Reflecting telescope, George Hearn, London, 1725
Inv V09620

A seventeenth-century telescope consisted of a tube with a number of lenses in it. This kind of telescope had two problems. Firstly the use of several lenses produced colour separation, so that the image you saw became blurred. Secondly for a large magnification you needed quite a long tube.As a solution Newton devised the reflecting telescope. This is also a tube, but with a concave mirror at the bottom. The light that falls through the opening at the top is reflected back by this concave mirror onto a small mirror. This mirror is in the top of the tube and reflects the light onto the side of the tube. With the aid of an eyepiece at the side you can see the night sky. This telescope has in addition a flap at the bottom, so that you can remove the mirror: it required regular polishing.