NO. 7 



CHRISTIANSEN LIGHT FILTER McALISTER 



with light of low intensity, such as in visual work, a carefully thermo- 

 stated water bath is sufficient. For use with intense light, such as 

 direct sunlight, other more direct means of cooling (discussed below) 

 are necessary. Weigert (1929), making use of this temperature co- 



4000 5000 6000 



WAVE LENGTH 



Fig. I. — Index of refraction curves of the components of a filter. 



7000 



efficient, constructed an ingenious filter using methyl benzoate with 

 crown glass particles. This filter transmits red light when at 18° C. 

 and blue at 50° C. 



These filters are not used like the ordinary colored-glass ones. The 

 " undesired " colors are not absorbed as in the case of colored glass, 

 but are scattered symmetrically in a halo about the center line through 



