32 CHAP. V. PHOTOSYNTHESIS AND INTENSITY OF LIGHT 



unfortunately, no universally accepted standard of illumina- 

 tion. The Geneva Conference, 1911, recommended lux as 

 the standard, which is the illumination produced by a 

 standard candle at a distance of 1 metre. In the follow- 

 ing experiments I generally employed light increasing from 

 about 100 to 5000 lux. The unit degree of variation of light 

 may be conveniently taken as 100 lux. 



Adjustment for variation of intensity. — A lens placed in 



Fig. 5. 



Diagrammatic 



Representation of Arrangement for 

 securing different intensities of Light 



Parallel beam of sunlight rendered divergent by a convex lens 

 seen to the left. Different intensities in terms of sunlight 

 marked on the table. Light from Pointolite lamp is simi- 

 larly made divergent by a lens. 



front of a 100 c.p. Pointolite produces a slightly divergent 

 beam. Since the radiant is a point, the intensity of light 

 varies inversely as the square of the distance. The positions 

 for intensities from 100 to 5000 lux are marked on a scale 

 fixed on the table. These values were independently 

 verified by a grease-spot photometer and a standard candle. 

 The sunlight reflected by a heliostat was similarly 

 rendered divergent by means of a lens (fig. 5). At S, the 

 area of the beam is the same as that of the circular opening 



