For obtaining a monochromatic spectral line of mercury arc as the source of excitation for Raman effect, a suitable filter is usually used. Various filters for the purpose have been reported, but as to their spectral characteristics no satisfactory description is given .
Absorption spectra of more than ten of those liquid solution filters have been examined spectro-photometrically from 3500Å to 5000Å. Most of these filters are found inadequate except sodium nitrite aqueous solution and iodine-carbontetrachloride solution; they are very efficient if their concentrations are kept properly adjusted.
Interference filter, which has lately been much improved, is tried. Examination of spectral charac- teristics shows its superiority over the above liquid solution filters, yet its application to practice need further investigation.
