NO. 7 CHRISTIANSEN LIGHT FILTER — McALISTER 5 



ble " angle for a given " undesired " color and this color is in evidence 

 in varying amounts at all angular positions about the axis of the filter. 

 Some means of intercepting these " undesired " wave lengths is neces- 

 sary. The simplest means is to use an optical system consisting of tv^o 

 lenses with the filter placed between them in parallel light. In this case 

 the " undesired " wave lengths are cut out by a diaphragm placed at 

 the image of the source of light. It is imperative that the opening in 

 this diaphragm should conform with the shape of the source of light. 

 If a filament is the source, the opening in the diaphragm should be cut 

 to conform to the shape and size of the image of the filament. If this 

 is not done, the maximum purity of color is not attained. Obviously, 

 the purity of the color obtained increases with an increase in focal 

 length or a decrease in numerical aperture of the optical system used. 

 (See, for instance, von Fragstein, 1933, pp. 33 and 34.) Thus it is 

 necessary to measure the transmission of the filter with the particular 

 optical set-up to be used. Another way of using the filter is in parallel 

 light — direct sunlight, for instance — employing a series of diaphragms 

 to intercept the halo of " undesired " wave lengths. In this case it 

 is necessary to place the filter at a considerable distance from the ob- 

 server, since the purity of color obtained increases with distance from 

 the filter. 



Owing to the fact that the " undesired " colors are not absorbed but 

 are scattered at various angles about its axis, the Christiansen filter 

 cannot be used in any optical system where sharp images are desired. 

 .For instance, it cannot be used before the lens of a camera in pho- 

 tography. The only way it could be used in this respect is with its 

 own optical system to illuminate the object (necessarily a small one) 

 to be photographed. 



Figure 2 shows the transmission characteristics of a set of five 

 filters at 20° C. They are all made of borosilicate crown glass particles 

 (i to 2 mm in size) immersed in mixtures of carbon disulphide and 

 benzene. The blue filter has about 4 percent (by volume) carbon di- 

 sulphide, the red one 20 percent, and the others have percentages 

 between these limits. These filters are 50 mm in diameter, 18 mm thick, 

 and the windows are fused on optical flats. Two of them are shown in 

 plate I. The transmission curves (fig. 2) were measured with the 

 filters in parallel light between two 20-cm focal length lenses. A 

 double monochromator and vacuum thermocouple were used to make 

 the measurements. This purity of color is obtained only in the image 

 of the filament used as a source. 



A battery of 10 such filters ranging from ultraviolet to infrared, 

 each selecting a spectral region about 150 A wide (at half maximum), 



