Up: Glassmaking
Lens Story: 10 of 28
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THE NORMAL EYE
The image of a distant object is
brought to focus on the retina
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THE NEAR-SIGHTED EYE
The image of a distant object is
brought to focus in front of the retina
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THE FAR-SIGHTED EYE
The image of a distant object is
brought to focus behind the retina
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be caught on a screen and one must look
through the lens in order to see it. Every lens has a principal axis
and a principal focus. The principal axis is a line passing through
the optical center of the lens and at right angles to both surfaces.
The point where all the rays parallel to this principal axis meet after
passing through the lens is the principal focus. On account of what is
called "spherical aberration" the rays near the edges of the lens are
refracted most and come to focus nearer the lens than the other rays.
This condition must be corrected in all lenses used in optical instruments.
The focal length of a lens is the distance between its optical center and
the principal focus. With converging lenses if an object is placed
outside the principal focus, a real, inverted image will result.
If placed inside the focus, a virtual image, erect and magnified,
is formed. The size of the real image will depend upon the distance of
the object from the lens, and may be smaller, larger, or of the same
size as the object.
The Human Lens
The most wonderful lens in the world is in the
human eye. No other lens has the power automatically to change its focal
length to meet the varying conditions of near and far objects. Back of
the cornea, the transparent window of the eye, lies the crystal-aqueous
humor, and, behind it, a gelatinous medium termed the vitreous humor.
At the back of the eyeball the optic nerve spreads out in the retina,
the part of the eye that is sensitive to light.
The crystalline lens and the aqueous and vitreous
humors have the power to refract light, just as any other lens has.
The rays of light entering the pupil of the eye are brought to focus on
the retina, where a real inverted image of the object is formed. That we
see objects erect, is due to our own mental interpretation of the images
on the retina. A normal eye when relaxed will focus clearly on the
retina objects which are at a considerable distance. This is because
the principal focus of the optical system under those conditions comes at
the retina. If the eye turns from a distant object to one near at hand,
in order still to bring the image on the retina, either the focal length
of the lens must change or the distance between the lens and the retina
must increase. In the camera we increase the distance between the plate
the the lens, but, in the eye, a ring of muscle around the crystalline
lens causes the lens to become more convex, thereby shortening its
focal length just the right amount to produce the image on the retina.
To a near-sighted person the image of a distant
object is brought to focus in front of the retina. This is due either
to too great convexity of the crystalline lens or to excessive elongation
of the eyeball. In order to throw the image back on the retina
PRODUCTION OF AN IMAGE BY
CRYSTALLINE LENS OF EYE
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and produce a distinct image, such a person must bring objects close to
the eye. If this defect is to be corrected, lenses that will diverge
the rays of light and prevent their focusing so soon must be used.
Therefore spectacles with concave lenses—thick at the edge and
thin in the middle—are employed.
In the far-sighted eye the image of a nearby
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