413 
in the Optical Phenomena of Mother of Pearl. 
Angles of incidence. Inclination of the faces m or h. 
00 
0 
1 
1 
to 
0 
0 
00 
70 
15 9 
65 
13 4.8 
bo - 
12 19 
12 - 
1 20 
But since the refraction cannot be completed when the ray 
reaches the surfaces m or ;z, having passed through only half 
the space of refracting activity, let us suppose the value of m 
corresponding to the partial refraction to be 1.300, and we 
shall have the following values of x: 
Angles of incidence. Inclination of the faces m» n. 
86° 4T 
- 
13*34' 
70 
- 
9 16 
65 
- 
8 19 
60 
- 
7 14 
12 
0 20 
If the light, therefore, suffers either a total or a partial refrac- 
tion, or if it suffers no refraction at all, the extraordinary 
reflection must be made from faces of variable curvature. 
The most unaccountable circumstance, however, accompany- 
ing the extraordinary reflection, is the difference of effect 
produced upon the surfaces m and ?i by removing the polish. 
The surface m retains its power of reflecting the primary 
extraordinary image, notwitlistanding tlie roughness which is 
thus superinduced, wliile the surface n loses the power of 
reflecting the secondary image, and acquires the faculty of 
transmitting the whole of the coloured pencil which composes 
it. The force therefore which reflects the primary extraordinary 
image would appear to be different from that which reflects 
the secondary extraordinary image, the latter being wholly 
dependant on the smoothness of the surface. 
3H 
MDCDCXIV. 
