405 
in the Optical Phenomena of Mother of PearU 
a point s in the extraordinary ray rs will describe a curve 
round S composed of two semielli pses having the same conj ugate 
axis, but having their semitransverse axes of different lengths. 
Thus, in fig. 3, if S be a point in the reflected ray ;*S ( fig. 2), 
and Ss, SN, S^, SM, different values of the angles of aberration 
when the pole A is in the directions S^, SN, &c. ; then the 
point s will describe the semiellipse M^N, and the semiellipse 
M/N which have their conjugate axis MN common, and their 
semitransverse axes S5, of unequal lengths. The conjugate 
axis MN is a constant quantity, while S5, St vary according to 
the law already mentioned. At different angles of incidence, 
therefore, the point s will describe other curves such as M/W, 
which approach to a circle as the angle of incidence dimi- 
nishes. 
The angles of aberration vary in different pieces of mother 
of pearl, but there is no deviation from the laws which have 
just been explained. 
On the outside of the extraordinary ray rs, fig. 1, a mass of 
coloured light rp makes its appearance nearly at the same 
distance from the extraordinary image, that the extraordinary 
image is from the common image : these three images are 
always in a straight line, but the angle of aberration of the 
mass of coloured light varies according to a law different from 
that of the extraordinary ray. At great angles of incidence, 
this mass of light is of a beautiful crimson colour. At an angle 
of about 37® it becomes green, and at less angles of incidence 
it acquires a yellow hue, approaching to white, and becomes 
very luminous. These colours which become more brilliant 
when the mother of pearl is polished, vary with the thickness 
of the plate. 
