Cosmology HOME
Jump to Ideas The Greek Worldview The Start of Scientific Cosmology The Mechanical Universe Island Universes The Expanding Universe Big Bang or Steady State? The Journey Continues
Naked Eyes The First Telescopes Early Reflectors Golden Era of Refractors Spectroscopy and the Birth of Astrophysics Rebirth of the Reflector New Tools
Galilean Telescope

A Galilean type refracting telescope. At the far left is the convex objective lens. It has a focal length f1 , the length at which it brings light from a distant object to a focus. The concave secondary lens (focal length f2) redirects the light towards the observer’s eye, where the lens in the eye itself — often not mentioned in descriptions like this! — brings it to a focus on the observer’s retina.

Adapted from an illustration in The History of the Telescope by H.C. King, Sky Publishing Corporation, Cambridge Massachusetts, 1955, drawn by Linda Wooliever.

Copyright ©.
Brought to you
by the
Center for History of Physics, a Division of the
American Institute of Physics

The First Telescopes