NO. 7 CHRISTIANSEN LIGHT FILTER McALISTER II 



shorter and longer than the wave length of maximum transmission. 

 In any case care must be exercised in determining the combined effect 

 of the shape of the transmission curve of the filter, the wave length 

 versus sensitivity curve of the phenomena under investigation, and, 

 in the case where the energy content of the beam from the filter is 

 measured with a photocell, the sensitivity versus wave-length response 

 of the detector. For instance, if the energy in the beam from an ultra- 

 violet filter is measured with a photocell that has its maximum sensi- 

 tivity in the blue, considerable error may come into the final result 

 owing to the long-wave-length " tail " on the transmission curve of 

 these filters. (See von Fragstein, 1933, fig. 8, p. 33.) 



The writer believes that these filters will be found of considerable 

 value as a source of monochromatic light for rough visual measure- 

 ments of refractive index, rotation of plane of polarization, etc., be- 

 cause one can set cross hairs on the wave length of maximum trans- 

 mission within ±10 angstroms. With a sealed filter and accurate 

 temperature control this wave length of maximum transmission is 

 sharp and reproducible. 



The large filter shown in plate 2, figure 2, will be used with sunlight 

 to irradiate a growing plant in an experiment to determine the wave- 

 length effect of photosynthesis. At great distance it will yield a trans- 

 mission curve comparable to that shown in figure 3. Two filters are 

 to be used — one to cover the range 4000 A to 6000 A, the other from 

 5500 A to 8000 A. The wave length of maximum transmission is 

 moved through these ranges by temperature variation. 



The possil)ility of substituting a high-dispersion glass for the 

 liquid — i. e., making the filter of a high-dispersion glass flowed around 

 the particles of low-dispersion glass — is interesting. Knudsen (1934) 

 has accomplished this, but gives no details. The resultant filter will 

 have only a very small temperature coefficient, which will considerably 

 enhance its usefulness. The writer has in his possession two suitable 

 glasses, but has not yet had an opportunity to complete the filter. 



The w^riter is grateful to A. N. Finn, of the United States Bureau 

 of Standards, who has kindly furnished the borosilicate crown glass, 

 and to L. B. Clark, of the Division of Radiation and Organisms of 

 the Smithsonian Institution, who constructed the glass cells with f used- 

 on optical windows. 



LITERATURE CITED 

 Christiansen, C. 



1884. Untersuchungen iiber die optischen Eigeiishaften von fein vertheilten 



Korpern. Ann. Phys. Chem., vol. 23, pp. 298-306. 



1885. Idem, vol. 24 pp. 439-446. 



