72 



F. E. Wright — Oblique Illumination 



in 



index for the red end of the visible spectrum and a higher 

 index for the blue and green rays. 



Figs. 12 to 17 are drawn on the assumption that the mar- 

 gins of the immersed mineral grain are more or less wedge- 

 shaped (prismatic) or lenticular ; theoretically these two cases 



Fig 



. 14a. 



Fig. 



D Q 

 _J m 

 ID X 



RED 



BLUE 

 BLUE 

 RED 





< 



are not different because any warped or curved surface can be 

 considered as resolved into a succession of very minute plane 

 surface elements. In practical work the above assumption of 

 more or less wedge-shaped margins on mineral grains is valid 

 and the relations depicted in tigs. 12 to 17 obtain. In figs 12a 



Fig. 15. 



Fig. 16. 



and Ida the mineral has a higher refractive index than the sur- 

 rounding liquid and a beam of centrally incident parallel light 

 emerges from the grain as a convergent bundle, while in figs. 

 125, 13b a similarly incident beam emerges as a divergent 

 bundle. In case the microscope be first focussed on the margin 

 of the grain AB and then raised, the intensity of the field illu- 

 mination is found to change continuously — the rays being con- 

 centrated toward the center of the grain in figs. 12a and 13a 

 where the mineral has the higher index, while in figs. 12b and 



