The Human Eye

62. EYE TENSION OR FATIGUE.

a.

Blinking of the eyes is an automatic process resulting from eye tension or fatigue. It will occur in the use of optical instruments because the eyes cannot focus steadily very long without relaxing. It is muscular rather than retinal and is least apparent when the eye is relaxed as when accommodated for distant objects. For this reason, in focusing an instrument, it should be focused for distance and the first distinct focus setting should be used.

b. Fatigue of the eye muscles will be experienced after comparatively short periods of continuous observation. This fatigue is usually greater with low illumination. Inasmuch as the eyes quickly recuperate, frequent rest periods are advisable.

c.

A particular type of fatigue results from the use of binocular instruments if not set at proper interpupillary distance. This is due to the fact that both eyes involuntarily adjust themselves so that a single image is formed when the image is focused on the macula of each eye where best vision is obtained. Under ordinary conditions this is done when viewing a distant object. In using an instrument, objects may be at different points and the eyes are placed in a condition of forced equilibrium by the expenditure of an amount of nervous force, resulting in rapid fatigue.

a. Types. Two of the most important types of optical glass are known as crown glass and flint glass. Crown is an alkali-lime glass. Boro-silicate crown glass contains a salt composed of boric and silicic acid. Flint glass contains lead. The refractive indices of these types of glass are listed in paragraph 14 c.

b. Characteristics. The characteristics affecting the values of all types of optical glass may be divided into the purely optical properties, which directly influence the light in passing through the glass, and the general qualities of the glass itself. The purely optical properties are constant density (homogeneity), transparency, refraction, dispersion, and freedom from color. The general features of optical glass are chemical stability, mechanical hardness, and freedom from internal strain and defects.

(1) Constant density or homogeneity is the most important property. The uniformity of the index of refraction of the glass is dependent upon its constant density.

(2) The greater the transparency, the less light is absorbed and the more light will pass through.

(3) The refractive and dispersive qualities are dependent upon the type of glass. Crown glass has nearly twice the dispersive power (ability to spread the colors of so-called white light) of flint glass.

(4) Freedom of color, while preferable, is not as essential as the other requirements. It is obtained by careful selection of raw materials which must be of the greatest possible purity.

(5) Chemical stability and hardness determine the resistance of the finished optical element to handling and contact with atmospheric moisture.

(6) Freedom from strain depends upon annealing or cooling of the glass in manufacture. Any large amount of internal strain may cause the glass to break during grinding or if subjected to shock at any time after grinding.

(7) Types of defects are known as stones, bubbles, seeds, and striae (layers). Stones are due to solid material that accidentally may get into the molten glass at the time of manufacture. Bubbles are air pockets in the glass. Seeds are very minute bubbles. Striae are wavy bands which appear to be of different density and color than the surrounding medium.

Optical Components, Coated Optics, and Construction Features

C.

Manufacture. Although an understanding of the manufacture of optical glass is not essential to a knowledge of elementary optics, some appreciation of the many processes involved will show why fine optical glass is considered a critical material. The finely divided materials are very thoroughly mixed, usually by hand. This mixture is melted in small charges, small quantities being fed to the melting pot at intervals regulated by the melting time of the charge previously introduced. Each charge is brought to a liquid state before the next charge is placed in the pot. The next operation, the fining process, consists of holding the molten glass mixture at a high temperature (sometimes as long as 30 hours) to drive off the bubbles contained in the liquid. To secure homogeneity and prevent striae, the molten glass is then stirred during the gradual cooling of the mass until it is so stiff that the stirrer cannot be moved. The mass is stirred from 4 to 20 hours, depending on the glass and the size of the pot. As soon as the stirring ceases, the pot is withdrawn from the furnace and allowed to cool to about the annealing temperature. It is then placed in an annealing kiln at a temperature of from 400 to 500 degrees centigrade, and slowly cooled to ordinary temperature. Annealing takes from 1 to 2 weeks, according to the size of the batch. When cooled, the pot is withdrawn from the annealing kiln and broken away from the glass.

64. LENSES.

a.

Single Lenses. Lenses are divided into two general classes (fig. 179). One class forms real images, and lenses in this class are termed convex, convergent, positive, or collective lenses. The other class can form only virtual images, and lenses in this class are termed concave, divergent, negative, or dispersive lenses.

(1) CONVEX LENSES. All convex lenses are thicker in the center than at the edges and will converge light from distant sources and objects. Both faces of a convex lens may be convex, one surface may be convex while the other is flat (termed plano- or plane), or one face may be convex while the other is concave.

(a) A double-convex lens (fig. 179) is one in which the convex curvature of one face is more powerful than that of the other face. Both surfaces contribute to the converging power of the lens. The greater the convexity of the surfaces, the shorter the focal distance.

(b) A plano-convex lens (fig. 179) has a plane surface and a convex surface. The plane surface is directed towards the light and does not contribute to the converging power of the lens.

(c) The convexo-concave or meniscus converging lens (fig. 179) possesses both a convex and a concave surface. The more powerful convex curve makes this a positive lens despite the fact that the concave surface tends to diverge the light, thus subtracting from the converging power of the lens. Meniscus lenses are mainly used for

spectacles because they permit undistorted vision through their margins as the eyes are rotated in their sockets.

(2) CONCAVE LENSES. Concave or divergent lenses (fig. 179) are thinner in the center than at the edges and will diverge light from distant sources and objects.

(a) One face of divergent lens may have a more powerful concave curvature (double-concave); one surface may be concave and the other plane (plano-concave); or one face may be concave while the other is convex (concavo-convex or divergent meniscus).

(b) The convex surfaces of either converging or diverging lenses of this type are often toward the light. The more powerful curve determines whether the lens is convergent or divergent.

b. Compound Lenses. As an optically perfect single lens cannot be produced, two, three, or more lenses ground from different types of optical glass are frequently combined as a unit to cancel aberrations or defects which are present in the single lens (see aberrations, ch. 2, sec. VII).

(1) The refractive power of a compound lens is less than that of the convex lens alone. For example, if a double-convex lens of crown glass is combined with a concavo-plano lens of flint glass, the latter would have little more than half the refractive power of the former, but sufficient dispersive power to neutralize the dispersion. The result would be that light passing through this compound lens would be brought to a practical focus at a point that would be about double the distance of the focal point of the crown lens alone (fig. 79).

(2) The elements are frequently cemented together with their optical axes in alinement. Two lenses may be cemented togther as

a doublet or three may be cemented together as a triplet, or each lens of the unit may be mounted separately (fig. 80). The cementing of the contact surfaces is generally considered desirable because it helps to maintain the two elements in alinement under sharp blows, it aids cleanliness, and it decreases the loss of light through reflection at the two surfaces in contact.

65. OBJECTIVES.

a. General. The lens nearest the object in any optical system of the refracting type is called the objective. Its function is to gather as much light as possible from the object and form a real image of that object. Objectives usually are of the compound type in order to reduce color and other aberrations to a minimum.

b. Construction. The majority of objectives are constructed of two elements, a double-convex converging lens of crown glass and a plano-concavo flint lens (A, fig. 80).

(1) When the elements of the objective are of large diameter, or when the faces of the elements are of different curvature, the elements are not cemented but are held in their correct relative positions in a cell by retaining and locking rings (B, fig. 80). This construction permits giving the inner surfaces of the two elements different values, and allows greater freedom in the correction of aberrations. An objective of this type is often called a dialyte or Gauss objective.

(2) Certain objectives are composed of three elements securely cemented together (C, fig. 80), or with two of the elements cemented and one mounted separately. Such objectives afford a total of six surfaces upon which the designer can secure the best possible correction for aberrations.