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 = i) 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. /Vhen 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 n = 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 



