404 WRIGHT— POLARIZED LIGHT IN THE 



density of a layer of unit thickness^) ; since the intensity varies with 

 the square of the ampHtude of vibration of the transmitted Hght, the 

 equation is vahd 



A = Aoe ^° ' = Aoe ^ "' , (2) 



in which A^ is the amphtude in vacuo. A, amplitude in the medium 

 after passage through thickness I, X, wave-length in material, Aq, 

 wave-length in vacuo, k, the absorption coefificient, k, the absorption 

 index. But 



therefore 



/\ I - — ■<./ --—-kj 



=— = gA =g^0 = g-ml. ^2a) 



k = HK and m = 



Xo X 



The significance of the absorption index can best be realized by a 

 transformation of equation (2a) 



luy = — l-n- K 

 I Ao 



or expressed in ordinary logarithms 



Jo 47r- 043429 

 logio^ = r l-n-K. 



1 Ao 



- The significance of these terms is clearly expressed in photography in 

 which the relative densities of the photographic plate serve to record more 

 or less accurately the relative intensities of the light impinging upon it. The 

 incident light produces changes of such nature in the silver bromide particles 

 in the emulsion that after development the exposed particles remain as opaque 

 specks of silver. The more intense the incident light, the greater the number 

 of affected silver particles. In the developed plate the more silver specks 

 there are per given area, the more light is stopped, and the more opaque and 

 dense is the plate. Density is measured in number of silver particles per 

 unit area (total weight of silver particles per unit area). If a layer is of 

 such density that it transmits the fraction a of the incident light, then two 

 such layers superimposed transmit aa or a~ of the light, three layers aaa or 

 o^; thus the density increases in arithmetical, but the transmission decreases 

 in geometrical progression. If ///o = percentage of light transmitted (=:f'''" 

 equation (i)) the reciprocal, h/I is the opacity = ^'"' and the density 

 Z) = log opacity or ml or simply m for /= i. A plate is said to have unit 

 density Z? =: i = log opacity = log /o//=:log 10, i.e., when it transmits only 

 Mo of the incident light. Similarly a crystal plate of absorption-modulus i 

 and of unit thickness transmits only Mo of the incident monochromatic light. 



