THE COMPLETE APPARATUS 157 



found to produce marked variation in the rate of the 

 photosynthetic evolution of oxygen. 



For securing conditions closely approximating to the 

 natural variation of light from the northern sky, the 

 specimen mounted in the Bubbler was taken to the roof of 

 the Institute, at a height of 40 feet (13 metres) from the 

 ground. No intervening buildings now interrupted the 

 northern light. The complete apparatus was placed in 

 front of the Observatory at the south-east corner, the plant 

 being thus protected from the sun. 



The apparatus for the simultaneous determination of 

 the hourly changes in photosynthesis and in the environ- 

 mental conditions consists of a base-board, 40 X 40 cm., 

 on which three separate instruments are fixed in the closest 

 proximity to each other, namely • 



(1) The Photosynthetic Recorder, in which the bubbling 

 apparatus containing the plant is provided with an electro- 

 magnetic arrangement for inscribing the record ; the drum 

 was placed in the Observatory a short distance from the 

 Bubbler, the two being in electric connection by a pair of 

 leading wires. 



(2) The Electric Photometer, previously described, was 

 placed by the side of the Bubbler, the line of sight of the 

 two being the same. The wires from the Photometer led 

 to the galvanometer, which was inside the Observatory ; 

 the intensity of light at any period was measured by the 

 galvanometric deflection. 



(3) The Thermograph consisted of a compound strip of 

 metal attached to a recording lever. The excursions of the 

 lever were recorded on a smoked glass plate allowed to fall 

 at a definite rate by clockwork. A sensitive thermometer 

 was suspended near the Thermograph to give independent 

 readings of the temperature. 



A glass cover was placed over the base-board bearing 

 the three instruments, the object being to protect the 

 instruments from disturbance caused by currents of air. 

 The complete apparatus thus permits the record of the 



