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