A device for safely reducing the intensity of light transmitted through a telescope during observation of the Sun. It consists of a thin prism with unsilvered faces. The first face is inclined at 45° to the optical axis and reflects 5 per cent of the incident radiation. A baffle is used to intercept the reflected beam from the second surface. The remaining radiation, which passes through, should be safely absorbed in a heat trap. An infrared rejection filter should also be used in conjunction with a Herschel wedge.