Provided that the difficulties associated with the high temperature coefficient of refractive index of the liquid component can be overcome, there appear to be certain advantages in the use of compound liquid-glass prisms (Zenger or Wernicke type), in Raman spectrographs. In particular, the curvature of the slit image is negligible, an important factor if photoelectric recording is used. The theoretical equations for dispersion, resolving power and image curvature for such systems have been derived. A small Littrow-mounted Zenger prism has been constructed, and a solution of mercuric bromide in aqueous barium bromide solution chosen as the liquid component after studying its optical constants. Without using elaborate thermostatic control, satisfactory spectrograms of the iron arc have been obtained. The practical resolution in the relatively crude experiments (1500 in the wavelength range λ=4200–5000A) was limited by the slit width used, and the reciprocal dispersion in the neighborhood of λ=4400A was found to be 25A/mm. No appreciable curvature of the slit image was observed with a slit height of 2.5 cm.
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