LXXVI
produced. This mirror may be used: a) as burning glass, 371;
b) also images may be formed in concave mirrors.
Remark. An image coming to existence by a real junction of two
rays is called a real (physical) image, if rays have only apparently a centre
of union they will produce a virtual (geometrical) image.
An object standing more distant than the focus will produce
an inverted real picture. The more distant the body, the nearer
and smaller the image. If the object is at an infinite distance,
the image being infinitely small, will stand in the focus. The
more the object approaches towards the focus, the more the in
verted image will draw back and increase to an infinite distance
and magnitude. In the centre of curvature the object and the
image coincide.
if the object proceeds from the focus towards the mirror, the
image behind the mirror decreases in magnitude from an infinite
magnitude to that of the object.
If the object stands between the focus and the mirror, only
a virtual enlarged image will be formed (just as in the looking-
glass). 372. These mirrors may c) also be used as Reflectors
by placing a luminous object in their focus. 373. (The concave
mirrors in light-houses. As the rays proceeding from the focus
are reflected in parallel lines, the light can be seen at a great dis
tance. By means of a clock-work the light and mirror turn
round a vertical axis so as to throw light over the whole surface
of the sea. Also in the lanterns of locomotives the light is
placed in the focus of a concave mirror).
§ 189. 2) Convex mirrors. If the light is reflected by
the external side of a spherical mirror, we have a convex mirror
before us. The magnitude and the place of the image are con
structed in the same manner as with the concave mirror. If the
object from an infinite distance approaches towards the mirror,
the image proceeds from the virtual (negative) focus (the point
