ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
267 
Between crossed nicols fig. 1 appears as fig. 3 * with a black cross, 
fig. 2 as fig. 4a or 4 b, dark or clear, according as the long “ Scliliere ” 
is parallel or inclined to one of the directions of vibration of the nicols. 
The distribution of the directions of extinction for the individual 
points of the drop is given by the curves represented in figs. 5 and 6. 
They correspond to the electric level surfaces and lines of flow in a 
conducting sphere in which the current enters and emerges at the ex- 
tremities of a diameter. Regarding a drop as composed of uniaxial 
crystal particles, the optic axes must be considered as arranged in posi- 
tions corresponding to the lines of flow, and the equatorial planes as 
representing the level surfaces. An optically uniaxial crystal in which 
the limit of elasticity is zero must assume such a form. For the surface 
tension between crystal and liquid will possess different values on the 
different crystal faces; but since there is no counteracting elastic 
force, the molecules will change their positions until the surface tension 
has everywhere the same value, and that the smallest possible, since the 
potential surface energy tends to a minimum. The molecules will 
accordingly arrange themselves so that they all present the same side 
outwards. 
By the use of the polarizer alone indications of dichroism are ob- 
tained. Thus, when the long “ Schliere ” is parallel to the short diagonal 
of the nicol, the crystals appear colourless and with faint outline ; when 
at right angles, yellow and sharply defined. 
Deformation of crystal drops. — By pressing the drop between stage 
and cover-glass the principal form (fig. 1) changes to that shown in 
fig. 7. In this the “ Schlieren ” have given place to a sharp nucleus at 
the centre and another around the circumference of the disc. Between 
crossed nicols the appearance is the same as before, except that the 
quadrants show colour differences. By suitable pressure on the cover- 
glass the central nucleus can be made to approach the edge and finally 
to take up a position between the broken ends of the marginal nucleus 
(fig. 8). By further deformation the two nuclei pass through various 
transitions until the symmetrical form of fig. 9 is obtained. By then 
diminishing the pressure until the drop once more assumes the spherical 
shape, the simple form of fig. 2 is obtained. Thus the total effect of 
the series of changes has been to turn the drop through 90° about a 
horizontal axis. 
More exact observation with greater magnifying power showed the 
peculiarity in the spaces between the two nuclei seen in fig. 10. This 
was found to be due to a rotation of the whole drop in a direction 
contrary to the hands of a watch. The rotation was so much more rapid, 
the greater the difference in temperature between stage and cover-glass. 
On account of the friction of the glass and because the rotating force 
mainly affected the circumference, the outer layers became distorted 
with respect to the inner. Fig. 11 represents a much twisted drop 
produced in this way. Between crossed nicols it showed concentric 
dark rings with alternating white and yellow ones. 
With very rapid rotation the ends of the nuclei bend towards the 
* The punctuation in the figures denotes, according to thickness, pale to dark 
yellow. 
