by simply rotating grating 22 about an axis parallel to the lines such rotation may be performed by an operator of the apparatus turning a simple micrometer lead screw as an external adjustment from grating 22 light is directed to a lens assembly designated by numeral 24 this lens assembly includes a first air spaced achromatic lens 25 light is focused by this lens to a diffraction limited spot on a slit 26 whose width is preferably between two and three times wider than the 1 e 2 width of the focal waist in one preferred design the lens focal length is 48 mm the focal waist at 488 nm is approximately 20 microns and the slit width is approximately 50 microns a second lens 27 is included in lens assembly 24 which can be adjusted so that the output beam divergence of the filtered laser line is appropriate for focusing simultaneously with the dye laser beam onto the stream of flowing particles 38 by means of lens 32 the monochromator optics such as the prisms and lens entrance and exit surfaces are all anti reflection coated for 0 995 or greater transmittance at the argon wavelength further the diffraction grating whether a holographically recorded or ruled type may be replicated on a copper substrate so that the energy densities even with a 5 0 watt pump should not present a threshold problem in order to appreciate how the diffraction grating of the present invention compares with a narrow line interference filter for nearby filtering such as described by arndt jovin et al the slit function half widths of the present invention may be compared with the bandwidth of narrow line interference filters.