an annular aperture 42 is inserted into the apparatus where the beam from the sample is collimated the inner and outer diameters of this annular aperture determine the solid angle 112 of detected scattered light fig6 is an extension of fig2 a and shows the focused radiation 106 passing through the aperture 13 in plate 14 and on to a fery prism 44 which disperses the radiation 116 and focuses a spectrum on a curved plate 45 the location and size of the aperture 47 in plate 45 determines the wavelength and wavelength interval of radiation detected by detector 46 a multiplicity of apertures and detectors may be used when it is desired to monitor different spectral intervals simultaneously similarly means other than a fery prism may be used to disperse radiation 106 alternately an array detector may be positioned in place of the aperture plate the signal from the array of detectors 46 may be processed in the usual manner to provide the dynamic changing spectra as samples flow through the system it should be apparent that any type of dispersing system may be employed in fig6 however the fery prism 44 affords a high efficiency along with low stray light when there is no need to filter the light from the.