THE DETERMINATION OF REFRACTIVE INDEX 
231 
light ^ is obtained by swinging the mirror to one side when no 
condenser is employed, or by sliding a piece of black paper or 
card just below the condenser or by holding a finger just below 
the condenser so as to cut off about one-half the lower aperture. 
In the chemical microscope slide a piece of stiff black paper 
between the condenser and the ring attached to its lower part.^ 
The preparation on the stage will then be illuminated by oblique 
light. The phenomena resulting can best be understood by 
consulting Figs. 138 and 139, in which the indicated directions 
Fig. 139. Contour Bands in 
Half Shadow Illumination. 
Fig. 138. Contour Bands in Half Shadow 
Illumination. 
of the passage of light -rays have been greatly exaggerated. The 
crystal H has a higher refractive index than the liquid surround¬ 
ing it; the rays passing through are therefore convergent, but 
only those at the left can enter the objective O; hence, the left 
side is bright and the right side dark. But in the case of the 
crystal L whose index is lesfe than that of the liquid the emerg¬ 
ing rays diverge, yet here again only part of the rays can enter 
the objective 0; in this instance those on the right; thus the 
right side is bright: the left dark or in other words, the opposite 
of the phenomena observed with crystal H. 
Conducting our observations with the condenser only very 
slightly lowered and the paper diaphragm inserted from the left 
1 See Wright; Oblique Illumination in Petrographic Microscopic Work; Amer. 
J. Sci. (4) 36 (1913), 63. 
2 Wright; J. Wash. Acad. 4 (1914), 389, suggests the use of safety razor blades 
for the half shadow method of illumination. 
