THE DETERMINATION OF REFRACTIVE INDEX 



189 



light. The phenomena resulting can best be understood by 

 consulting Figs. 108 and 109, in which the indicated directions of 

 the passage of light rays have been greatly exaggerated. The 

 crystal H has a higher refractive index than the liquid surround- 



FIG. 108. Contour Bands in Half Shadow 

 Illumination. 



FIG. 109. Contour Bands in 

 Half Shadow Illumination. 



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 less 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 

 until the dark shadow extends approximately to the center of 

 the field, the phenomena seen will be as indicated in Fig. 109. 

 The crystal H of higher index than the liquid appears dark on 

 the dark side of the field and bright on the light side of the 

 field; but the crystal fragment L of lower index than the liquid 

 appears bright on the dark side of the field and dark on the bright 

 side of the field. This is as it should be from Fig. 108, since in 

 the image formed in the microscope the directions are reversed. 



If we now lower the condenser a reversal of all the above 

 phenomena takes place. It is therefore always wise to check 

 the results recorded with condenser raised by lowering the con- 



