168 



BIOLOGICAL EFFECTS OF RADIATION 



100 A per degree, and consequently the cell must be subjected to accurate 

 thermostatic control. (6) It has the same objections as the monochro- 

 mator in that it produces only a limited area and solid angle of effective 

 source. 



If such a Christiansen filter is used with extended source and without 

 a small restricting aperture, one secures a wave-length distribution fairly 



Wave- Length 

 ^000 6,000 7.000 



8,000 A 



Wave-Length 

 6,000 



4,000 



^^ I.OxlO'^ 



5,000 



aoooA 



eiOOO 7000 8,000 A 5,000 6,000 ^000 



Wave - Length Wave-Length 



Fig. 14. — Effectiveness of Christiansen filter: Upper left — transmission of filter: 

 1. small area illumination; 2. large area illumination. Upper right — radiation trans- 

 mitted by filters. Lower left — absorption coefficient a base 10 of chlorophyll A in ether. 

 Lower right — radiation absorbed by chlorophyll A solution from transmitted beam, 

 showing maxima both in region of maximum transmission and also in region of maximum 

 absorption. Note wave-length shift with increased area of illumination. 



narrow at one-half intensity, but spreading out rapidly at low intensities. 

 This is more aggravated on the long wave-length side. In favor of the 

 Christiansen filter, wave-length bands may be secured anywhere in the 

 visible or near ultra-violet. Wave-length change can be made over a 

 limited range by change in temperature, and over a wider range by vary- 

 ing the liquid. For work in the visible, ordinary optical crown glass, 

 index of refraction 1.52 to 1.54, serves satisfactorily as the transparent 



