I90 CHAP. XXII. EFFICIENCY OF THE SPECTRAL RAYS 



Pointolite was a 30-c.p. lamp mounted on the same base- 

 board which carried the plant-vessel, so that the distance 

 between the two was constant. This fixed distance was 

 33 cm., or 0-33 metre, and the intensity of light to which 



30 

 the plant was subjected was therefore 7-^—^2 = 2 75 mx - 



This was slightly above the verge, and the evolution of 

 oxygen by the plant was at a rate of 23-5 c.mm. per hour. 



The plant-vessel and the auxiliary light were then 

 placed on the experimental table, different points in which 

 had been marked previously as 500, 750 and 1000 lux with 

 a stronger Pointolite lamp of 100 c.p. After observation 

 of the additive effect with 500 lux, the experiment was 

 repeated with 750 and 1000 lux. The results given in the 

 following table show that the combined effect of two distinct 

 lights is additive. 



Table XXVIII. — Effect of Incident Light of increasing 

 Intensity superposed on Auxiliary Light 



c.mm. O per hour 

 Effect of auxiliary light . . . . . . = 23-5 



Effect of 500 lux (without auxiliary light) . . = 42-8 



Effect calculated with addition of auxiliary light : 



42-8 + 23-5 = 66-3 



Observed effect . . . . . . . = 66-7 



Effect of 750 lux . . . . . • • = 64-3 



Effect calculated with auxiliary light: 64-3 + 23-5 = 87-5 

 Observed effect . . . . . • • = 87-5 



Effect of 1000 lux = 92-3 



Effect calculated with auxiliary light: 92-3 + 2 3 "5 = H5* 8 

 Observed effect = 116-0 



These results show (i) that, when a plant is exposed to 

 light from two sources, the activity of photosynthesis is the 

 sum of the effects, which are therefore additive ; and (2) that 

 an auxiliary light may be employed to bring the plant to the 



