LIGHT ABSORPTION OF PHOTOSYNTHESIZINO CELLS 61 



emission of tungsten is not the same function of temperature at differ- 

 ent wavelengths. It seems also difficult to ascertain whether the ab- 

 sorption of the compensating beam does not change upon illumination. 

 There is in our opinion no reason to assume that an apparatus of the 

 first type is inferior to one of the second type; the former is, however, 

 less complicated and less expensive. 



Calibration. The calibration of the apparatus required to express 

 deflections as changes in optical density can be carried out in two 



ways: 



1. By measuring the deflection brought about by a known change 

 in the measuring beam. This can be done by inserting a glass plate 

 (1) or wire screen (see below) into the measuring beam. 



2. By measuring the photocurrent produced by the measuring 

 beam. This can be done by comparing this current by means of a 

 "Brown converter" with a known current. Various checks on Unearity 

 of amplifier and multipher and on influence of compensating beam 

 are needed to make certain that the results are correct {cf. 3). 



Apparatus using blank. An apparatus based on the same principle 

 as a conventional spectrophotometer such as the Beckman DU 

 could conceivably be used, provided a "blank" is used of approxi- 

 mately the same absorption as the cells to be measured. However, 

 owing to technical difficulties, the sensitivity generally is not better 

 than about 1 X 10"^ unit of optical density and may in general be 

 insufficient for measuring spectral changes in photosynthetic tissue. 

 Lundegardh (4) used an automatic recording apparatus of this type, 

 but only in a narrow region where the absorption was low so that a 

 dense suspension could be used. 



Illumination of cells. Two methods of illumination for exciting the 

 changes in absorption can be used. 



1. The cells are illuminated in the vessel through which the measur- 

 ing beam passes (1,2). 



2. A batch of cells is illuminated outside the absorption vessel and 

 quickly moved into this vessel to replace the nonilluminated cells. 

 The cells are pumped around in a closed circuit, and a transparent 

 part of the circuit at a short distance from the absorption cell is 

 illuminated or darkened (5). In a variation of this method, the flow 

 is stopped after the illuminated cells have passed into the absorp- 

 tion vessel (4). 



Illumination in the absorption vessel gives the least variation in 



