112 CHAP. XIV. THE ELECTRIC PHOTOMETER 



of sunlight is, as we have seen, practically uniform be- 

 tween ii a.m. and i p.m. I have explained how the in- 

 tensity of sunlight can be decreased in a known manner 

 by the employment of a double convex lens which gives 

 a divergent beam of light. For moderate intensity of 

 light, Pointolite gives, as we have seen, a range of from 

 ioo to 5000 lux. 



Speaking generally, sunlight is most suitable for experi- 

 ments with strong light. The light from the Pointolite is 

 employed when we wish to use light of moderate intensity 

 which may be increased or decreased in a definite manner. 

 Finally, we have the light from the sky, which may be secured 

 practically constant under proper conditions. 



The most serious difficulty in the quantitative deter- 

 mination of the activity of photosynthesis has thus been 

 removed, and the constancy of the various sources of light 

 secured. 



Summary 



By means of the Portable Photometer immediate deter- 

 mination of the intensity of light can be made from the 

 galvanometer deflection. 



Widely different intensities of light can also be measured 

 with the same apparatus by suitable reduction of sensitive- 

 ness by shunting the galvanometer or by reduction of the 

 aperture for the incident light. 



By taking readings through intermediate stages, the 

 intensity of sunlight may be compared with that of 

 Pointolite. 



The diffuse light from the northern sky remains practi- 

 cally constant about midday, when the light is most intense. 



