Animated model of the reflecting telescope designed and built by the English scientist Isaac Newton in 1671-2. Unlike earlier telescopes, which used lenses to focus light, Newton's designs used a concave mirror to focus light, helping to prevent the splitting of colours seen with lenses. This telescope was the third built by Newton, and had a mirror some 5 centimetres in diameter. Newtonian telescopes remain popular, as a large mirror is cheaper and lighter to construct than a lens giving the same light-gathering and magnifying power. Newton used his telescope to observe the phases of Venus and the moons of Jupiter.