APPENDIX 8 



KEPORT ON THE DIVISION OF RADIATION AND 



ORGANISMS 



Sir: I have the honor to submit the following report on the 

 activities of the Division of Radiation and Organisms during the 

 year ended June 30, 1935 : 



It is a pleasure to acknowledge further financial support for the 

 Division during the past year from the Research Corporation of 

 New York. 



An important improvement of the Christiansen filters used for 

 selecting desired spectral rays for caiTying on plant growth exper- 

 iments was perfected. The difficulty hitherto has been that when 

 powerful beams of white light enter a Christiansen filter, the central 

 parts of the filter, farthest from the control of the water jacket, rise 

 considerably above the temperature of control. This spoils the 

 selective properties of the filter and gives rise to a broad, indefinite 

 spectral band. The defect was remedied by inserting parallel with 

 the transmitted beam a grill of thin aluminum strips intimately in 

 contact with the outer wall of the filter. In this way, without much 

 loss of light, the excess heat at the center is conducted away and the 

 selective properties are greatly improved. 



Christiansen filters thus equipped have been used to repeat exper- 

 iments on the dependence of the growth of algae and of wheat on 

 the wave length of radiation. In the experiments on wheat a 

 further improvement was made by setting up the great coelostat 

 referred to in the Smithsonian Report for 1903, constructing far 

 use with it a pair of very large Christiansen filters and using sun- 

 light in place of electric light, thus multiplying the available intensi- 

 ties. By controlling the temperature of the water jacket it was then 

 possible to select from the solar spectrum any desired color from 

 the extreme red to the deep violet. 



With these improvements, studies of wave-length influence on the 

 growth of unicellular algae and on photosynthesis of wheat have 

 been repeated with great success, much improving earlier results. 

 The study of the lethal effects of ultraviolet rays on unicellular algae 

 has also been repeated and carried to a wave length of 2,250 

 Angstroms, with highly accurate results. Further experiments in 

 phototropism are in progress, and new results of especial interest 

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