THE PHYSICAL CHEMISTRY OF VISUAL PURPLE 



photocell ^yas a thin circular disc rotated by an electrical synchronous 

 motor at exactly 1 r.p.m. A concentric slot of 180°C cut in this disc 

 allowed the light transmitted by the visual purple to pass to the 

 photocell for half a minute and screened it for the following half- 

 minute. A galvanometer scale reading was taken 15 sec. after the 

 shutter had opened and a *zero' reading 15 sec. after it had closed. 

 The difference between these readings was proportional to the output 

 current of the photocell and hence proportional to the hght trans- 



60 



l^ 



40 



30 



20 



1 — I — I — I — j — I — I — I I 1 \ I I J ! !.. 



'inciden'riight (1) 



Finally transmitted (If) ^ 



J I 1 I I I 1 L 



120 

 Tinne (minutes) 



Fig. 3.3. Transmission/time curve of a visual purple solution when 

 bleached by a constant-intensity light. 



mitted by the visual purple solution. As the visual purple bleached, 

 its transmission increased and there were corresponding increases in 

 the galvanometer deflection. 



The transmission/time curve for the first visual purple solution 

 studied by this method is shown in Fig. 3.3. In this experiment the 

 bleaching was carried out with blue-green light (A = 506 mju). The 

 initial transmission of the solution, expressed in galvanometer scale 

 units, was 20-5 and the final transmission, i.e. after the solution had 

 been completely bleached by a prolonged exposure, was 57-0. The 

 intensity of fight actually incident on the solution (allowing for that 

 reflected from the front surfaces of the cell) was 66-9 in the same 



67 



