SCIENCE— ADVERTISEMENTS 



How Intense Is Sunlight 



Its intensity, of course, depends upon latitude, season, time of 

 day, atmospheric conditions ; but whatever it happens to be, it can 

 be measured readily with 



Leeds & Northrup Company's 



MACBETH ILLUMINOMETER 



Illumination of any intensity, artificial or natural, is within the 

 range of this useful instrument. Its scale reads foot-candles directly. 

 It is provided with a reference standard lamp with which each ob- 

 server standardizes the instrument according to his own visual per- 

 ception. This is done wherever he happens to be — no photometric 

 dark room is required. 



The illustration shows one method for making a measurement. 

 The illumination on the test-plate, mounted on a tripod, is being 

 measured. A translucent plate, mounted in 

 a "horn", can be fitted to the instrument, and 

 by pointing the "horn", in any direction, up, 

 down or sidewise, the illumination due to the 

 light from that direction is measured. 



The Macbeth Illuminometer is especially 

 suited for investigations con- 

 cerned with the reactions of liv- 

 "^ ing organisms to light. 



Bulletin S680 describes the 

 Macbeth Illuminometer in detail 



Leeds & Northrup 



Company 



Electrical Measuring Instruments 



4901 Stenton Ave. Philadelphia, Pa. 



