344 merwin: chromatic reflection in covellite 



and violet light is reflected;^ 15 percent of the green; 12 of the 

 yellow; 5 of the orange; and none of the red. The resulting 

 color is blue. At angles less than 60° the colors are reflected in 

 about the same proportions as at 60°. At larger angles blue and 

 violet are less dominant. 



Light incident at 60° from water has a little more than half as 

 much of its green, blue, and violet reflected as in case of air; and 

 all of the red and about half of the orange is entirely reflected, be- 

 cause the angle of incidence is greater than the critical angle. 

 Yellow is reflected scarcely at all because for it, water and covel- 

 lite have nearly the same refractive index. As the angle of 

 incidence decreases orange and then red cease to be totally re- 

 flected, but the relative proportions of the other colors do not 

 change greatly. For more oblique rays total reflection extends 

 a little further into the orange. The total color effect is purple. 



Similarly, oblique reflection from covellite in benzene (n = 

 1.50) contains still more of the colors of the red end of the spec- 

 trum and less of the violet end, and in methylene iodide {n = 1.74) 

 orange is the predominating color. In the latter at 60° only 

 about 5 per cent of the blue and violet are reflected as against 

 20 per cent in air. 



The light reflected in air from surfaces perpendicular to the 

 cleavage is blue, but brighter than from those parallel to the 

 cleavage. This stronger reflection indicates higher refraction 

 for at least the red end of the spectrum. Red is totally reflected 

 at large angles of incidence in liquids of higher refraction than 

 1.64. Therefore the index of refraction €Li is about 1.5. A 

 large amount of blue is also reflected (not totally) in liquids 

 of even higher refraction, indicating that e for blue must be 

 considerably higher than 1.75. Thus throughout the visible 

 spectrum e>co. The differences in color of the light reflected 

 parallel and perpendicular to the cleavage are independent of 

 the pleochroism observed in transmitted light. 



* Calculated by the formula for transparent substances, for it has already 

 been shown that absorption in this case effects the reflection of different colors 

 about the same amount. The fraction reflected 



_ 1 jsin- {i — r) tan^ (i — r)) 



~ ^ I sh^(i + r) "^ tan2~(T+7y } 

 where i and r are the angles of incidence and of refraction, and sin i = n ain/'- 



