in the Optical Phenomena of Mother of Pearl. 411 
the pearl, and the various hues are thus blended into a white 
unformed light, which gives to this substance its high value 
as an ornament. 
Having thus ascertained that the surface of mother of pearl 
is actually grooved, and different in this respect from all other 
bodies that have yet been examined, we must now seek, in this 
peculiarity of structure, the cause of its optical properties. 
The facts which have been detailed in the beginning of this 
letter will enable us to draw several important conclusions of 
a general nature, but they leave us in the dark respecting the 
immediate cause of the phenomena. 
Let us now suppose that fig. 5. represents a section of a 
plate of mother of pearl, having Kanmhcl^> for its upper sur- 
face, and Wa*n'm'b'c'A! for its lower surface. Let OP be the 
line at which the attracting or refractive force ends, and where 
the repulsive or reflecting force begins, and let the reflecting 
force terminate at MN according to the Newtonian theory. 
We have already seen that when the surface AB of the mother 
of pearl is ground as flat as possible, and brought to a high 
polish, the light which is incident on the repulsive stratum 
MNOP is reflected as in all other bodies, and affords a 
perfect image of the object from which it radiates. Hence it 
follows that the light which forms the extraordinary images 
has escaped reflection, and penetrated the attractive stratum 
OP; and that its separation into colours and extraordinary 
reflection are produced by one or more causes residing between 
OP and the surface AB of the mother of pearl. 
Let us first attend to the aberration of the extraordinary 
images. Since the real surface of AB is composed of faces 
inclined to the general surface AbcB, we are led to suppose 
