These great lights come in a set of 5 different colours, they measure 36mm long x 17mm in diameter. Powered by small flat watch type batteries (supplied). To operate simply push down the white tops and hey presto they change to match the colour of the light case. Once the batteries are flat, simply push on the white lug on the side and the batteries will drop out ready to be replaced. Great for any steampunk project. The photographs do not quite show how rich the colours are in real life.