THE CHRISTIANSEN LIGHT FILTER: ITS ADVANTAGES 

 AND LIMITATIONS 



By E. D. McALISTER 

 Division of Radiation and Organisms, Smithsonian Institution 



(With 2 Plates) 



INTRODUCTION 



Since the Christiansen hght filter is Httle known in this country, 

 it is believed that a brief description of the filter and a discussion of 

 its possibilities may be useful. The object of the present paper is 

 threefold: i, to report an improvement in the construction of the 

 filter, which allows its use in an intense beam of light; 2, to discuss 

 the advantages and limitations of these filters for general usage ; 

 and 3, to give some " practical suggestions " concerning the construc- 

 tion of these filters. The improvement mentioned has arisen from a 

 need (in our laboratory) for an extensive beam of reasonably mono- 

 chromatic light intense enough to produce an easily measured amount 

 of photosynthesis in a higher plant. The second and third purposes 

 of the paper are to answer numerous inquiries the writer has received 

 during the past year. 



REVIEW OF LITERATURE 



In 1884 C. Christiansen discovered that a mass of glass particles 

 immersed in a liquid transmitted freely that color for which the liquid 

 and glass particles had the same refractive index. He pointed out in 

 two papers (1884, 1885) that any desired color could be obtained and 

 that a color complementary to the one directly transmitted was seen 

 at oblique angles. He also showed that the wave length of the trans- 

 mitted ray decreased rapidly with an increase in temperature. After 

 a paper with comments and improvements by Lord Rayleigh in 1885, 

 the subject lay dormant for nearly 50 years with the exception of a 

 descriptive paragraph in all editions of R. W. Wood's " Physical 

 Optics." In a series of three papers F. Weigert and collaborators 

 (1927, 1929, 1930) show the necessity of accurately controlling the 

 temperature of the filters and the advantage of a refined optical system, 

 and also describe a single filter that transmits red light when at 18° C. 



Smithsonian Miscellaneous Collections, Vol. 93, No. 7 



