colours which accompany them in calcareous spar. 287 
the resulting pencil VX with a prism of calcareous spar, we 
shall find certain positions of the prism in which the fringes 
are invisible. In some positions they are finely displayed 
across the first image, and are not visible across the second ; 
while in other positions the fringes across the first image 
vanish, and appear distinctly across the second. All these 
phenomena arise from the alternate evanescence of the images, 
which causes the fringes to be seen across the remaining 
image formed by light polarised in one plane. 
The explanation which has now been given of the iridescent 
phenomena of calcareous spar, enables us to account for the 
origin of the colours peculiar to the agate, which I have de- 
scribed in a former Paper.* These colours always appear in 
veined agate, and are, undoubtedly, produced by the inter- 
position of a vein between two equiangular prisms. 
Sect. V. Description of new instruments for exhibiting comple- 
mentary colours. 
A simple instrument for exhibiting the opposite or comple- 
mentary colours has long been a desideratum in the arts, as 
well as in the sciences. To painters, and to artists of almost 
every description, it is of very extensive use, while in many 
optical inquiries its advantages cannot be sufficiently appre- 
ciated. 
The method of showing these colours which I have pointed 
out in a former Paper, consists in the separation of polarised 
light into two pencils, by the action of a crystallized plate, 
and in the subsequent analysis of the pencil, by a doubly re- 
fracting crystal. The simplest way of fitting up an instru- 
* See Philosophical Transactions, 1813, p, 102, 103, and 1814. p. 197, 199. 
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