Note A — Chapter I · EARLY LIFE OF WILLIAM HERSCHEL.
Reading edition
In “Newtonian” telescopes the image formed by the large speculum is obliquely reflected from a small plane mirror to the side of the tube, where it is viewed with an ordinary eye-piece. With a “Gregorian,” the observer looks straight forward, the image being thrown back by a little concave mirror through a central perforation in the speculum where the eye-piece is fitted.