Chapter XIV: Fire Control Equipment

SIGHTS.

The instruments provided for sighting and laying the gun include a line sight, a rear sight, a front sight, a panoramic sight, and a range quadrant.

=Line sights.=—The line sight consists of a conical point as a front sight and a V notch as a rear sight, located on the top element of the gun. They determine a line of sight parallel to the axis of the bore, useful in giving general direction to the gun.

=Front and rear sights.=—The front and rear sights are for general use in direct aiming. The front sight carries cross wires. The rear sight is of the peep variety, constructed as follows: To the sight bracket is attached the shank socket upon which a spirit level is mounted for the necessary correction due to difference in level of wheels. The sight shank consists of a steel arc, the center of which is the front sight. It slides up and down in the shank socket and is operated by a scroll gear. A range strip is attached to the face of the shank and is graduated up to 6500 yards, least reading 50 yards. To the left side of the shank is an elevation spirit level, permitting approximate quadrant elevations to be given with the sight shank when the quadrant is out of order.

The peep sight and its deflection scale are mounted above the shank. This peep traverses along a screw operated by a knurled head. A socket and ratchet are also provided for the attachment of the panoramic sight.

Nomenclature of the important parts of the Rear Sight:—

Peep sight
Elevation level
Deflection scale
Peep sight screw and head
Range strip
Shank
Shank socket
Cross level
Leveling screw
Scroll gear and handle
Rear sight bracket
Panoramic sight socket and ratchet

PANORAMIC SIGHT, MODEL 1917.

Description.

=The panoramic sight= is a vertical telescope so fitted with an optical system of reflecting prisms and lenses that the gunner can bring into his field any point in a plane perpendicular to the axis of the telescope. The optical characteristics of the instruments are as follows:

Power = 4.
Field of view = 10°.

=The rotating head prism= has a movement of 600 mils in a vertical plane; movement is obtained by turning elevation micrometer. The amount and direction of rotation is indicated on a scale in the head by the elevation index and micrometer. The scale is graduated in 100-mil intervals, the micrometer in mils. One complete turn of the micrometer is equivalent to one space on the sale. The head is level when the index is opposite 3 and micrometer at zero.

Movement in azimuth is obtained by turning =azimuth worm=. The amount of rotation is read from the scale on the azimuth circle and the azimuth micrometer. =The azimuth micrometer= may be turned independently of the azimuth worm to set any desired deflection. Figures in black are for right-hand deflection and in red for left-hand deflection. The scale on the azimuth circle is graduated in 100-mil divisions from 0 to 32 in each half circle. The micrometer is graduated for every mil. For larger angular deflections, by turning the throw-out lever the azimuth worm is disengaged, permitting the head to be turned to any desired position.

=The reticule= is provided with a horizontal and a vertical cross line. The horizontal line is graduated in mils.

=An open sight= attached to the side of the rotating head is for approximate setting of the instrument.

No disassembling or adjustment of the panoramic sight, except as described herein, is to be made, except by ordnance personnel detailed for such work.

The panoramic sight is seated in a T slot in a socket of the sight, model of 1916, in firing, and is carried in a panoramic sight case on the shield when traveling.

Use of the Panoramic Sight for Direct Fire.

Level rocker with zero on range scale opposite 300 on angle-of-site scale and gun at center of traverse. Set azimuth scale at zero, azimuth micrometer knob at zero, micrometer index at zero, elevation scale at 3, and elevation micrometer knob at zero. By means of cross-leveling knob on sight socket bring cross-level bubble level.

Correct for deflection in azimuth by turning azimuth micrometer until required deflection is opposite fixed arrow pointer; bring zero on micrometer index to zero on azimuth micrometer by means of micrometer-index knob.

Elevate gun by means of angle-of-site handwheel and traverse until cross hairs of panoramic sight are on target.

For Indirect Fire.

Level rocker and set scales for zero setting as directed in first paragraph under “direct fire.”

Lay off required deflection in azimuth by means of micrometer index and azimuth worm knob, so that deflection may be read from azimuth index and azimuth micrometer. Traverse gun until vertical cross hair of panoramic sight is on aiming point.

Vertical angles may be read by means of elevation scale and micrometer scale. Zero point of elevation scale is 3. Each division on elevation scale represents 100 mils.

All scales are graduated in mils.

The open sight on side of rotating head is used to obtain preliminary direction of sight.

In turning azimuth angles greater than 100 mils the throw-out lever may be pressed and rotating head turned to nearest division in even hundreds desired. Each unit on azimuth scale represents 100 mils.

Panoramic Sight, Model of 1915.

=The panoramic sight= is a vertical telescope so fitted with an optical system of reflecting prisms and lenses that the gunner with his eye at the fixed eyepiece in a horizontal position can bring into the field of view an object situated at any point in a plane perpendicular to the axis of the telescope.

The rays coming from the object are reflected downward from the rotating head prism into the rotating prism. The rotating prism rectifies the rays; after their passage through the achromatic objective lens, the lower reflecting prism reflects them in such a way that there is presented to the eyepiece a rectified image, which the eyepiece magnifies. A glass reticule marked with graduated cross lines is located in the focal plane of the instrument, with the intersection of the cross lines coincident.

The instrument has a universal focus, =a magnifying power of 4= and =field of view of 180 mils=.

The principal parts of the panoramic sight are the rotating head, the elevation device and its micrometer, the azimuth mechanism with limb and micrometer, the rotating prism mechanism, the deflection mechanism, R and L scale and micrometer, the shank and the eyepiece.

The limb or azimuth scale is divided into 64 parts, each division representing 100 mils.

The azimuth micrometer is divided in 100 equal divisions or mils, numbered every 5 mils. One complete revolution of the azimuth micrometer is equal to the distance between divisions on the azimuth scale. The limb of the deflection scale is divided into six divisions; three on each side of the zero, red for right and black for left, each division representing 100 mils. The deflection micrometer, engraved upon the front end, is graduated into 100 equal divisions, numbered every 10 mils, red and black in opposite directions.

THE RANGE QUADRANT

The range quadrant consists of the quadrant spring fastening and bracket, rocker, body, scroll gear, range disk, range and cross levels with suitable leveling screws, and a micrometer to set off the angle of site.

The range disk is graduated to 6500 yards, least reading every 50 yards, scale numbered every 500 yards. This disk is operated by a scroll gear. Large changes in range may be made by pulling out the handwheel, thus disengaging the scroll gear, and moving the body and index to the approximate range, whereupon the handwheel is released, and the range accurately set by again turning the handwheel.

The micrometer has 100 divisions and is operated by a milled head. The limb of the micrometer, called the level scale, is graduated from 2 to 5, each division corresponding to one complete revolution of the micrometer. The arbitrary reference point or mean position of the level holder is 300 which corresponds to a point on the same level as the gun.

_REVISED JUNE 4, 1908, JULY 30, 1910, FEB 6, 1911._
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CARE AND INSPECTION OF SIGHTS.

=Rear sight Bracket.=—Should not be bent, broken or cracked. Must be seated firmly in the bracket support.

=Shank Socket Mechanism.=—Not bent or burred; interior of shank socket, scroll and worm gears free of scratches, burrs or deformed threads. Sight shank easily inserted and moved up and down in the socket. Scroll gear securely held in mesh with rack of sight shank. For large movements of shank, should be easily disengaged by pulling out scroll gear handle; must be securely held in place by spring when released. Keep interior well lubricated. See that level vials of cross and elevation levels are intact and bubbles not too large.

=Sight Shank.=—Rack on right side must engage with scroll gear; keep lightly oiled. Sight shank should remain in fixed position except when disengaged or operated by scroll gear handles. For changes in range of 300 yards or less use only the scroll gear; over 300 yards pull scroll gear handle and slide shank up or down in socket by hand, making accurate setting with the scroll gear.

=Panoramic Sight.=—Lug must fit snugly in T slot of rear sight and then held fast by clamp screw and ratchet. When making ready for march order, first set the azimuth and R & L deflection scales at zero, and the elevation scale at 300. Release clamp screw and ratchet. In removing panoramic sight, grasp at center below the azimuth gear case, lift vertically out of the socket, lower the top of sight to the left and replace in panoramic sight box.

=Quadrant.=—Must fit snugly in its bracket, spring catch engaging. Nut on range disk must be tight to prevent slipping. To remove quadrant, press on spring catch before sliding off bracket.

=The sights= are correctly adjusted when, at zero elevation and deflection, correction having also been made for difference of level of wheels, the line of sight is parallel to the axis of the bore.

The =range quadrant= is correctly adjusted when, with the range disk set at zero, level set at 300, axis of gun horizontal and corrected for difference of level of wheels, the bubble of the range level is centered.

In adjusting sights, the panoramic sight should _first_ be corrected. If the rear sight is adjusted first, it will require readjustment if the subsequent adjustment of the panoramic sight causes a change in position of the rear-sight range strip.

=To adjust the panoramic sight.=—Select a well defined point at least 2000 yards distant. If the bore sights are not available, stretch two threads or hairs from the tail of a horse across the grooves marked on the face of the muzzle and fasten them by a strap or rubber band. Remove the firing-lock case from the hub of the block-carrier. By sighting through the hole in the vent bushing of the breechblock, adjust the intersection of the cross hairs on the distant point, using the elevating and traversing mechanisms of the gun. Now without moving the gun or disturbing the laying of the cross hairs, bring the cross wires of the panoramic sight on the same distant point by means of the azimuth scale worm-knob and the scroll gear of the rear sight (_on panoramic sights_, M1915, _to the deflection R & L scale should first be made to read 0, and the elevation scale of the objective to read 300_). The cross hairs of the gun and the cross wires of the panoramic sight will thus be laid on the same point. Now adjust the azimuth micrometer scale to read 0 by means of unscrewing and tightening the screw on the milled head. Loosen the nut on the range strip; move it up or down until it reads (0) 100. Tighten the nut again. Verify the laying. For this adjustment, it is not necessary to have the wheels or the carriage on a level platform; cross level should be leveled.

=To adjust the rear sight.=—Having adjusted the panoramic sight and the rear sight range strip as described above, and without disturbing the laying of the gun, shift the deflection scale of the peep sight and raise or lower the front sight until the cross wires of the latter are laid upon the same distant point. Now loosen the two screws of the deflection scale and place the 0 of this scale opposite the index of the peep sight. The front sight is raised or lowered by removing the split pins and then turning the front sight in the sight bracket sleeve either up or down.

=To adjust the quadrant.=—(a) If an adjusted B. C. telescope or another gun with its quadrant in proper adjustment is available, the angle of site of some distinctly visible and distant point is measured by either of these means. The gun to be adjusted is then laid upon this distant point either with the bore sights, or with the tangent sight set at 0 (or some other convenient) range. The measured angle of site is then set off on the level scale of the quadrant and the bubble of the range level is centered by turning the handwheel of the range disk. By using the quadrant wrench, the range disk is then adjusted to read 0 range, (or the convenient range previously set off on the tangent sight).

(b) If no means are at hand to correctly measure the angle of site of a distant point, the quadrants may be adjusted by using two guns as follows: Unlimber two guns at about the same level, first seeing that the sights are in adjustment (par. 75). Lay both guns upon some distant point by means of the panoramic sight set at 0 range. Now set both quadrants for the same angle of site (roughly estimated A. S. of the distant point) and center the bubbles of the range level by turning the handwheel of the range disks. By means of the quadrant wrench, adjust the range disk of one of the quadrants to agree with the other. Whatever error exists will now be the same in each quadrant.

Now move one of the guns about 100 yards away and turn the muzzles toward each other. With the sights set at 0 range, lay the panoramic sights upon each other and measure the angle of site of each gun. Half the difference of the two readings will be the slope of the line of sight between the two guns. On the level scale of the quadrant which read the greater angle of site, set off 300 plus the half-difference, on the other quadrant set off 300 minus the half-difference. Now center the bubble of the range level by turning the handwheel of the range disk. By using the quadrant wrench, adjust the range disk until it reads 0 range. Having thus adjusted two guns, the others may be adjusted by the first means described.

OBSERVING INSTRUMENTS.

B. C. Telescope, M. 1915.

=The B. C. Telescope, M. 1915=, is a binocular observing instrument of the scissors type. The two tubes of the telescope may be clamped either in a vertical or a horizontal position. In the former position the objectives are 12 inches above the eyes of the observer, and in the latter position they are 24 inches apart and at the same height as the eyes of the observer. In both cases they permit the observer to take advantage of some shield or other cover and still obtain a full view of the sector of observation. The tubes may be adjusted for the observer’s interpupillary distance in either the vertical or the horizontal position. The eyepieces may be adjusted to the eyes of the observer by screwing in or out.

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=The principal parts of the telescope are=:—Leveling mechanism, azimuth mechanism, elevating mechanism, angle of site mechanism, the telescopes and the tripod. A carrying case is provided separately for the instrument and for the tripod. In garrison a storage case is also provided.

_The leveling mechanism_ consists of a ball and socket joint operated by the vertical spindle clamping screw.

_The azimuth mechanism_ consists of the azimuth worm knob with its lever, operating the azimuth worm and worm wheel; the adjusting or slow motion knob and the azimuth clamp. The azimuth limb is divided into 64 parts, each division representing 100 mils. The azimuth micrometer is divided into 100 equal parts or mils, numbered every 10 mils. One complete revolution of the micrometer is equal to one division of the limb. The scales therefore correspond to those on the panoramic sight, 6400 mils to the circumference.

The leveling mechanism is operated by the small elevation worm knob.

_The angle of site mechanism_ consists of the level, the angle of site scale and micrometer with its worm knob.

_The telescopes_ consist of the eyepieces, telescopic tubes with their optical systems. In the right eyepiece is a graduated cross wire which can be rotated for either the horizontal or the vertical position.

_The tripod_ is similar to the usual telescopic instrument tripod.

The optical characteristics of the instrument are as follows:—Power 10; field of view 75 mils; focal length of objectives 11½ inches; the field is flat, free from chromatic and sphercial aberration, coma and distortion.

Care and Instructions.

=To set up the telescope.=—First set up the tripod, clamping and propping the legs so as to obtain the desired cover and view. By means of the vertical spindle clamping lever, approximately center level on azimuth worm case and clamp tightly. Carefully take out B. C. telescope from case and while pressing on locker plunger, place telescope on vertical spindle so that the projection on the azimuth worm case will fit into the corresponding slotted segment of the telescope. Release the locking plunger.

=To focus the eye pieces.=—Adjust each eye piece separately by turning the same until the image of a distant object appears sharply defined. Read the diopter scale, plus or minus, and note for future use.

=To adjust the interpupillary distance.=—The eye pieces having been focussed, loosen the large friction clamp knob in front. Grasp both tubes with the hands and separate or close them in (either in the vertical or horizontal position) until the fields of view of the two eye pieces are exactly coincident and present a single image to the eyes. This can be tested by alternately closing one eye and then the other, noting any movement in the image. Tighten the large friction clamp knob. Read the interpupillary scale and note for future use.

=To lay 0 on any point.=—Level the instrument by means of the vertical spindle clamping screw. Bubble must remain approximately centered while instrument is turned 1600 mils. Set both the azimuth index and the micrometer to read zero. Release the azimuth clamp shaft knob; turn the telescope toward the point and tighten the azimuth clamp shaft knob. Bring the vertical wire accurately on the point by turning the azimuth adjusting worm knob.

=To measure the deflection and site of a target.=—Lay the zero on the aiming point as above. If the line of sight must be moved through a large angle, press down the azimuth worm lever as far as it will go and while holding it down move the azimuth mechanism until the line of sight is approximately directed upon the target; then release the worm lever and bring the cross wires accurately on the target by turning the azimuth worm knob (for deflection) and the elevation worm knob (for elevation). Center the site level. Read the deflection and site.

For carrying the B. C. telescope assembled on the tripod, clamp tightly the vertical spindle clamping lever; close in and clamp the tubes of the telescope; slide up and clamp the lower tripod legs; then carry the telescope over the shoulder by grasping the tripod legs, tube bases of telescope resting on the shoulder.

=To dismount the B. C. Telescope and to pack in carrying case.=—Screw in both eye pieces. Press on the locking plunger and lift the telescope vertically off the spindle. Unclamp the large friction clamp knob, bringing the tubes together and insert in the carrying case; close and lock the lid. In dismounting the tripod the leg separators of the upper sections should first be unclamped. After the lower legs have been assembled, they should be clamped. To set up the tripod the operation is reversed.

Aiming Circle, M, 1916.

=The Aiming Circle= is an angle measuring instrument only and consists of a telescope, leveling mechanism, angle of site device, elevating mechanism, azimuth circle and compass, mounted on a tripod.

Its leveling, azimuth and angle of site mechanisms are similar to those in the B. C. Telescope, as is also the tripod. The elevating mechanism consists of elevating worm knob and gear connecting it with the telescope. The telescope has a universal focus, magnifying power of 4, and a field of 180 mils. The cross wires are illuminated by a window. The compass is secured by the needle release button.

Care and Instructions.

Glass compass cover should fit tightly. Compass needle when clamped should not rotate while instrument is revolved or tipped. When release button is pressed, compass must swing freely on pivot and again remain clamped when button is released. _Compass should be released only when aiming circle is set up and horizontal._

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Bubble must remain approximately centered while instrument is turned 1600 mils.

=To lay 0 on any point.=—Set both the azimuth index and the micrometer to read zero. Release the wing nut; turn the telescope toward the point and again tighten the wing nut. Bring the vertical wire accurately on the point by turning the adjusting worm wheel.

=To measure the deflection and site of a target.=—Lay 0 on the aiming point as above. If the line of sight must be moved through a large angle, press down on the azimuth worm lever as far as it will go and while holding it down, move the azimuth mechanism until the line of sight is approximately directed upon the target; then release the worm knob (for deflection) and the elevation worm knob (for elevation). Center the site level. Read the deflection and site.

=To measure the compass deflection of a target.=—Set both the azimuth index micrometer to read 40. Release the wing nut; turn the telescope until the N and S poles of the compass are respectively near the N and S points marked on the compass box. Make the coincidence accurately with the adjusting worm wheel. Now proceed to measure the deflection and site of the target as described above.

=The Tripod and Azimuth Gear Case.=—The leather cover protecting the ball and socket joint must fit snugly both above and below, and be free of rips or holes. The ball and socket joint and the tripod legs should be easily adjusted and moved, but should remain fixed when clamped by the clamping levers.

For carrying the aiming circle assembled on the tripod, the wing nut and the vertical spindle clamping lever should be clamped tightly and the instrument carried over the shoulder by grasping the tripod legs, head of tripod resting on the shoulder.

Adjustments B. C. Telescope and Aiming Circle.

The B. C. Telescope and the Aiming Circle are correctly adjusted when the following conditions prevail: Telescope properly focussed; plane of level perpendicular to the vertical axis of the instrument; angle of site scale reading 300 when the line of sight is horizontal; lost motion on worm gears eliminated. In principle, the adjustments of the B. C. Telescope and of the Aiming Circle are exactly the same. The leveling and focusing have already been described. Detailed instructions of the operations in eliminating lost motion in the worm gears will be found in the _Handbook for F. A. Fire Control Equipment_, 1916.

=To make the site scale read 300 when the line of sight is horizontal.=—Set up the telescope or aiming circle. Level the instrument so that the bubble on the azimuth worm case will remain centered while the instrument is turned 1600 mils. Lay on some point of a stake or other vertical linear object which is at a convenient distance but not closer than 100 yards to the instrument. The point selected should be at such a height that the telescope can later be set up close to it, with the objective at the same height as the selected point. Read the angle of site. This reading will be equal to (300 + S + E), in which S is the angle of slope of the line of sight, and E is the error in the site adjustment. Before leaving this station, set up a second stake near the telescope and mark on it a point which is at the height of the objective.

Now move the telescope to the first stake; set up and level the instrument with the objective at the height of the point marked on this stake. Lay on the marked point of the second stake and read the site. This angle will be equal to (300 -S × E). Subtracting one reading from the other we have:—(300 × S × E) - (300 - S × E) = 2S; or one-half the difference of the two readings is equal to the angle of slope of the line of sight. Therefore with the cross wires directed upon the marked point of the second stake, center the bubble of the site level. Now loosen the angle of site locking screw and turn the micrometer to read (300 - S), being careful to keep the bubble centered. Screw up the locking screw. The instrument is now in adjustment.

After one telescope has been adjusted, other telescopes, aiming circles and quadrants may be adjusted by merely measuring the site of some distant point. The other telescopes and guns, being in position near the adjusted telescope, are then made to read the proper site when laid upon the distant point.

FIELD ARTILLERY RANGE FINDER, 1 METER BASE

(Bausch & Lomb.)

The instrument and accessories consist of the range finder proper, the tripod mount, the tripod, the adjusting bar, the carrying-case and the storage box.

The optical parts of the =range finder= are embodied in a seamless, steel tube (A) covered with canvas and asbestos, to minimize the effects of the change of temperature. The eyepiece (B) is located in the center of the instrument and is equipped with a focusing device graduated in diopters. A soft rubber eye cap is furnished to protect the observer’s eye from shocks and stray light. A ray filter having two sets of glass is operated by a small lever (C) situated to the lower left of the eyepiece housing.

The objective openings on the end boxes are opened and closed by means of rotating shutters (D). Buffers (E) are provided on the ends as a protection against shocks.

The range finder is of the type known as the fixed base, invert, single coincidence. The magnifying power is 15; actual field of view 50 mils; shortest distance measurable 400 yards. The instrument weighs about 20 pounds. Under favorable conditions and with expert operators the average errors are:

1000 yds. 5 yds.
2000 yds. 15 yds.
3000 yds. 30 yds.
4000 yds. 55 yds.
5000 yds. 90 yds.
6000 yds. 130 yds.
7000 yds. 175 yds.
8000 yds. 225 yds.

In practical use under ordinary conditions and with average operators, the errors are three times as great.

=The tripod= mount consists of a spring catch (a); clamping lever (N); elevation worm case (b); elevation worm knob (c); worm wheel support (d); angle of site micrometer (e); angle of site housing (f); angle of site vial holder (g); clamp screw handle (h); azimuth worm knob (i); azimuth micrometer (j); azimuth worm lever (k); azimuth scale (m); adjusting worm knob (n); and the vertical spindle clamping lever (p). The tripod, consisting of spindle bushing locking screws (t); tripod legs upper (r); tripod legs lower (q); clamping wing nuts (w); and locking clamp arms (x), is similar to that provided for the B. C. telescope and the aiming circle.

Care and Instructions.

=To set up the Range Finder.=—Set up the tripod as heretofore prescribed, take the range finder from its case, holding it with the eye piece toward the body, hook down. Place the instrument on the support and engage the spring catch (a). The instrument is then firmly seated on the tripod. Remove the protective hood from the eye piece and the rotating shutters (D) from the objective apertures. Loosen the locking lever so that the range finder may be made horizontal, then turn toward the target and clamp the lever. To dismount the range finder the operations are executed in an inverse manner.

=To Measure a Distance.=—Focus the eye piece. In very bright light or in thick haze use the amber ray filter in the eye piece. By means of the clamps and worm knobs of the azimuth mechanism and the worm knob of the elevation mechanism, lay the range finder on the target roughly by looking through the open sight on top of the instrument. Final adjustment in deflection is made by the adjusting worm knob (n) and in elevation by the elevation worm knob (c).

Now, look into the instrument. The field of view is divided into two parts by a horizontal line. In the lower part the image is erect, in the upper part inverted. By turning the elevation worm knob (c), the images are lined up so that the same points will touch the dividing line. Now, by turning the measuring roller (M) on the right hand top side of the range finder, the upper image is shifted laterally until the same vertically disposed points of the target are exactly opposite each other. The range is then read on the range scale (H), which is protected by a sliding shutter (Q).

In case the target is without prominent vertical features, such as a crest line, the distance is measured by first turning the instrument to a vertical position (Plate XV). This is accomplished by the clamp screw handle (h) on the tripod mount. Final adjustment in deflection is then made by the elevation worm knob (c), and in elevation by the clamping lever (N).

Adjustments.

=Adjustment for Height.=—The erect and invert images sometimes do not touch the dividing line with similar point so that one image reaches this line before the other. In this case, lay the range finder on an object having a sharply defined horizontal line or very prominent point, and bring the images of this point exactly opposite each other by means of the measuring roller (M). The two images are then brought to the dividing line, the lower image by means of the elevation worm knob (c), and the upper image by means of the halving adjusting roller (J).

=Adjustment for Range.=—Three methods are available for adjusting the range finder for range; the artificial infinity method, using the adjusting bar; the actual infinity method using the sun, moon, star, or a very prominent distant point; the known range method, using a point the range to which is known. Of the three, the first is the best and should habitually be used.

=By the Adjusting Bar.=—Set up the range finder; take the adjusting bar and place it 100 yards from the instrument, sighting through the peep sight (y) of the adjusting bar and moving it until the range finder appears in the center of the field of view. Set the range scale at infinity by turning the measuring roller (M), then make an observation on the adjusting bar. The right hand line of the upper image of the adjusting bar and the left hand line of the lower image should now be coincident, in which case the instrument is in adjustment (Fig. 10). If not, bring these lines in coincidence by turning the key which fits the square shaft (K), which operates the range correction dial (L). This operation should be repeated at least three times, the reading of the range correction dial being noted each time. Now, set the range correction dial at the mean of the three readings. Remove the key. The instrument is now in adjustment. Before using, always note the reading of the range correction dial.

=By the Actual Infinity Method.=—This method is in all respects the same as the one with the adjusting bar, except for the images. After the instrument is set for infinity, actual coincidence is made as in paragraph 99.

=By the Known Range Method.=—Operate the measuring roller (M) until the range dial reads the actual distance to the object. After the range finder has been set for this known range, actual coincidence is obtained by using the key which operates the range correction dial.

FIELD GLASSES.

The instruments for focusing the eyepieces and for adjusting the interpupillary distance are the same as for the telescope.

The two barrels should revolve easily about the central pivot and clamp in any position of the interpupillary scale.

After being once adjusted, the field glasses should fit into the case without being changed.

Carrying strap, button strap and rain shield should always be part of the equipment. Amber shades and camels hair brush should be carried in the case for use.

The vertical scale in the Type EE Field Glass represents the Infantry range scale and does not apply to Field Artillery.

Compass should always be securely clamped except when in use.

THE BRACKET FUZE SETTER, MODEL 1905 M.

The bracket fuze setter is attached to the rear end of the fuze setter bracket on the caisson. It consists of the following principal parts: Base, housing, corrector-worm case, guide, range and corrector worms, rings, and scales.

The corrector scale reads from 0 to 60, numbered every 10 points, 30 being the normal or mean arbitrary point. The range ring is graduated to 6400 yards, numbered every 500 yards, least reading 50 yards. These rings are graduated for the F. A. fuze on one side and for the Ehrhardt fuze on the other. Care should be taken that the proper side is up when firing the two different kinds of shrapnel.

THE HAND FUZE SETTER, MODEL 1912.

The hand fuze setter is provided for the same purpose as the bracket fuze setter, and is intended to supplement the latter. One hand fuze setter is issued in a leather case and is carried in the trail box of each gun. It is intended for the use in case the bracket fuze setter should become disabled, or in case the gun should for any reason be separated from its caissons. The principal parts are the case, the range index mechanism, range mechanism, corrector mechanism, and guide plate.