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 sciagraph 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 result of a 5-second 

 exposure to full sunlight at meridian, and a reading which corresponds in 

 color requires 100 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 .05. With the 

 sciagraph 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 Jo be preferred, as it 

 represents the maximum light values of the y^r. LLincoln has been taken 

 as the base station for the plains and mountains. ' It is desirable, howeverj 



