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 tppermost 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 
the 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 pin so that it can not rotate. By means of 
