EXPERIMENTAL METHODS AND APPLIANCES 



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sunlight reflected by a heliostat enters the experimental 

 room after having passed through a thick stratum of water 

 for absorption of the heat-rays. The experiment was carried 

 out in the morning, when the intensity of light is about 

 0-4 S, i.e. less than half the intensity at midday. There 

 is no advantage in the employment of a stronger light, 



Fig. 55. Apparatus for Determination of Photosynthetic Efficiency 



Sunlight enters by rectangular aperture a, and falls on plant-vessel 

 B (only one shown in figure) . Energy of radiation measured 

 by the calorimeter c, or by the radiometer r. Thermo-electric 

 couple shown in the upper right-hand figure (see text) . 



since there is little further increase in photosynthetic action 

 above the intensity of 0-4 S. 



The Bubbler. — Two Bubblers in every way similar (of 

 which only one is shown in the figure) are successively 

 interposed in the path of light. They are mounted side by 

 side on a stand which slides on a rail. Both the Bubblers 

 are filled with water, containing 8 mg. of C0 2 in 100 c.c, but 

 one of them, b', is blank, while the other, b, contains the 

 plant. Interposition of the blank Bubbler b' allows 



