400 Dr, Brf.wster on new Properties of light exhibited 
angle of incidence, the coloured ray rs will have a motion of 
rotation about rS, the common section of the plane srS and 
the surhice of the mother of pearl being invariable. 
The line AB may be called the axis of extraordinary reflection; 
the extremity A towards which the coloured ray rs is reflected, 
the primary pole of extraordinary reflection ; srC the angle of 
extraordinary reflection ; and ^rS the angle of aberration. 
If the ray Rr is now reflected from the opposite surface of 
the mother of pearl as represented in Fig. 2. the same pheno- 
mena will be observed ; but the coloured ray rs will now be 
reflected towards B, and will be seen at s' above the common 
image S', being formed by rays reflected in the direction rs. 
The extremity B therefore of the axis AB will be the primary 
pole of extraordinary reflection for the lower surface. Hence 
the two surfaces of mother of pearl have always their poles in 
opposite directions, unless in specimens where a change of 
structure takes place. 
Let the plate AB be now brought into the position in Fig. 1. 
where the plane r^S coincides with the plane of ordinary re- 
flection RrS, and let it be placed upon a goniometer so that we 
may ascertain by measurement the changes which take place 
by varying the angle of incidence RrC. It will then be found 
that the angle of aberration 57S regularly increases with the 
angle of incidence. The variations which it undergoes are 
represented with tolerable accuracy in the following table, but 
owing to the elongation and indistinctness of the coloured 
image at large angles of incidence, the measures are not 
susceptible of great correctness. The first column contains 
the angle of incidence; the second, the complement of that 
angle ; the third, the angle of aberration as determined by 
