Home Index Site Map Up: Glassmaking Navigation
Up: Glassmaking

First: The Mentor · The Story of the Lens · Front Cover Last: The Mentor · The Story of the Lens · Back Cover Prev: The Mentor · The Story of the Lens · Page 7 Next: The Mentor · The Story of the Lens · Page 9 Navigation
Lens Story: 10 of 28
·Front Cover
·Page 1
·Page 2
·Page 3
·Page 4
·Page 5
·Page 6
·Page 7
·Page 8
·Page 9
·Page 10
·Page 11
·Page 12
·Gravure 1 Front
·Gravure 1 Back
·Gravure 2 Front
·Gravure 2 Back
·Gravure 3 Front
·Gravure 3 Back
·Gravure 4 Front
·Gravure 4 Back
·Gravure 5 Front
·Gravure 5 Back
·Gravure 6 Front
·Gravure 6 Back
·Back Cover
The normal eye
The image of a distant object is
brought to focus on the retina

The near-sighted eye
The image of a distant object is
brought to focus in front of the retina

The far-sighted eye
The image of a distant object is
brought to focus behind the retina

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
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