Herschelian telescope

A Herschelian telescope was invented in the 1780s by William Herschel. It is similar to a Newtonian telescope, except using only one mirror. 
In the 1780s, telescopes used metal mirrors which were not very reflective. Each mirror would absorb light and make the image dimmer. So Herschel invented a telescope with a tilted mirror that put the eyepiece in the front.
When mirrors became more reflective, Herschelian telescopes stopped being useful.