A comparison fluorimeter, utilizing a secondary emission multiplier as the sensitive element, has been built. It consists of an optical system, an electronic switching system and power supply, and a housing for the entire assembly;A single secondary emission multiplier was used to alternately determine the intensity of the samples to be compared;A simple dual cathode follower system was designed to switch the multiplier tube, and to simultaneously provide impedance matching between the multiplier tube and the meter being used in the circuit;Tests on the cathode follower circuit show that it is a highly stable, comparatively linear system for detecting potential differences up to thirty volts;The entire unit was housed in an aluminum box designed to have all necessary controls readily accessible;The fluorimeter was found to be considerably more sensitive than existing devices using older types of phototubes;The fluorimeter has considerable range, and when used as recommended, was found to be nearly linear in its response to variations in concentration;Many of the design features are readily adaptable to similar devices, such as turbidity meters.