The optical intensity correlation technique developed by Hanbury Brown and Twiss has been used to measure the linewidth of 4358-\AA{} light from a mercury discharge lamp which was filtered with a Fabry-Perot interferometer. A time-to-height converter was used to convert the delay spectrum of photoelectric counts to a pulse-height spectrum which was recorded by a multichannel pulse-height analyzer. Correlation times of 1.0 to 3.4 nsec were observed, corresponding to linewidths of 208 to 66 MHz.