39^ -Dr. Brewster on new Properties of light exhibited 
mother of pearl from every other substance, and the succes- 
sive developement of fresh tints, by every gentle inclination 
of the plate, have always been ascribed to the lamellated 
structure of the shell, and have been regarded as a fine proof 
of Newton's theory of the colours of natural bodies. This 
explanation I had no disposition to call in question, nor did the 
general train of my experiments lead me to such an inquiry. 
In examining the coloured rings which mother of pearl, like 
the topaz, exhibits by polarised light; and in ascertaining the 
relation between its refractive power, and the angle at which 
it gi^^es polarity to the reflected ray, I was under the necessity 
of grinding and polishing, with the utmost care, various plates 
of this substance. The developement of new colours, and the 
extinction of others which took place during these processes, 
indicated the operation of some unknown and extraordinary 
cause, and encouraged me to pay the most minute attention to 
all the phenomena which were presented. The results of this 
investigation I shall endeavour to explain under the four 
following heads. 
I. On the optical properties peculiar to mother of pearl. 
II. On the communication of these properties to other 
bodies, 
III. On the causes by which these phenomena are produced. 
IV. On a new species of polarisation peculiar to mother of 
pearl. 
I. On the Optical Properties peculiar to Mother of Pearl, 
Mother of pearl sometimes possesses a regular, and some- 
times'an irregular structure, and has a striking resemblance 
to the agate, in the immense variety of forms which it exhibits. 
