rhomboid into two prisms ABE, ABF, Fig. 4, 
having equal refracting angles ; and I interposed a thin plate 
of sulphate of lime between the two prisms. When the light 
was incident on the first surface EB at an angle of between 12 0 
and 14 0 , so that the two images a, B, had vanished, I shifted the 
sulphate of lime till it ceased to depolarise the light, or restore 
the vanished images a and B. I then cemented it in this position 
to the two prisms, and thus obtained an artificial rhomboid, 
which imitated with the utmost exactness all the phenomena 
of the natural one. The extreme images a, B, became coloured, 
while the double image A h remained white, and the colours 
varied by varying the inclination of the plate to the incident 
rays. The images a and B approached to, and receded from, 
the middle image as in the natural crystal, and at particular 
incidences the middle image exhibited colours complementary 
to those of the extreme images, and of the very same kind 
with those in the natural rhomboid. 
Phil. Trans, for 1814, p. 210. 
O O 3 
