THE DETERMINATION OF REFRACTIVE INDEX 



231 



light l 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. 2 

 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 



\ \\ K\ \ 



Fig. 138. Contour Bands in Half Shadow 

 Illumination. 



Fig. 139. 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 less than that of the liquid the emerg- 

 ing rays diverge, yet here again only part of the rays can enter 

 the objective O; 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) 35 (1913), 63. 



2 Wright; J. Wash. Acad. 4 (1014), 389, suggests the use of safety razor blades 

 for the half shadow method of illumination. 



