Achromatic telescope with two eyepieces

This is an achromatic telescope. A telescope is achromatic (= colourless) if the lens system in the telescope makes all colours of the incident light travel the same path. Until the middle of the eighteenth century, the image of a lens telescope (refractor) was not sharp. This was caused by chromatic dispersion: the lenses refracted the light unevenly, which resulted in a blurred image with coloured edges. In 1758, the English John Dollond came with the idea to use two different kinds of glass, each with a different refractive index. By adjusting the shape and the kind of glass of the lenses, they counteracted the prismatic effect of each other. Dollond's invention meant a breakthrough in the development of the telescope. One hundred and fifty years after the invention of the first telescope, there was finally a successful solution to the problem of chromatic dispersion in lens telescopes.