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Properties of Light

47. NEWTON'S RINGS.

a.

When positive and negative lenses of slightly unequal curvature are pressed one against the other, irregular bands or patches of light and darkness or color appear between the surfaces. This pattern is called Newton's Rings (fig. 68), after Sir Isaac Newton who directed attention to it. This condition is a defect if it occurs in a compound lens. It may be utilized to advantage as a means for testing the accuracy of grinding and polishing lenses.

b. When testing lenses for accuracy of grinding and polishing, a test lens having the desired curvature is used. The test lens is placed in contact with the lens to be tested, care being taken that both surfaces are perfectly clean and dry. When the lenses have been squeezed together, if the lens being tested is perfect, the air film between the two lenses will be of uniform thickness and the color will be uniform all over the surface. Irregular bands or patches of color show that the surface of the tested lens is not perfect and point out clearly the parts of the lens needing attention. If the lenses are viewed by daylight, the rings will be colored; if viewed by pure white light, the rings will be alternately black and white.

48. LIGHT LOSS.

a.

Whenever light rays strike the surface of any lens or prism, a certain amount of light is lost by reflection. Light is absorbed by every element it strikes or travels through. The more elements in the optical system, the more light is lost by absorption.

b. Light loss has been greatly reduced by coating the surfaces of elements which are intended only to refract light (chap. 4, sec. II). The additional light transmitted by instruments in which various elements have been coated produces brighter, more effective images.

49. PARALLAX.

a.

Parallax is apparent motion of the reticle with respect to the field of view of an optical instrument. It is due to incorrect adjustment of objective position, so that the focal points of distant objects and the reticle are different. When the cross line or reticle is not in the focal plane of the objective, the image and the reticle are not magnified simultaneously and both are not in focus at the same time. This displacement will render all measurments made with the reticle inaccurate.

b. Correction for parallax in some instruments can be made by focusing the objective. If the instrument is provided only with a focusing eyepiece, as is the case of most fire control instruments, the objective is adjusted during manufacture or maintenance to eliminate parallax for an object at a specified distance from the instrument. This distance is chosen so that parallax will be negligible for all average operating distances.

CHAPTER 3

THE HUMAN EYE

Section I
GENERAL

50. COMPARISON OF EYE AND CAMERA.

а.

A high-grade came a closely resembles the eye in basic essentials. Each has a compound lens to refract light rays and to project them to a definite point by means of focusing. Each has a means of shutting out all light, a diaphragm to regulate the entering light, a sensitized surface to receive and record optical images, and a lightproof chamber to protect the receiving and recording mediums (fig. 69).

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a.

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The eye is a bulbous organ which rotates through quite a wide range in a socket in the bone structure of the head. Muscles control the eyes in associated movements to a considerable extent and in individual movement to a lesser degree. Protection and shade are furnished by the eyelids, lashes, and brows.

b. The eye has three coats which afford it protection, supply it with nutrition, and transmit visual impressions to the brain (fig. 70).

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Figure 71 - Action of Iris Compared With Camera Diaphragm It has a refracting mechanism consisting of the crystalline lens, cornea, aqueous humor, and vitreous humor; together, these form a compound lens. The shape of the lens is changed by the ciliary body to change the focus of the lens. The iris serves as a diaphragm to regulate the amount of incoming light.

C. The response mechanism of the eye is centered in a small area called the macula on the inner coat or retina and in a still smaller spot in the center of the macula called the fovea. Sensations of light are received by tiny cones and rods attached to nerves and distributed over the fovea, macula, and retina. These sensations are transmitted to the brain by the optic nerve at the lower rear of the eyeball.

52. THREE COATS OR TUNICS.

a. The outer of the three coats or tunics of the eye is the sclera (fig. 70). It is tough, white, and flexible and is the white portion of the eye normally seen. The slightly protruding transparent portion at the center front of the eye, the cornea, is part of this coat. The cornea and the transparent liquid or aqueous humor behind it are parts of the refracting mechanism of the eye.

b. The middle coat, the choroid, is a deep purple layer made up of veins and blood vessels which supplies nourishment to the eye tissues. The coloring of the choroid forms a dark housing and prevents internal reflection of light rays within the eye.

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C. The inmost coat of the eye is the retina, a highly sensitive layer of nerve fibres by means of which visual impressions are trans

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mitted to the brain. The retina is a part of the response mechanism. The interior of the eye is filled with a jelly-like substance, the vitreous humor, which is transparent.

53. REFRACTING MECHANISM.

a.

The chief refracting surfaces of the eye are those of the cornea and the crystalline lens. The cornea provides the greater portion of the refraction. The crystalline lens supplies a variable degree of refraction which permits the eye to focus on near or distant objects. This gives to the eyes the ability to see far or near objects distinctly and is termed the power of accommodation.

b. The iris is a diaphragm which contracts and dilates to regulate the amount of light entering the eye (fig. 71). The iris is the colored portion of the eye around the pupil. The pupil is the circular opening in the center of the iris. Its size is limited by the contraction or dilation of the iris. It appears black because of the darkness of the inside of the eyeball.

C.

The crystalline lens is transparent, and is suspended by the ciliary body which encircles it (A, fig. 72). The front of the lens rests against the aqueous humor, the rear against the vitreous humor. The suspensory ligaments of the ciliary body draw upon or release the outer edges of the lens to change its refracting power by altering its shape.

d. The lens is double-convex and when the eye is relaxed, the front surface is flatter than the rear (B, fig. 72). With the lens in this condition, the eye will focus upon distant objects. To increase the refractive power of the lens for focusing the eye upon a close object, the surfaces are made more convex by the suspensory liga

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