LIGHT 53 
for exposure is 6 mm. square and is controlled by a photographic shutter. 
The latter is constructed so that it may be set for 5, 10, or 20 seconds, since 
a single period of exposure can not serve for both sun and shade. The 
shutter is tripped once every two hours, by means of a special wheel revolv- 
ing once a day. Each exposure is 6 mm. square, and is separated by a 
small space from the next one. Twelve exposures are made every 24 hours, 
and 84 during the week, though, naturally, the daytime exposures alone are 
recorded. Comparisons with the multiple standard are made exactly as in 
the case of the simple photometer. The selagraph is made by the C. H. 
Stoelting Co., Chicago, Illinois. 
Standards 
82. Use. The light value of each exposure is determined by reference to 
a standard. When the photometer carries a permanent standard, each ex- 
posure is brought to the tint of the latter, and its value is indicated by the 
time ratio between them. Thus, if the standard is the resuit of a 5-second 
exposure to full sunlight at meridian, and a reading which corresponds in 
color requires Ioo seconds in the habitat concerned, the light of the latter 
is twenty times weaker or more diffuse. Usually, the standard is regarded 
as unity, and light values figured with reference to it, as .o5. With the 
selagraph such a use of the standard is impossible, and often, also, with the 
photometer it is unnecessary or not desirable. The value of each exposure 
in such case is obtained by matching it with a multiple standard, after the 
entire strip has been exposed. The further steps are those already indicated. 
After the exact tint in the standard has been found, the length of the 
reading in seconds is divided by the time of the proper standard, and the 
result expressed as above. 
83. Making a standard. Standards are obtained by exposing the photo- 
meter at meridian on a typically clear day, and in the field where there is 
the least dust and smoke. Exception to the latter may be made, of course, 
in obtaining standards for plant houses located in cities, though it is far 
better to have the same one for both field and control experiment. Usable 
standards can be obtained on any bright day at the base station. Indeed, valu- 
able results are often secured by immediate successive sun and shade read- 
ings in adjacent habitats, where the sun reading series is the sole standard. 
Preferably, standards should be made at the solstices or equinoxes, and at a 
representative station. The June solstice is much to be preferred, as it 
represents the maximum light values of the year. Lincoln has been taken 
as the base station for the plains and mountains. It is desirable, however, 
