in the Optical Phenomena of Mother of Pearl. 407 
In order to shew that in these cases no part of the mother 
of pearl is detached, I plunged the wax, after it had received 
the impression, into nitric acid, which had no effect either in 
destroying or diminishing the colorific property of the sur- 
face. In soft cements made of bees' wax and rosin, the 
slightest degree of heat destroyed the superficial configura- 
tion by which the colour is produced. In sealing wax, gum 
Arabic and realgar, a much greater heat was requisite to re- 
move the colour; and in tinfoil and lead this could only be 
effected by the temperature at which they cease to become 
solid. 
Let us now examine more minutely the phenomena which 
present themselves when the light is reflected from the sur- 
face of wax ; and let us suppose that the impression upon the 
wax is made by the lower surface, when rough, of the mother 
of pearl, as represented in fig. 2, where B is its primary, and 
A its secondary pole. When the light Rr of a candle is re- 
flected from the surface of wax AB, fig. 3, the extraordinary 
image, instead of being reflected towards the primary pole B 
as in fig. 2, is reflected from it, and A is the primary pole of 
the wax, whereas B was the primary pole of the mother of 
pearl. By polishing the mother of pearl, and taking a new 
impression from it, the wax will now reflect the other extra- 
ordinary image in the direction rt, and therefore B is the se- 
condary pole of the wax. Hence it follows that mother of pearl 
communicates to wax and other bodies the optical properties of the 
surface opposite to that from which the impression is taken. 
At different angles of incidence the two coloured images 
formed by the wax follow the same laws as those produced 
by the mother of pearl ; but the mass of green and crimson 
3 G 2 
