SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



distilled water. This water cell (F H ) is connected to a small reservoir 

 kept at room temperature and by thermosiphon action the accumulation 

 of heat is minimized. By means of these filters the radiation utilized in 

 the transmission measurements is restricted to the range from 6240 A. 

 to just beyond the visible in the near infrared. 





Fig. 1. — Diagram of apparatus used for the determination of small amounts of 

 chlorophyll. Fi, Corning heat-resistant, heat-absorbing, light shade glass filter, 

 2.66 mm. thick; F 2 , heat-resistant, pyrometer red, No. 241, 48 percent filter, 4.85 

 mm. thick; F 3 , glass water cell, 2 cm. thick: C, glass absorption cell 5 cm. long 

 used for holding chlorophyll solutions ; T, vacuum thermocouple. 



A thermocouple (T) is employed as the energy receiver. The brass 

 thermocouple housing is thermally insulated from the lamp housing 

 by a cylinder of Bakelite whose interior diameter is decreased in steps 

 to reduce internal reflections. 



The thermocouple has been developed and built in this laboratory 

 by L. B. Clark. The receiver is a circular disk 1 mm. in diameter and 

 0.00127 mm. thick. The couple is permanently evacuated to a pressure 

 of less than io~ 4 mm. mercury which increases its response 20- to 



