THE DETERMINATION OF REFRACTIVE INDEX 



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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 be- 

 comes 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. When 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 



FIG. 106. Oil Globule and Air Bubble illuminated with Axial Light. (Gage.) 



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. 106 and 

 107. With air, n < liquid, the emerging rays are divergent; 



