106 CHAP. XIII. THE AUTOMATIC RADIOGRAPH 



resistance induced by diurnal variation of temperature. In 

 order to determine the extent of this variation, a spark 

 record was also obtained before exposure to light. The 

 dotted record near the base-line shows that the resistance 

 remained practically constant throughout the day, in spite 

 of the variation of the temperature. 



In order to determine the diurnal variation of light 

 and of temperature and their periods of maximum and 

 minimum, records were simultaneously taken one day in 

 summer with the Thermograph and the Radiograph. The 

 two curves are given in fig. 31. It will be seen that, while 

 the maximum intensity of light is at 12 noon, the thermal 

 maximum is at about 2 p.m. The thermal noon is thus 

 two hours later than the light-noon. Light disappears at 

 night from 6 p.m. to 6 a.m. ; that is to say, the period of 

 minimum is prolonged for twelve hours. But the fall of 

 temperature is gradual, and the minimum is attained at 

 about 5 A.M., which is the thermal dawn. The characteristic 

 variations of these two important factors should be borne 

 in mind, since it is known that the diurnal movements of 

 plants are modified by the algebraical summation of the 

 effects of light and of temperature. 



It is sometimes desirable to carry out researches during 

 a period when the intensity of light remains approximately 

 constant ; this period is found to be between 11 a.m. and 



T P.M. 



The above record of the diurnal variation of light is true 

 of days when the sky is clear. But the passage of a cloud 

 causes change in the intensity which is accurately recorded 

 by the Radiograph. A record of such irregular variation 

 on a stormy day is given in the lower record of fig. 30. 



Summary 



The Radiograph gives a record of the diurnal variation 

 of light. On a clear day in January the intensity was found 

 to increase rapidly from 6 a.m. to 12 noon, when it reached 



