C. 5. FRENCH 



457 



the same convenience thai commercial instruments record absorption 

 sjiectra. A block diagram is given in Fig. 8. 



The principle oi the action spectrophotometer is to use mono- 

 chromatic light slowly changing in wavelength and having its in- 

 tensity continuously adjusted to keep the rate of photosynthesis at a 

 constant value as the spectrum is traversed. This intensity adjust- 

 ment is done automatically by a servo-driven optical wedge con- 



Photosyn+hcsis Action Spec+rophofomclcr 

 Operating principles 



Wavelength sweep 



CH 



Lamp 



Moro- 

 chromafor 



Servo adjusts 

 intensity to keep 



ptiotosynthesis rate . 



Amplifier 



IS 



± 



Correction for 

 photocell 

 wavelength 

 sensitivity 



S=fU) 



Reciprocal 

 unit 



Photocell measures intensity needed 

 to keep photosynthesis rate constant 



Pen 

 drive 



Gloss plate 



Light beam 



■^ 





o 



"5 



a: 



Paper 

 drive 



Recorder 



Algae on _ 

 electrode 



constant os 

 wavelength is varied 



/ 



Pt electrode 



Action spectrum 



for photosynthesis 



SCO eoo 700 mu 

 Wavclengtti.x 



Amplifier 



(Motor 



Difference 

 amplifier 



Voltage is proportional 

 to rate of photosynthesis 



Reference 

 voltage 



Fig. 8. A block diagram of the recording action spectrophotometer (From 27). 



trolled by the rate of photosynthesis itself. A platinum electrode 

 (Haxo and Blinks, 33; Haxo and Fork, 34) gives a current propor- 

 tional to the photosynthesis rate and is used to control the light in- 

 tensity. A fraction of the light beam is continuously measured by a 

 photocell, and a value proportional to its reciprocal (expressed in 

 relative numbers of incident quanta and corrected for the wavelength 

 sensitivity of the photocell) is plotted against wavelength on a re- 

 cording potentiometer. Thus the action spectrum is plotted directly 

 without calculation. The system works reasonably well for very 

 thin layers of algae that do not swim. A record is shown in Fig. 9. 



