164 



The température. 



The température was always kept constant in ail the 

 experiments. This was ensured by joining the thermo- 

 regulator with an accumulator and a relay in an electric- 

 circuit, so that the current of the heating lamps was 

 immediately interrupted when the température had suffl- 

 ciently risen '). In addition a second relay was included, 

 in order to prevent discharge of the accumulator as much 

 as possible. 



The source of light. 



À simple electric carbon filament lamp served as the 

 source of light for the phototropic experiments and its 

 luminosity was determined by Weber's^') photometer and 

 as a distance of one mètre, was found to equal 5 M.C. 

 (mètre candies). For this détermination I placed the lamp 

 in the thermostat in the position which the boxes with 

 the seedlings occupied during the experiments. The space 

 between the inner and outer case was filled with water 

 and I placed the photometer at a distance of 1 mètre 

 from the lamp. Thus in determining the luminosity, the 

 absorption of light by the glass-walls and layer of water 

 was taken into account. The lamp was so arranged in 

 the experiments that it was at a distance of exactiy one 

 mètre from the middle of a box of seedlings, which was 

 placed in the thermostat, in such a way that illumination 

 took place through the side-walls of the thermostat. 



M For a detailed description of this arrangement as well as of the 

 thermoregulator, see Rutgers, l.c. pp. 46 and 48. 



2) Prof. Snellen was kind enough to place the photomètre at my 

 disposai for this purpose. 



