74 F. E. Wright — Oblique Illumination in 



over solids enters the problem and gives rise to characteristic 

 color fringes along the margins of the mineral grain. Under 

 these conditions the mineral has a higher refractive index than 

 the liquid for the red rajs, while for the blue rays the liquid 

 has the higher refractive index, with the result that the red 

 rays emerge as a convergent bundle (figs. 12$ and 14$) while 

 the blue rays are divergent (fig. 125, 145). This difference in 

 behavior between the blue and red rays can be clearly seen 

 on properly immersed grains illuminated by a narrow central 

 cone of light from the condenser ; on raising the objective, a 

 red marginal line appears to move toward the center of the 

 grain while a similar blue marginal line moves out and away 

 from the center. 



These marginal fringes can be studied to still better advan- 

 tage on grains illuminated from a strong point source of light 

 (Nernst glower) placed approximately in the plane of the sub- 

 stage reflector, the condenser and reflector having been pre- 

 viously removed ; the dispersion colors obtained with this 

 arrangement are remarkably clear and beautiful. By shifting 

 the Nernst glower to one side oblique illumination can be 

 obtained and characteristic phenomena produced (fig. 145, 17C) 

 which are best shown by a low power objective (E.F. about 

 16 mm and N.A. 0*65). On shifting the Nernst glower away 

 from the axis of the microscope we soon reach a limit beyond 

 which direct light no longer enters the objective (limit of 

 angular aperture of objective in object field) ; this limit is 

 not sharp, but a hazy, transition edge, grading into the bright 

 field on the one side and into the black, non-illuminated field 

 on the other. It is in this semi-dark transition zone that 

 the phenomena of color dispersion are best observed. In fig. 

 145 the effect of the color dispersion between a mineral grain 

 and the enclosing liquid are illustrated (the drawings are 

 angle-true for the refractive indices given above on page 71) ; 

 the angle between the blue and red rays on emergence is 4° 57' 

 at the right margin of the grain and 3° 36' at its left margin. 

 In view of the small angle between these rays it might seem 

 difficult to explain why the dispersion colors are so clearly 

 marked along the margins of the grains. But the reason is 

 obvious when we note that these colors are practically confined 

 to the semi-dark zone between darkness and light in the field. 

 This narrow zone is itself less than 5° in width, and in it cor- 

 respondingly small angular differences between entrant rays 

 are rendered visible. Thus in the grain of 145 the red rays 

 from the left margin are less steeply inclined than the blue 

 rays and would accordingly be transmitted and visible through 

 an objective whose angular aperture is just large enough to 

 admit them' but too small to admit the blue rays. Under these 



