Note:  Do not rely on this information. It is very old.

Herschel Telescope

Herschel Telescope is a special form of reflecting telescope, possessing the advantage of great simplicity, but only adapted for large instruments and for observation of nebulae or of such other objects as chiefly require light-giving power. The large concave reflector is placed at the end of the tube, so that light from the heavens is reflected back; a slight tilting is given to the mirror, so that the reflected image is brought to one side of the tube, and the observer introduces his head actually into the tube to examine this image. The amount of light intercepted by his head is a fatal objection if the aperture of the telescope is small. Herschel's instrument of 1789 had a four-feet aperture. The obliquity of the mirror spoils the definition of the image, and heat from the body of the observer is also found to introduce difficulties. [Telescope.]