in the Optical Phenomena of Mother of Pearl, 
of eminences and depressions, is capable of reflecting light 
with perfect accuracy. 
3. Since a particular configuration of surface, independent 
of chemical composition and crystalline structure, is capable 
of producing the most brilliant colours, may not the colours 
of ail natural bodies be owing to the arrangement of their 
superficial particles ; and may not the changes which these 
colours undergo by the action of light, heat, and atmospheri- 
cal causes, arise from a corresponding change in their super- 
ficial structure? — I have endeavoured to communicate to wax 
the faculty of producing colour possessed by Labrador spar, 
the metallic oxides, and various other bodies ; but though I 
have’^ not succeeded in this attempt, it by no means follows 
that the colour is not produced by the configuration of the 
surface. The structure mav in these cases be so minute, that 
fluid wax cannot be forced into the grooves or depressions ; 
and we have an approach to this delicacy of conformation in 
some specimens of mother of pearl, where the grooves cannot 
be seen by the most powerful microscopes. 
4. Since a particular structure of surface is always accompa- 
nied with a new repulsive force, residing nearer the body 
than the common repulsive force which produces ordinary 
reflection, may there not reside also, near the surface of all 
crystallised bodies, a new refractive force which produces 
double refraction? And is not this supposition countenanced by 
the fact, that the extraordinary pencil formed by Iceland spar 
suffers the ordinary as well as the extraordinary refraction ? 
I shall now conclude this section with a few remarks on 
the crimson and green light which always accompanies the 
primary coloured image. This mass of light is never pro- 
duced by the wax, and as it appears, even when the rays are 
3H2 
