LXXXIV
catoptric telescope), the image produced by a concave mirror
placed at the end of a long wooden tube, is caught by a plane
mirror or unrefracting prism and directed either in front of a
tube, containing a series of magnifying glasses, on the side of
the large tube (Newton, Earl of Rosse) or it is beheld by an
ocular near the front of the telescope (Gregory, Herschel). 393.
§ 205. 5). Microscopes for observing objects too small to
be seen by the naked eye. The simplest kind is nothing but a
double-convex lens and the object is put between the lens and
its principal focus (see § 197. 198).
§ 206 The Compound Μicroscope consists of a small ob
jective at the bottom of the tube, below which the object is laid
and an eyepiece at the top. The image already magnified by
the first lens is magnified again by the second. To increase
the power of this instrument two or three object-glasses and
oculars may be applied and thus the magnifying power may be
carried to 1801 times. To increase illumination a concave mir
ror below (see No. 101) (g), or if the object is an opaque body a
condensing lens (k) above is in use. 392.
§ 207 The Solar Microscope consists of a double convex
lens (l) with a small focal distance (the distance between the
focus and the mirror), which produces of the small object (h)
placed outside of the focal distance a magnified real image (o-p)
on the white wall in a dark room (m-n). But as the rays of
