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toward each other like the surfaces of a common house shingle. Some wedges used in fire control instruments appear to be plates or disks of glass with parallel surfaces because the angle between the surfaces is so slight it cannot be detected except by careful examination or by actual measurement.

C. Because divergence by wedges is produced by refraction instead of reflection, a certain amount of dispersion or separation of color results from the use of wedges. For this reason achromatic wedges, composed of two different kinds of glass to neutralize the dispersion, must be used where the angles through which the rays are bent are relatively large or where the work performed by the wedges is of the most exacting nature.

d. All wedges cause a certain deviation of the path of light. Instruments employing wedges are therefore designed so that the path of light entering the wedge has a certain amount of initial deviation, which is termed the constant deviation. Were it not for this factor, the wedge could be made only to increase the amount of deviation. The constant deviation makes it possible for the wedge to neutralize the path of light or divert it at a minus angle.

ē. A wedge may be caused to change the direction of a path of light by rotation of the wedge (fig. 94). The extent to which a wedge may be made to divert the path of light may be varied by changing the position of the wedge with relation to the other elements of the optical system (fig. 95).

f. Another means of varying the deviation of a path of light is the use of pairs of wedges which are geared to rotate in opposite directions. The wedges of such a system, which may consist of two or four elements, are referred to as rotating wedges or rotating compen

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sating wedges. When the thicker edges of two of these wedges are together (A, fig. 96), they produce a deviation twice that of one wedge, and in the direction of the thicker edges of the wedges. When rotated until the thick edge of one wedge is toward the thin edge of the other (B, fig. 96), the wedges neutralize one another and the wedges produce no deviation. Intermediate positions cause a deviation which increases as the thick edge of one wedge rotates farther from the thin edge of the other. When both wedges are rotated through 180 degrees (C, fig. 96), the thick edges again are together and the wedges function as in A, figure 115, except in the opposite direction.

69. ERECTING SYSTEMS.

a.

Types. Erecting systems are of two general types, lens and prism. While most lens erecting systems are of the same general design, differing only in the types of lenses used and the spacing of the elements, the prism erecting systems employ elements of a wide variety of shapes, causing the rays of light to be diverted in different paths in the different systems.

(1) A lens erecting system greatly increases the length of an instrument. It is often used where length in the optical system is a distinct advantage, as in periscopes. This system also is employed in instruments with variable magnification because different degrees of magnification can be obtained by changing the relative positions of the lenses.

(2) Where compactness of instrument is a desirable feature, a prism erecting system is used. Such a system may consist of a single element, or several. The great reduction of length resulting from the use of a prism system is due to the doubling back of the path of light.

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The prism erecting systems in most general use employ Porro and Abbe prisms, the Amici or roof prism, the Dove or rotating prism, and single and double right-angle prisms.

b. Lens Erecting Systems.

(1) The simplest form of lens erecting system places a second convex or converging lens in the proper position to pick up the inverted image formed by the objective, and form a second erect image which is magnified by the eyepiece (upper, fig. 97). In fire control instruments, a lens erecting system of two, and sometimes three, lenses is substituted for the single erecting lens (lower, fig. 97). The entire lens erecting system functions as a single lens.

(2) In a two-element lens erecting system, both lenses usually are of compound (achromatic) type. In a three-element system, the third lens is of plano-convex or double-convex type and serves as a collective lens. It converges the light rays of the image formed by the objective so they will pass through the erecting lenses. It increases the field of view and brightness of image for a given diameter of the lenses of the erecting system. Inasmuch as the collective lens is sometimes placed at the point where the image of the objective lens is formed, and a reticle must be placed at this same point, the reticle markings are simply engraved on the flat face of the planoconvex lens used as the collective lens.

(3) According to the placement of the erecting lenses, a lens erecting system may increase or decrease the magnifying power of the instrument, or it may not affect magnification. Moving these lenses closer to the focal point of the objective lens and farther from the eyepiece increases the magnification, but cuts down the field of

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Figure 98 - Erecting System Using Two Double Right-angle Prisms

view. When these lenses are located at equal distances from the focal points of the objective and the eyepiece, there is no magnification. This method of changing the degree of magnification is employed in instruments of variable power.

c.

Prism Erecting Systems. Prism erecting systems function by internal reflection. As a single reflection will cause reversion of an image, reflections must be in pairs to overcome this condition. If the path of light is to continue in an unchanged direction, a total of four reflections is required, two to revert the image and two to invert it (inversion in two planes). The use of prisms does not alter the magnification of the instrument.

(1) PORRO ERECTING SYSTEM. This system is employed in prism binoculars and in a number of telescopes. It tends to decrease curvature of the field, and its use is particularly advantageous when a compact instrument having a large field of view is desired as is the case in observation telescopes. This system is not well suited to telescopes which are to be mounted on gun carriages as the prisms are of awkward shape and it is difficult to clamp them so tightly that they will not shift their positions when the gun is fired.

(2) ABBE PRISM SYSTEM. This erecting system (fig. 98) employs two double right-angle prisms (fig. 93). The two prisms are assembled in the instrument in the same general form as the Abbe prism (fig. 87), except that the two prisms are not joined, a slight amount of space being left between them. The travel of the light rays through this erecting system is shown in figure 98.

(3) AMICI OR ROOF PRISM. When used in an elbow telescope, this single prism (fig. 88) serves as an erecting system as well as a right-angle prism. By its use, the light rays are diverted through the

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