404 Dr, Brewster o?i new Properties of light exhibited 
The fracture of mother of pearl always reflects the extra- 
ordinary ray rs, as if the surface of the fracture were parallel 
to the real surface ; but when the fracture is ground flat, no 
extraordinary reflection takes place. 
When the extraordinary ray rs is reflected from another 
piece of mother of pearl, it experiences, as might have been 
expected, both an ordinary and an extraordinary reflection. In 
virtue of the ordinary reflection, an image is formed exactly 
like the extraordinary image, but in virtue of the extraordi- 
nary reflection the highly coloured image is sometimes ren- 
dered more highly coloured, and at other times converted 
into a greenish white image, according as the second reflection 
conspires with or opposes the first. 
Hitherto we have attended to the phenomena only when 
the surface is rough and unpolished. When a slight degree 
of polish, however, is communicated to it, a new coloured 
image appears on the opposite side of the common image 
formed by the rays rt, fig. 1 and 2. This new image resembles 
in every respect the other coloured image, and follows the 
same laws ; and after a high degree of polish is induced upon 
the mother of pearl, it is almost as bright as the first coloured 
image which has its brilliancy somewhat impaired by polish- 
ing. If the polish is removed by grinding, the second coloured 
image vanishes, and the first resumes its former brilliancy. 
As this second image is reflected towards the pole Ain fig. 1. 
and the pole B in fig. 2, they may be called the secondary poles 
of extraordinary reflection. 
, If we now examine the light transmitted by the mother of 
pearl, we shall perceive phenomena analogous to those which 
liave been described. A coloured image will appear on each 
