The average transmission of one of the Fabry‐Perot mirrors in a ruby laser has been varied by changing the diameter of a hole in the center of one of the aluminum end coatings. Measurements of the output of the laser as a function of average transmission with fixed input energy show that the output is maximum for a particular value of transmission. The optimum transmission depends upon the input energy. A relationship between the optimum transmission and threshold for stimulated emission is shown.