264 RADIATION BIOLOGY 



per erg of radiant energy received, irrespective of the wave length of the 

 light. Energies from 500 to 50,000 ergs/sec/cm- cover the range of ordi- 

 nary experimentation in photosynthesis. Bright sunlight has an energy 

 of over 5 X 10^ ergs/sec/cm-. 



Although the calibrated thermopile and bolometer are accepted as the 

 primary standards in energy measurements of photosynthesis, che^mical 

 actinometers are often more convenient and sometimes more suitable. 

 The uranyl oxalate actinometer (ibid.) is the best chemical actinometer, 

 but it is responsive only to blue and to ultraviolet light. The amount 

 of oxalate decomposed by the light is measured by titration with potas- 

 sium permanganate, 0.57 molecule being decomposed by each photon of 

 light absorbed. It is useful in checking the calibration of any radiation- 

 recording instrument, but its absorption spectrum is so different from 

 that of chlorophyll that it cannot be used directly in photosynthesis. 



The ethyl chlorophyllide actinometer, developed by Warburg, Burk, 

 and Schade (1951), Warburg and Schocken (1949), and Gaffron (1927), 

 has also been studied by Evans (1951). It is ideally suited for energy 

 measurements in photosynthesis because its absorption is nearly the same 

 as that of chlorophyll. The actinometer as developed by Warburg and 

 Schocken (1949) is read by measuring the consumption of oxygen in the 

 same type of Warburg manometer used for measurements of respiration 

 and photosynthesis. The light absorber is about 2 mg of ethyl chloro- 

 phyllide extracted by a special process from leaves of the stinging nettle, 

 dissolved in 7 ml of pyridine containing 200 mg of allyl thiourea. 



The quantum yield * was determined by Evans (1951) under a variety 

 of conditions and found to average 0.96 molecule of oxygen consumed in 

 the solution per photon absorbed when the solution is shaken (150 times 

 per minute) in a Warburg manometer. There is a sUght dependence on 

 the intensity of light. The use of this actinometer in light transmission 

 measurements by Warburg and Burk is a distinct advance. 



A serious problem in measurements of photosynthesis is the scattering 

 of light by algae. If a beam of light is passing directly through a clear 

 aqueous solution onto a receiver, this light beam will be altered by the 

 introduction of a suspension of algae. Some of the light will be absorbed, 

 and some of the light that is not absorbed will be scattered. The more 

 concentrated the suspension of algal cells, the longer the path of the light, 

 and the smaller the size of the algal cells, the greater the amount of the 

 scattering. If the thermopile or bolometer receiver is considerably larger 

 in area than the normal beam, most of the scattered light energy will 

 still be registered, because the wide-angle scattering of light is less prob- 

 able than the scattering of light that departs only slightly from the main 

 beam and still falls on the large-area thermopile. In the case of a large- 

 area photocell and receiver, there is a tendency, too, for some of the 

 scattered light to be scattered back again. The closer the large-area 



