Volltext: Optic Projection: Principles, Installation and Use of the Magic Lantern, Projection Microscope, Reflecting Lantern, Moving Picture Machine

Ch. XIV] 
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.


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