228 
ELEMENTARY CHEMICAL MICROSCOPY 
ration will probably be flooded with light. Close the diaphragm 
two-thirds or even more. If the crystal fragments are not now 
clear and distinct with sharply defined contours lower the con¬ 
denser a trifle, but only a trifle. It is of course possible that in 
selecting the liquid one of the same refractive index as that of 
the solid may have been chosen; this is, however, very unlikely. 
Contour bands appear whether the solid has either a higher or 
a lower index than the surrounding liquid. The next step must 
therefore be to ascertain whether it is higher or lower than the 
liquid employed. This is accomplished by slowly raising the 
microscope tube by means of the coarse adjustment, at the same 
time closely observing the change in appearance, direction of 
motion or change in color of the contour bands and halo-like band 
of light bounding the crystal fragments. When the solid pos¬ 
sesses a higher index than that of the liquid, the contours are 
usually dark and well defined with a halo or band of light within 
the black bands; as the microscope tube is raised this band of 
light will appear to move inward, i.e., toward the center of the solid. 
If, on the other hand, the solid possesses a lower index of refrac¬ 
tion, the black contours are relatively weak, with the bright 
halo outside the black bands, and upon raising the objective the 
band of light or bright halo appears to move outward or awayfron, 
the center. This difference in behavior is due to the fact that 
when the fragment has a higher refractive index than the liquid 
it causes the rays leaving it to converge, but if the solid has a 
lower refractive index the emerging rays are divergent. In order 
to obtain the best results by this method, always screen the 
preparation upon the stage with the hand; thus none but trans¬ 
mitted light rays can enter the objective. 
By employing oblique instead of axial light it becomes still 
easier ^ to determine whether the solid possesses a higher or a 
lower refractive index than the liquid in which it is immersed. 
Before considering the method of procedure in this case let us 
study several simple yet instructive experiments.^ 
Place a small drop of mucilage or thin gum upon an object 
1 Schroeder van der Kolk, Zeit. anal. Chem., 38 (1899), 615. 
^ Gage, The Microscope (1920), p. 112, 13th ed., Ithaca, N. Y. 
