42 



PROBLEMS IN PHOTOSYNTHESIS 



Fig. 22. Measurement of the energy of the 

 incident Hght beam. T: thermostat. V: 

 manometer vessel. M \: movable mirror. 

 A/2.- mirror with known refliection power. 

 LB: light beam. B: bolometer (Warburg). 



lens, concentrated on the bolometer. The permeability of this mirror-lens 

 system is a constant, dependent on the wave-length. For example, it is 

 0.78 for red light, which has a wave-length of 6440 A. If i' is the intensity 

 measured bolometrically, then the incident intensity i„ of the cell suspension 

 is?V0.78or 1.28r. 



The determination of the light intensity by means of a bolometer (Fig. 23) 

 is based on the absorption of radiation by thin blackened platinum strips. 

 The absorption increases the temperature and gives rise to a change in the 

 electrical resistance which is then measured in a Wheatstone bridge. Calibra- 

 tion of the bolometer is effected by standard carbon filament lamps supplied 

 by the U.S. Bureau of Standards.* 



The incident intensity i^ can be measured with the bolometer, taking into 

 consideration the corrections made for permeability. However, the cell 

 suspension absorbs only part of the incident intensity. In early experiments 

 complete absorption was realized by using dense cell suspensions of 10 jul 

 cells or more per ml. This has become unnecessary to-day. With the aid 

 of Ulbricht spheres, it is possible to measure the fraction a of the incident 

 intensity (O absorbed by the cell suspension. For this reason, the density 

 of the cell suspension can be considerably decreased to about 0.5 to 1 fx\ 

 cells/ml. 



When dense cell suspensions are used, the cells at the bottom of the manom- 

 eter vessel absorb more light than those at higher levels. As the manometer 

 vessels have to be shaken rapidly, cells in such suspensions change continuously 

 from the light to the dark regions, resulting in photochemical inductions and 

 consequent loss of energy. According to Warburg (35), the passage of light 

 through the vessels remains optimal only if the intensity is kept nearly con- 

 stant. This condition can be realized only with thin suspensions which ab- 

 sorb only 3 to 4% of the incident light in the manometer vessels. 



Ulbricht spheres are hollow vessels impermeable to light, with a diameter 



* Bolometry, introduced by the American physicist S. P. Langley, was developed and adapted 

 to photochemistry by Lummer and Kurlbaum. It was Lummer's bolometer that played a decisive 

 role in the discovery of light quanta. The bolometer of Lummer and Kurlbaum was used to measure 

 light energy in the experiments of E. Warburg that laid the foundations of quantitative photochem- 

 istry. It is the same bolometer that has now solved the problem of the energetics of photosynthesis 

 (24, reference 9). 



