THE VISUAL PIGMENTS 



units. Thus the initial optical density was 0-51 (logjo 66-9/20-5) and 

 that of the fully-bleached solution, 0-07. 



ANALYSIS OF TRANSMISSION/tIME CURVES 



When a solution of visual purple is 'bleached' its transmission may 

 decrease or increase, depending on the wavelength of light used for 

 measurement. Since the bleaching results in the formation of a 

 yellow product, the transmissivity to violet light (for example) will 

 decrease while to light of longer wavelength it will increase, as 

 bleaching proceeds. In alkahne solution absorption by the yellow 

 product is very small for wavelengths greater than 460 m// and, when 

 the bleaching light is of this, or longer, wavelength the visual purple 

 can be regarded as bleaching to a colourless product. 



It is convenient to consider first the case where the product of 

 bleaching does not appreciably absorb light of the wavelength used 

 for bleaching and to proceed therefrom to the general case. The 

 following symbols and definitions are used : 



/ intensity of light, expressed as number of quanta per second 



per square centimetre, incident on the solution; 

 I^ intensity of light transmitted by the solution at any given 



time, t\ 

 /f intensity of light absorbed by the solution at the time, t\ 

 If intensity of Hght finally transmitted by the solution, i.e. after 



complete bleaching; 

 t time (in seconds) from the beginning of the exposure ; 

 / internal length (cm) of the optical cell ; 

 A exposed area (cm^) of the solution; 

 V volume (cm^) of the solution ; 

 n number of centres of absorption (chromophores) in the volume, 



F, of the solute; 

 c concentration of chromophores; thus c = «/F; 

 y quantum efficiency, a ratio defined as 



number of chromophores destroyed , 

 number of quanta absorbed 



a extinction coefficient per chromophore, expressed in cm^ and 

 defined by the equation 



log, II It = CLCL 



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