406 



WRIGHT— POLARIZED LIGHT IN THE 



graphically in Fig. i. From these the pronounced effect of k in cut- 

 ting out and absorbing the transmitted light energy is clearly evident. 

 For any given thickness /, refractive index n, and absorption index k, 

 the amount of light absorbed can be obtained directly. From Fig. i 

 it is evident that for a plate only i micron thick and for k > 0.5 the 

 amount of transmitted light is almost negligible and the plate is prac- 

 tically opaque. The effect of using a plate %o as thick is the same 

 as that produced by a plate of the original thickness but with an 

 absorption index %o as large. Thus a plate of refractive index 

 n = 2, thickness / = 0.001 mm. and absorption index k = o.5 trans- 

 mits 10'" of the incident light while a plate of the same refractive 

 and absorption indices but ^0 as thick ( 0.000 1 mm.) transmits lO"^ of 

 the incident light. If we assume an intensity of illumination of 1,000 

 meter-candles and a threshold limit of vision o.ooi meter-candles, 

 then 10"*^ is the least amount of light which can be detected; under 

 these conditions for an ordinary rock section of thickness 0.02 mm., 

 equation (2b) reduces to n.K = o.OT,; therefore a mineral in a thin 

 rock section whose absorption index k exceeds 0.02, will appear per- 

 fectly opaque. The influence of even a very small absorption index 

 is therefore exceedingly great in absorbing light energy. 



TABLE I. 



In this table are listed the absorption indices k of absorbing crystal plates 

 of thickness / = o.ooi mm. and refractive index n Avhich transmit given quan- 

 tities ///o of the incident light. Thus a plate of this thickness, of refractive 

 index n = 2.0 and absorption index k = 0.030 transmits 0.25 of the incident 

 light. 



The characteristic feature of waves traversing an absorbing medium is 

 the decrease in their vibration amplitude with distance of penetration; the 

 waves are damped; the result is that for obliquely incident waves the vibra- 

 tion amplitudes along each wave front are not constant. In other words, the 



