A Christiansen filter is a narrow bandpass or monochromatic optical filter which consists of an optical cell which is stuffed with a crushed substance (e.g. glass) and a (mostly organic) liquid.
The liquid is chosen according to the substance, so that the dispersion curves coincide at one wavelength.
For this wavelength the filled optical cell behaves like a plane-parallel, homogeneous disk and allows transmission.
All other wavelength ranges of the spectrum are reflected, scattered as well as refracted at the many interfaces between substance and liquid.
A typical filter is composed of glass particles immersed in a mixture of benzene and carbon disulfide.