( 232 ) 



of light, all possible intensities were obtainable, from 100 Hefner 

 candles downwards. The intensity was measured by means of a 

 Weber photometer. The gaslamp was outside the room containing 

 the experimental plants, so that the latter were protected against any 

 harmful effect of coal-gas. 



For greater light intensities the electric arc-lamp of a lecture lantern 

 was used, and by concentrating its light through lenses, strengths up 

 to 48000 Hefner candles were obtainable. 



The period of illumination varied from 13 hours to 0.001 second ; 

 the very short periods were obtained by means of a photographic 

 instantaneous shutter with slit. 



The plants were now placed at various distances from the source 

 ot light; they were illuminated for a given time and were then left 

 in the dark and were examined for phototropic curvature after about 

 2 hours. When the distance and time had been properly chosen, a 

 well-marked limit was found to occur, so that below a certain strength 

 of light no curvature occurred, whereas above that strength all 

 or nearly all the seedlings were bent towards the light. It may be 

 said, therefore, that with a given exposure-time, a certain minimum 

 intensity of light is required for perception, or, more correctly, for 

 the production of a reaction, since of the actual perception of a light 

 stimulus we know nothing. 



It was already a striking result, that while, as stated above, the 

 presentation time was assumed to be 7 minutes, Mr. Blaauw in his 

 experiments still obtained a reaction when the exposure was diminished 

 to 0.001 second, provided the light was very strong. 



The results become still more important if expressed numerically, 

 as in the following table. The first column gives the length of the 

 exposure, the second the corresponding intensity of the light (in 

 Hefner candles) which just sufficed for a phototropic reaction; the 

 third column gives the product of these two magnitudes, the time 

 being expressed in seconds, so that the product might be called 

 candles-seconds. In other words, the third column indicates in every 

 case, how much light should have been allowed to fall on the plant 

 during one second, in order to give the same amount of light as in 

 the experiment. 



I (Exposure). II (Intensity of light). Ill (Candles-seconds). 



13 hours 0,000439 H.C. 20,6 



10 „ 0,000609 „ 21,9 



6 „ 0,000855 „ 18,6 



3 „ 0,001769 „ 19,1 



