A modification to a multiple anvil device is described which allows optical transmission through a specimen whilst it is subjected to a pressure of up to 50 kb and a temperature exceeding 1000°C. The pressure cell is a pyrophyllite cube which is squeezed by six anvils located between a pair of wedge cones. The specimen is contained in the cubic cell between a pair of optical windows. Holes along the axis of two opposing anvils, and through the cones, form the path for optical transmission axially through the pressure system.