JULY 19, 1923 WRIGHT: DISTINGUISHING CULTIVATED PEARLS 283 
to the pearly layers. The iridescence (luster, orient) of the pearl is 
due to interference of waves of light at the different pearly layers’ 
which are remarkably uniform in thickness; the combined thickness of 
the carbonate layer and the conchiolin layer is 0.0004 to 0.0006 mm. or 
about equal to a wave length of ight. At the surface of each layer 
some light is reflected and this interferes with a certain part of the 
meident light. The final result is the reflection of a relatively large 
amount of light and a correspondingly low transmission of the mother- 
of-pearl for rays of light incident normal to the pearly surface. The 
reflecting power on sections normal to the layers is appreciably less 
and the transmission is relatively much higher. This difference in 
reflecting power and in transparency with direction is easily seen on a 
bead of mother-of-pearl. Held in one position the characteristic pearly 
luster appears; turned through 90° the luster is less and the bead is 
noticeably more transparent. In strong sunlight this difference is still 
more striking. If now the bead of mother-of-pearl is enclosed in con- 
centric layers of pearly substance, the lack of transparency of these 
layers, especially when viewed along a diametral direction, tends to 
mask the mother-of-pearl phenorhnena; but if the cultivated pearl be 
viewed under proper conditions of illumination the phenomena charac- 
teristic of mother-of-pearl are readily seen. 
1. Test in reflected light. To test a pearl by this method examine the 
pearl first in reflected light. Stand with the back to the window, to 
the sun, or to some strong source of light. Hold the pearl so that it is 
illuminated by rays from the rear and observe the change in intensity 
of reflected light as the pearl is rotated. This rotation is accomplished 
most readily if the pearl is mounted on a string or a piece of thin wire. 
At the position for which the characteristic mother-of-pearl sheen is 
reflected by the nacre-bead, this sheen is clearly visible shining out from 
inside the pearl (Fig. 1c). It appears again on rotation of the pearl 
through 180°. After a little practice the eye catches quickly this 
phenomenon. Its appearance, which adds to the interest of the pearl, 
brands the pearl definitely as a cultivated pearl with a mother-of- 
pearl center. . 
2. Test in transmitted light. The pearl is examined either in air 
or while immersed in a liquid, such as water; the purpose of the 
liquid is to reduce to a minimum the amount of light reflected at 
the surface and thus to render more easily visible any lack of 
’ Optics. Sir David Brewster, pp. 137-149. 1853: A H. Pfund, The colors of mother- 
of-pearl. J. Franklin Inst., 453-464. 1917. 
