Abstract: A photocoagulator is described incorporating a selected light source. The selected light source is a waveguide laser of the type using an active liquid comprising a dye in solution.The laser is so constructed that a cell containing the active liquid and a reservoir, rigid with the cell, for replenishing the liquid in the cell, form a single unit which can be readily removed from the remainder of the laser. An optical pumping source which forms part of the remainder of the laser is arranged to set up a heat gradient in the cell which tends to collimate the stimulated emission in the cell thus avoiding the need for accurate alignment of the reflectors at opposite ends of the cell.