220 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, No. 10 
distinguishing between cultivated and fine pearls must be based on some 
property that brings out the inhomogeneity between the foreign nucleus 
and the pearl periphery in the cultivated pearl. If the nucleus is a bead 
of mother-of-pearl the fact that the mother-of-pearl reflects the light strongly 
along a direction normal to the pearly layers and weakly along other direc- 
tions can be used as a criterion. Under proper conditions of illumination 
the characteristic mother-of-pearl sheen can be seen shining out as a sub- 
dued glow from the nacre bead of a cultivated pearl when held in certain 
positions. In transmitted light the corresponding differences in degree of 
transparency for directions normal and parallel with the pearly layers of 
the nacre bead can be used to reveal its presence. Under intense illumina- 
tion the banding of the mother-of-pearl nucleus gives rise to a series of 
bright lines on a dark field when the pear] is held in a certain position. In- 
tense illumination is obtained by use of a strong artificial light or the sun 
focussed by means of a condenser lens on the pearl. The pearl may be 
examined in air or immersed in a refractive liquid. Polarized light was 
found to be of no assistance except for cutting out glare due to extraneous 
light. 
A third method is a modification of the method of Galibourg and Ryziger 
for examining the walls of the hole drilled through the pearl. In place of 
the mercury column which they employed a bead made by fusing the end of 
a pure gold wire 0.2 mm. diameter is used as a reflecting mirror. By its 
use any irregularities or changes in the substance lining the hole in the pearl 
can be seen reflected by the gold bead when examined under proper condi- 
tions of illumination through a low power microscope or binocular. 
882ND MEETING 
The 882nd meeting was held jointly with the Washington Academy of 
Sciences, the Washington Society of Engineers, and the American Society 
for Steel Treating in the auditorium of the Interior Building on Saturday, 
March 31, 1923. 
The chair was taken by President Wuitrr of the Philosophical Society 
with 150 persons in attendance. 
Dr. WattreR Rosenuatn, F. R. 8., of the National Physical Laboratory, 
addressed the meeting on The structure and constitution of alloys. The 
address was illustrated with lantern slides and was followed by discussion. 
J. P. Auut, Recording Secretary. 
