1920] HURD—FUCUS SPORES 31 



of whatever vibrations fall upon it, be they long and therefore 

 heating in their physiological effect, or short and therefore perceived 

 as light, is converted into heat energy upon being absorbed by the 

 exposed junction of the thermopile. In other words, the light of the 

 blue end of the spectrum produces an electromotive force much 

 less than that of the infra-red, but no less measurable. 



The instruments used in the energy calibration of these screens 

 were a Hilger thermopile with junctions of bismuth and silver, and 

 a moderately sensitive galvanometer (d'Arsonval). An electric 

 arc similar to the one later used in the experiments themselves 

 was the source of light. The thermopile with the open end 

 screened by the red filter was exposed to the light until the galva- 

 nometer indicator reached a maximum deflection, which ordinarily 

 took about 30 seconds. The number of divisions through which 

 the spot of light reflected from the galvanometer mirror was 

 displaced on the scale was noted. This was repeated six times, 

 and the average deflection recorded. The other filters were then 

 used in turn to screen the thermopile, and thermopile and screen 

 moved to such a distance from the arc that the displacement 

 of the galvanometer indicator was approximately equal in each 



i 



case to that produced by the red filter. This distance was also 

 found for the thermopile when screened by a piece of clear glass, 

 which represented the distance of the control from the source. 

 For the experiments the quantity of light used could be varied for 

 the whole set of screens by multiplying or dividing these distances by 

 the same number, and the intensity in all the boxes would remain 

 equal. The actual amount of light in meter-candles can be measured 

 by means of a photometer. Then from the law of inverse squares, 

 namely, that the intensity of light per unit surface varies inversely 

 as the square of the distance from the source, the intensity at any 

 distance from the arc can be computed. 



The calibration of the set of screens was repeated seven times, 

 or until satisfactory checks of the distances were obtained. With 

 some thermopiles of less rapid action than the one used here, it is 

 impossible to get results by waiting for the galvanometer indicator 

 to come to a steady state. In such a case the deflections produced 

 by exposure to the light for equal intervals of time can be compared. 



