.. w 
\ 
412 Dr, Brewster on new Properties of light exhibited 
that the primary extraordinary image is reflected from the 
face m, while the secondary extraordinary image is reflected 
from the face n. Now this could only happen from two causes, 
either in consequence of the mother of pearl having a repul- 
sive force different from the ordinary repulsive force which 
produces reflection ; or from its possessing the power of 
reflecting light from its actual surface. That this extraordi- 
nary force is, in other respects like the ordinary reflecting 
force, is manifest from a portion of the extraordinary pencil 
being transmitted, while the other portion suffers reflection. 
The existence of such a force being unquestionable, we have 
next to consider the form and position of the surfaces to which 
it belongs. The changes in the angle of aberration at different 
angles of incidence, is a proof that the surfaces m, w, present 
different inclinations to the incident ray ; and hence their form 
must becurvilineal, as represented by the dotted lines above m 
and 71 . If we suppose that the ray has been refracted before 
it experiences the extraordinary reflection, the angles of 
aberration still require that the faces have a curved form. 
Taking the index of refraction m = 1.653 and making A == 
angle of incidence ; a = angle of refraction, and consequently 
the new angle of incidence upon the faces m,ox n\ b = angle 
of extraordinary reflection, and %= the inclination of the 
reflecting face tu or n, then we shall have 
. sin. A 1 a + b b — a 
Sin. a = and % = — a = 
m 22 
By calculating the value of z for different angles of incidence, 
we obtain the following results : 
