110 CHAP. XIV. THE ELECTRIC PHOTOMETER 



may be regarded as a point. As the intensity of light 

 varies inversely as the square of the distance, the inten- 

 sity of the light at various distances is known. We place 

 the selenium cell at different distances and measure the 

 corresponding deflections of the galvanometer. 



From these results we are able to construct the calibra- 

 tion-curve from which the absolute value of the intensity of 

 light in lux is obtained corresponding to any particular 

 deflection of the galvanometer. The calibration-curve is 

 practically straight for moderate variation in the intensity 

 of light. It is sometimes necessary to measure widely 

 different intensities of light, such as sunlight, sky light, or 

 the light of Pointolite. When the galvanometer is made 

 highly sensitive for the measurement of light of moderate 

 intensity, sunlight sends the index out of the scale. We 

 may get over this difficulty by two different means : (i) by 

 reducing the sensitiveness of the galvanometer to one-tenth 

 or one-hundredth by the employment of suitable shunts ; 

 (2) by sufficiently reducing the aperture which admits light 

 to the selenium cell. 



Again, through intermediate reduction of sensitiveness 

 of the apparatus it is possible to compare the intensity of 

 a known fraction of sunlight — say, o-oi S — with a definite 

 intensity in lux given by Pointolite. 



Best Aspect of Sky for Uniform Light 



The variation of intensity of the light from the sky 

 vertically above has been given (fig. 30). In the actual ex- 

 periments, however, the plant received light either from the 

 eastern, western or northern aspects. From November to 

 February the sun rises south-east and sets south-west. 

 The plant cannot be exposed to the south, since sunlight 

 would fall directly on it. When the plant faces the east 

 or west, the light which falls on the plant changes very 

 rapidly. The results given below show that the light from 

 the northern sky remains practically uniform from n a.m. 



