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ALTHOUGH rock salt has been commonly used in the construction of absorption cells for infra-red spectroscopy, it has the inherent disadvantages of being opaque beyond 15 µ, and of being attacked by water. Silver chloride, however, is transparent up to 25 µ and is completely resistant to water.

The Christiansen filter effect in slurries of organic crystals is reported for the first time, in the infra‐red. The main difference between the effect in molecular and in ionic crystals is that the Christiansen (transmission) peak and the ``Reststrahlen'' (reflection) peaks are much closer together for organic crystals since they are produced by a single frequency instead of a wide band of lattice frequencies. The Christiansen effect is much more selective in organic crystals since it occurs for some bands but not for others. This is thought to be due to the nature of the coupling between the particular vibration and the atoms or groups in the molecule which make major contributions to the polarizability.

A double beam recording infra-red spectrometer is described which employs a monochromator and servo system of novel design. The double beam principle adopted is similar to that of Lehrer and Wright. The monochromator is compact and uses relatively cheap and readily available components. The performance of the instrument is shown by reproductions of typical spectrograms.