Ch. XIV]
LENSES AND THEIR ACTION
58i
the axis. It results from this that the border rays cross the axis
considerably nearer the lens than the central rays, hence, with
parallel rays, instead of one focus, there are many foci drawn out in
a line. This is shown by the bright core in the photograph of the
cone of rays in fig. 322.
Except with a symmetri¬
cal, double convex lens the
amount of spherical aberra¬
tion depends upon which face
of the lens receives the inci¬
dent light, and whether the
incident light is parallel,
diverging or converging.
■. With plano-convex lenses,
\ as shown in fig. 320-323, the
spherical aberration with
parallel incident fight is less
when the parallel fight is
incident on the convex face
than when the lens is turned
iFiG.*3i9. The Principal Focus of a
Convex and of a Concave Lens.
(From The Microscope)
Axis, Axis. The principal optic axis of
the lenses.
F The focus. In the convex lens it is
where the light rays actually cross the axis.
In the concave lens it is where they would
.... cross if produced backward as indicated
so that the light is incident by the broken lines.
upon the plane face.
For diverging rays the plane face should receive the incident
light, and for converging rays the convex surface should receive the
light to insure minimum spherical aberration. With all lenses,
the general rule to follow is that for minimum spherical aberration,
the fight rays should be equally bent on entering and on leaving the
lens i. e., at both refracting surfaces. Furthermore, with the same
light beam, the aberration is greater for lenses of large curva¬
ture than for lenses of small curvature.
To overcome this aberration, a concave lens is combined with a
convex lens, and so proportioned that the too great converging
effect of the outer zone of the convex lens is just counterbalanced
t>y the diverging effect of the concave lens in its various zones (fig.
324). A perfectly corrected, or aplanatic combination brings all
the parallel rays to one focus.