JULY 19, 1923 WRIGHT: DISTINGUISHING CULTIVATED PEARLS 285 
observer is that which passes throught the pearl. Any differences in 
degree of transmission between center and periphery of the pearl are 
then clearly visible and enable the observer to see the shadow of any 
foreign nucleus. If the nucleus consists of mother-of-pearl the trans- 
parency differences for different directions of transmission through the 
bead can be observed on rotating the pearl. In natural pearls the 
central part appears opaque because of the high reflecting power normal 
to the concentric lamellae; but this central disk is not so large as the 
bead of a cultivated pearl and does not change its appearance as the 
pearl is rotated. 
Under these conditions of illumination the pearl can be examined 
by the observer looking at it from any direction, either facing the 
source of light or at right angles to the incident beam or from any 
NX 
Nal 
Fig. 2. Simple device for distinguishing cultivated pearls from natural pearls. S. 
strong point source of light; L. aspheric condenser lens; R. reflecting mirror; B. metal 
plate in which a small hole 3mm. diameter has been drilled. Above this aperture, which 
can be made of different size by means of a sliding stop diaphragm, the pearl, P, rests. 
intermediate position (Fig. 1,e.f). Itis advantageous thus to examine 
the pearl end on and from the side because, as it is turned about, 
certain differences in homogeneity are more readily seen along one 
direction than another. Under these conditions any flaws or imper- 
fections in the pearl} whether natural or cultivated, are clearly shown. 
Cultivated pearls exhibit many imperfections and patches of different 
reflecting power and degree of transparency. Many natural pearls 
show minute spots and irregularities, but the best pearls are free from 
flaws of any kind. 
3. Examination of the walls of the hols drilled through the pearl. 
Illuminated by a strong beam of light from the side, the walls of the 
hole drilled through the pear! exhibit the boundary between the outer 
pearl substance and the mother-of-pearl nucleus. This is rendered 
visible not only by a difference in intensity of illuraination but also 
in color; the pearl shells are noticeably blue in color while the nucleus 
