410 Dr. Brewster on new Properties of light exhibited 
structure, these grooves or lines are always parallel, but when 
there is any irregularity of configuration, the grooves vary 
their direction, and are arranged in all possible forms like the 
veins of agate, or like the lines upon the coast of a map, where 
there are numerous inlets and islands to be represented. 
Sometimes the spaces between the grooves are so wide, that 
they can be seen with a magnifying power of six or eight times, 
and in one or two specimens I have observed them with the 
naked eye. At some parts of the surface the distance between 
the grooves is so small, that I have counted more than 3000 in 
an inch ; and in some pieces they can scarcely be detected 
with any magnifying power which I have been able to apply. 
When the space between the grooves is large, a new groove 
often commences, and there is frequently a sudden change 
from a space with a series of distant grooves, to another space 
with a series of very close ones. Similar appearances were 
also seen in the structure of pearls. When the mother of pearl 
is scratched or indented, the bottom and the sides of the scratch 
are grooved exactly like the parts that are polished. The 
same grooved structure is likewise distinctly seen in wax, gum 
Arabic, and the metals, after they have received the impression 
of the mother of pearl. 
In every case the grooves are at right angles to the axis of 
extraordinary reflection, and hence in irregularly formed mother 
of pearl, where the grooves are often circular, and have every 
possible direction, the axes of extraordinary reflection have 
also every possible direction, and the coloured images appear 
irregularly scattered round the ordinary image. In the real 
pearl, these coloured images are crowded into a small space 
round the common image, on account of the spherical form of 
