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flected twice towards the base of the prism; but as we see the
image in the direction of the ray at the moment it enters the
eye, the object seen through a prism appears deflected towards
the summit (the edge opposite the base). 380.
§ 197. A disc of glass bounded by two curved surfaces is
called a lens. The two chief kinds of lenses are: the double con
vex lens (both surfaces convex) and the double concave lens
(both surfaces concave). The path of rays in double convex
lenses may be easily understood, for it may be deduced from
that through a prism (the surface of a lens may be supposed
as being formed by an infinite number of small-plane surfaces).
A double convex lens is the portion common to two balls which
intersect each other; the centres of these two balls are the centres
of curvature of the lens; the centre of the lens itself is the
optical centre and a ray passing through this centre is not re
fracted at all, because it would be the same as if a ray traverses
a glass with parallel faces. Luminous rays parallel to the axis,
after having traversed this lens, unite in a single point, called
the principal focus. As great heat can be produced in this
point, the double convex lens (like the concave mirror) may also
be used as a burning glass or as a reflector in lanterns and light
houses. 381.
§ 198. As to the images produced by this lens we may
say in short: when the object stands outside the principal focus
a real image is formed, which is the smaller the more distant
the object is. When the object is between the lens and the
focus only a virtual (as in the looking-glass) multiplied image
will be produced behind the object. 382.
§ 199. A double concave lens is the place between two
balls almost neighbouring each other; the same centres and foci
may be distinguished as in the double convex lens. This lens
