THE DETERMINATION OF REFRACTIVE INDEX 
229 
slide, beat it with a knife blade until full of air bubbles. Cover 
with a cover glass and place upon the stage of the microscope. 
Use an 8-millimeter objective and center a tiny air bubble whose 
image appears to be not over i to 2 millimeters in diameter. 
Focus sharply. The image obtained will consist of a tiny disk 
of light surrounded by a black ring. We are here dealing with a 
sphere of less refractive index (air n = 1) surrounded by a liquid 
of higher refractive index (gum solution or water n = 1.3 +). 
Remove the condenser and slowly swing the mirror to one side, 
looking into the microscope at the same time. As the light 
becomes oblique the bright disk in the image of the air bubble 
moves in the opposite direction from the movement of the mirror. 
Move the mirror back and the reverse phenomenon will be 
observed. A^hen the light is exactly axial the bright spot will 
be exactly at the center of the black circle This constitutes 
one of the simplest and best tests for axial light that we possess. 
Now slowly raise the objective; the bright disk will be seen to 
grow larger and larger and the black ring will appear to move 
outward and the disk will become indistinguishable before the 
surrounding ring vanishes. 
Take a drop of water and mix very thoroughly with it by gentle 
beating a tiny.droplet of oil. There are thus obtained tiny 
spheres of oil of a refractive index higher (oil # = 1.4 +) than 
that of the surrounding liquid (water n = 1.33). Again we 
obtain as the image of a tiny globule, a bright disk surrounded 
by a dark ring. With axial light this disk is concentric; with 
oblique light eccentric. As the mirror is swung aside the disk 
of light in the image appears to move in the same direction as 
the mirror. Upon raising the objective the disk of light grows 
smaller and smaller, the black annular contour band appears to 
move inward and the bright spot is the last to disappear. These 
phenomena are readily interpreted by referring to the diagrams. 
Figs. 136 and 137. With air, n < liquid, the emerging rays are 
diverging; with oil, n > liquid, the emerging rays are converg¬ 
ing. In Fig. 137 the solid line arrows indicate the direction of 
the moving mirror, while the dotted line arrows that of the cor¬ 
responding direction of movement of the disk of light. These 
