NO. 7 CHRISTIANSEN LIGHT FILTER McALISTER 9 



Aluminum was chosen as the metal for the cell and vanes because it is 

 least attacked by the various liquids used. The outside of the alumi- 

 num case must be carefully covered with several coats of waterproof 

 paint. 



PRACTICAL CONSIDERATIONS IN THE CONSTRUCTION AND 

 USE OF THESE FILTERS 



The Christiansen filter is little known in this country. For this 

 reason the writer believes it will not be amiss to pass on to those in- 

 terested some practical points concerning the construction of these 

 filters and their uses. In this connection the writer is drawing on the 

 literature cited and his own experience with these filters. 



The type of cell chosen to hold the components depends upon the 

 use to be made of the filter. For visual work and other uses where 

 only moderate intensities are necessary, a glass cell with parallel win- 

 dows fused on is suitable. If a permanent filter is desired, a small 

 expansion chamber should be provided on the filling " neck," and 

 after filling, the cell should be sealed off above the expansion chamber 

 in a flame. To do this safely the expansion chamber should be packed 

 in carbon dioxide snow. When high intensities of light are. used, such 

 as direct sunlight, the cell needs to be of the type detailed in figure 4. 

 To be sure, a thin glass cell may be used for high intensity work, but 

 the purity of color will be very inferior to that obtained with a metal- 

 cased filter equipped with vanes. 



The glass particles for the filter should be of the best optical quality 

 obtainable — preferably low-dispersion borosilicate crown glass. Fused 

 quartz is also suitable and of course necessary for ultraviolet work. 

 However, the quartz should be free of bubbles and inclusions, as these 

 lower the transmission of the filter and give it a muddy appearance. 

 In preparing the glass particles, the writer has used the following pro- 

 cedure. If the glass or fused quartz is in large fragments, it is ground 

 up with an iron mortar and pestle until the larger particles are 2 or 

 3 mm in size. This should be done with a minimum of grinding. A 

 damp towel should be wrapped about the top of the pestle and draped 

 over the top of the mortar to prevent the " dust " from flying. The 

 operator should use some protection over his nostrils to avoid breath- 

 ing the dust. The glass particles are graded by running them through 

 several sizes of sieves. Before using, the particles must be carefully 

 cleaned. This is best accomplished by boiling in chromic acid cleaning 

 solution. The particles are then washed many times in clear water, 

 then in distilled water, and finally dried completely. The particle size 

 found suitable by previous workers and the writer ranges from 0.5 



