26 Introduction 



bolometer, the electrical response of the bolometer remains constant; whereas, 

 if a ray is shifted along the surface of a thermopile, the deflections of the galvano- 

 meter change so considerably that absolute radiation measurements are impossible. 



We have simplined (11) the calibration of the bolometer by substituting for the 

 Hefner lamp the calibrated carbon-filament lamp of the Bureau of Standards in 

 Washington. If, during calibration, the bolometer opening is covered by means of 

 a lithium fluoride window rather than a quartz window, no corrections need be made 

 for selective absorption; correction must be made only for the reflection of the 

 Standard light. 



As light sources we use the mercury and xenon high-pressure lamps manufac- 

 tured by the Osram A.G., or 500-watt metal-filament lamps that have a small 

 area of intense brightness. Nowadays we rarely employ monochromators to isolate 

 the spectal regions but, most of the time, use colored glass filters and interference 

 disks. We test each spectral region before using it for measurements of photosyn- 

 thesis, employing a Solution of 1 mg/cm 3 ethyl chlorophyllide in pyridine; the So- 

 lution should not transmit any light in a layer of 1-cm thickness. 



Another contribution to the development of radiation methods (11) is the 

 Splitting of light beams by means of prisms or of mirrors that permit only partial 

 transmission; and the bolometric adjustment of the two parts to the same intensity, 

 as is required in the two-vessel method. This technique has been improved in 

 such a way for the measuring the absorption of the blue-green action spectrum of 

 photosynthesis, that it became possible to add, to two red beams of equal intensity, 

 two blue-green beams of equal intensity. 



IV. Radiation measurement (Quantum actinometer) (4) 



In addition to using the bolometer for absolute radiation measurements, we deve- 

 loped the quantum actinometer for special purposes, In this instrument, oxygen 

 is transferred to thiourea by chlorophyllide or pheophorbide and the oxygen 

 absorbed is measured manometrically. If the quantum intensities are below a cer- 

 tain threshold, the amount of oxygen consumed when one light quantum is ab- 

 sorbed is almost exactly one molecule. 



If monochromatic light beams are to be measured, the bolometer is to be preferred 

 to the actinometer. But if diffuse light is to be measured, the actinometer is prefer- 

 red. The actinometer may also be used to measure the sum of light intensities 

 that are not constant, since the actinometer integrates the intensities of the 

 absorbed light. 



V. Radiation measurement (Absorption measurement) (25) 



The most difficult part of the determination of the photosynthetic yield by 

 radiation measurements was the measurement of the light absorption in the 

 turbid media of the cell suspensions. Our first Solution of the problem consisted 



