NO. 13 FUNCTIONS OF RADIATION BRACKETT AND JOHNSTON 9 



temperature T' relative to T\ Such new air as is required is bled into 

 the circulating system from a compressed-air supply line. 



Since the water which is circulated into the walls determines the 

 temperature of the air in the upper control tank, both water and air 

 enter the tanks at the same temperature, T\ As the four chambers 

 may be illuminated by different intensities of radiation, the air may 

 experience a different rising temperature in each chamber. In order 

 to overcome this difficulty the water which is passed through the 

 upper filter and also the lower plate, can be adjusted differently for 

 the different chambers. The flow is adjusted and the temperature of 

 the discharge observed as shown by the indicators in the lower left- 

 hand corner. Since the upper filter and the lower plate are the surfaces 

 which receive most of the overhead illumination this arrangement 

 yields a very satisfactory control over the rise in temperature. As 

 this compensating cooling is required only while the lights are on, a 

 solenoid valve (No. 2) has been provided, which is connected to the 

 lighting circuit so that the water is allowed to flow only when the 

 lights are on. With the lights off, the side walls are still maintained 

 at the same temperature by the main recirculation system. This insures 

 that during periods of darkness the four chambers have the same 

 temperature. 



GENERAL 



The chief advantage of the apparatus which has been described is 

 that a satisfactory degree of control has been secured without loss of 

 flexibility. It is readily adapted to all types of artificial light sources. 

 Plants may be exposed to overhead or lateral illumination at will. Half 

 of the entire solid angle may be utilized for illumination. Humidity can 

 be controlled over a wide range. The temperature of three-fourths 

 of the walls is controlled by water circulation. Gas concentration may 

 be varied, as the entire enclosure can be hermetically sealed ; the pres- 

 sure of the gas phase may even be modified. Observations of growth 

 and color may be readily made through convenient ports. Finally the 

 apparatus lends itself readily to measurement of gas phase concentra- 

 tion, temperature, humidity, and light intensity. Measurements of 

 carbon dioxide assimilation in connection with these various observa- 

 tions may be made under the influence of different spectral distribu- 

 tions, but at present this type of investigation is being carried out 

 only in tubular glass growth chambers with young wheat plants. 



In planning the apparatus the following types of experiments have 

 been contemplated: i, investigations of the effect of the infra-red; 



