THE ' HYDRILLA ' PLANT AS A PHOTOMETER 45 



intensities at different positions on the table being marked 

 in the usual manner. There was a certain amount of loss of 

 light by scattered reflection from the heliostatic mirror, and 

 also by reflection from the surface and absorption in trans- 

 mission through the lens. For the exact determination of this 

 loss I employed the Electric Photometer, which was at first 

 exposed to light at the point marked S inside the room, when 

 the photometric deflection was found to be 77 divisions. 

 The photometer was next taken outside and pointed directly 

 to the sun, the deflection being now 118 divisions. This 

 showed that nearly one-third of the light was lost in reflec- 

 tion and absorption, the intensity of the light S inside the 

 room being two-thirds of that of direct sunlight. 



The physiological and physical measurements of sunlight 

 were carried out as follows. Sunlight entered the room 

 from the right side, while a divergent beam from Pointolite 

 (with the various intensities in lux marked on the table) 

 came from the left. A specimen of Hydvilla, mounted in its 

 vessel with the Bubbler, was placed between the divergent 

 beams. The specimen was first exposed to Pointolite, the 

 sunlight being shaded. The experiment was repeated with 

 sunlight, this time shading the Pointolite. The plant was 

 moved in one direction or the other till the two photo- 

 synthetic activities under Pointolite and under sunlight 

 were found to be the same, i.e. 296 c.mm. per hour. The 

 effect of 1700 lux was found to be the same as that of 0-04 

 S, the two intensities being physiologically equivalent to 

 each other. 



A grease-point photometer was next employed for an 

 independent comparison of the two lights ; the intensity of 

 1700 lux was found to be exactly equal to 0-04 S. The 

 Hydrilla is thus found to be a reliable indicator of the 

 intensity of light. For the most sensitive condition of 

 balance, the intensity of light should be chosen to be at or 

 near the turning-point of 1200 lux. 



Reference to results obtained in January (Table VII.) 

 shows that while on a dull day in January o • 12 S was physio- 



