SUNLIGHT AND ARTIFICIAL LIGHT 



III 



to I p.m. Since none of the experiments undertaken with 

 the light from the northern sky lasted for more than an 

 hour, the intensity of light employed may be taken to be 

 constant. As an additional precaution, as already stated, 

 frequent readings of the Photometer were taken. The 

 experiment was rejected if the light did not prove to be 

 constant. 



The place chosen for the experiment is the garden of the 

 Institute, open to the northern sky. There is a low building 

 which extends from the east to the west, which protects the 

 place of the experiment from the direct ravs of the sun. 



Table XIV. — Readings of the Electric Photometer at Different 



Hours of the Day 



Sunlight and Artificial Light 



In certain experiments we require strong light the in- 

 tensity of which may be maintained uniform or increased or 

 decreased in a definite and known manner. The intensity 



