Abstract: A slow scan, frequency limited spectrometer having a screw mechanism for driving a rotatable grating is improved by incorporating and interlinking the following new constituent members with the screw mechanism: a cylindrical cam member with a groove and a constraint to attain a predetermined pitch, so that rapid scanning of a preselected wavelength range is achieved with the grating; a high torque, variable speed, direct current electric motor, so that the rapid scanning may be varied in speed; and, a liner potentiometer with a sliding contact, so that a remote wavelength readout of the variable rapid scanning can be obtained. Cylinder cam members of various pitches are also disclosed for use in scanning different wavelength ranges. At a low cost for improvements, the resulting spectrometer provides a variable fast scan, and is frequency selective.