VISIBLE AND NEAR-VISIBLE RADIATION 

 Table 11b 



167 



T.\BLE lie 



obtained through the diaphragm aperture. If great purity is to be 

 secured, source and final aperture must be small. Such an optical system 

 must be just as precise as that of a spectrograph, and of the same general 

 characteristics, the Christiansen filter taking the place of the prism. It 

 will be seen, however, that source and emergent aperture must be limited 

 in both directions, while in a spectrograph the sUt need only be narrow. 

 Consequently, the Christiansen filter set-up is in no sense superior to a 

 monochromator. It has the following objections: (a) It is critically 

 dependent upon the temperature-sensitive dispersion curve of the liquid. 

 In most cases, the wave-length of coincidence of indices varies some 



