VARIATION OF INTENSITY 33 



in the shutter ; the intensity at S is therefore taken as that 

 of normal sunlight. The various intensities at different 

 positions of the scale are marked in terms of S, which is the 

 intensity of sunlight in the town during unclouded days 

 from November to March. Sunlight increases from morning 

 and attains its maximum at noon : in the afternoon it 

 undergoes rapid diminution. It is necessary to state that 

 the intensity of sunlight in a town is not the same as that 

 in the open country or in the hills. The atmosphere, on 

 account of floating dust particles and slight haze due to 

 diffuse smoke, is not absolutely clear. The sunlight becomes 

 relatively more intense immediately following a rainy day. 

 The sunlight employed will be described in general terms as 

 bright, moderate or dull. 



For investigations on the effect of variation of intensity 

 of light, Pointolite offers special advantages. The light 

 emitted by this lamp is perfectly constant ; the value of 

 intensity in lux at any distance from the source of light can, 

 moreover, be easily determined. The effect of varying 

 intensity of sunlight will be described in the next chapter. 



In the experiments with Pointolite, the plant-vessel with 

 the Bubbler was moved gradually nearer the light, and the 

 activities at different intensities, say from ioo to 3000 lux, 

 were measured. 



The period for physiological adjustment to changed inten- 

 sity of light. — When the plant is subjected to increasing 

 intensity of light, a certain period must elapse before 

 photosynthesis attains a steady value under the increased 

 intensity. The following experiment was undertaken to 

 determine the minimum period for the physiological readjust- 

 ment, the successive evolution of equal volumes of gas being 

 automatically inscribed by the Electromagnetic Recorder. 

 The plant was first subjected to an intensity of 750 lux 

 (7-5 units), and the bubbling-period was found to be prac- 

 tically uniform at intervals of 60", 61", 62" and 60". The 

 plant-vessel was then quickly brought nearer the source of 

 light, the intensity being 1000 lux ; the automatic record 



