284 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, No. 13 
homogeneity within the pearl. As a rule the liquid is unneces- 
sary and is rarely used because it may possibly stain the pearl. The 
method consists in sending a narrow beam of intense light into one 
side of the pearl and noting any differences in illumination, along differ- 
ent directions in the opposite half, which are due to a foreign nucleus 
such as as a mother-of-pearl bead. Adequate illumination is attained 
by imaging on the pearl an intense light source, such as a pointo- 
lite bulb or a small are or the sun, with the aid of a condensing lens 
lig. 1. A. Diametral section of cultivated pearl showing mother-of-pearl bead illumi- 
nated from the side. Magnification 5x. B. Section of the same pearl viewed in re- 
flected light. C. Cultivated pearl viewed in reflected light showing on the left the illum- 
inated spot due to the mother-of-pearl sheen. D. Natural pearl illuminated from the 
side. EH. Cultivated pearl illuminated by strong light from the rear. /’. Natural pearl 
photographed under the same conditions of illumination. 
(Fig. 2). If possible a vertical beam of light travelling upward is used. 
To avoid extraneous light, the pearl is placed on a thin sheet of metal 
directly over a small hole (c Fig. 2) drilled through the metal. The hole 
serves as an aperture, somewhat smaller in diameter than the pearl.‘ 
The intense beam of light passes through the aperture, impinges on the 
pearl, and illuminates its interior. The only light that reaches the 
‘A special piece of apparatus for the testing of pearls by this method is now being 
made available by the Bausch and Lomb Optical Company of Rochester, New York. 
