LIGHT 



51 



standard beside it. A second method which is equally advantageous and 

 sometimes preferable does away with the permanent standard in the field 

 and the need for a stop-watch. In this event:, the strip is exposed until a 

 medium color is obtained, since very light or very deep prints are harder to 

 match. This is later compared with the multiple standard. In both cases, 

 the date, time of day, station, number of instrument and of exposure, and 

 the length of the latter in seconds are carefully noted. The instrument is 

 held with the edge toward the south at the level to be read, and the open- 

 ing uppermost in the usual position of the leaf. When special readings are 

 desired, as for isophotic leaves, reflected light, etc., the position is naturally 

 changed to correspond. In practice, it is made an invariable rule to move 

 tiie strip for the next exposure as soon as the slide is closed. Otherwise 



Fig. 12. Dawson-Lander sun recorder. 



double exposures are liable to occur. When a strip is completely exposed 

 it is removed in the dark, and a new one put in place. The former is care- 

 fully labeled and dated on the back, and put away in a light-tight box in a 

 cool place. 



80. The Dawson-Lander sun recorder. 'The instrument consists of a 

 small outer cylinder of copper which revolves with the sun, and through 

 the side of which is cut a narrow slit to allow the sunshine to impinge on 

 a strip of sensitive paper, wound round a drum which fits closely inside the 

 outer cylinder, but is held by a p-n so that it can not rotate. By means of 



