The Quantum Efficiency of Photosynthesis 81 



fuge at the lowest possible speed giving nearly complete settling in 10 minutes and 

 were taken up, with or without further washing, in fresh nutrient medium at a 

 concentration of 30 — 50 //l cells per ml. 



II. Monochromator 



A Steinheil glass 3-prism spectrograph operated with a focal lenght of 195 mm 

 at F 3.5 for the collimator and a focal length of 710 mm for the telescope was 

 used as a monochromator. The slit was illuminated with a 750-watt projection 

 lamp. The image of the coiled filament at about 20° to its plane was projected onto 

 the slit with an auxiliary lens. A 1000-watt voltage regulator was used to supply 

 power to the lamp which operated at constant current. 



The width of the entrance slit was about 2 mm, corresponding to about 20 mi< 

 in the red. A slit was placed in the focal plane of the telescope and was adjusted 

 to have a width of about 30 m// covering the region 630 to 660 m/i. A lens was 

 placed behind this slit to throw, in a weakly convergent beam, an image of the exit 

 prism face on the bottom of the manometer vessel. 



The area of the beam at the vessel was about 3 cm 2 and the energy flux was 

 about 0.6 microeinsteins/min. This intensity was decreased when desired by 

 placing in the light beam, just before the exit slit, blackened wire screens cali- 

 brated by the National Bureau of Standards. 



III. Measurement of Light Energy 



The energy of the light beam was measured by the recently developed chemical 

 actinometer 3 whereby for each quantum of visible light absorbed one molecule of 

 O2 is consumed. In the same or similar rectangular vessel as used for the yield 

 determinations were placed 2 mg ethyl chlorophyllide, 200 mg thiourea, 7 ml 

 Pyridine, and Oz gas. The actinometer vessel was shaken in the thermostat at 

 20° C in the same manner and in the same cross-section of the light beam as the 

 vessels with the cell suspensions were shaken during the yield determination. The 

 total intensity of light, absorbed by the actinometer, should not exceed 0.3 micro- 

 einsteins per minute under our working conditions. Higher intensities, as used for 

 the yield determinations, were diminished for this purpose by the calibrated screens. 

 Several 10 minute periods were observed for every actinometer determination. 

 When in t minutes the pressure change in the actinometer vessel is ho 2 tnm, the 



total energy flux in the light beam in t minutes is ° 2 ° 2 or -~~- microeinsteins 



(micromole quanta), where the vessel constant ko 2 is expressed in mm 2 . Then, 

 when the oxygen developed by illuminating the green algae is n u\ and the oxygen 

 absorbed in the actinometer for the same time and beam of light is n' ///, the quan- 

 tum requirement per mol of 0> developed in photosynthesis is simply I/9? = n'jn. 



6 Warburg, Zellphysiologie 



