It may readily be seen that if the eye is placed at a certain distance behind a ring an angle is subtended by the diameter which is proportionl to it and which increases or decreases as the eye is moved nearer to or farther away from the ring. In making use of a ring for a machine-gun sight, any size of ring, the distance from the eye can be determined so that if the gun is fired just as an enemy machine cuts the ring when flying at right angles to the line of sights, a hit will be scored, assuming, of course, that the elevation of the gun is correct for the range.

It must be remembered that the bead is used only to align the eye with the center of the ring and may be placed either behind or in front of the ring and at any distance from it.

In the above figure. the eye is placed at A, a distance behind the center of the ring B, such that when the enemy machine appears at D, just cutting the ring at E, the gun is fired and scores a hit at C.

Let AB Distance of eye from ring in inches. D Radius of the ring sight in inches.

AC Range, which is:

Inches projectile will travel in I second.

CD =Distance the enemy machine will travel in 1 second at 110 miles per hour.
From similar triangles:

For a 3-inch ring, range of 2,650 feet, and enemy speed of 110 miles per hour: 1.511 . x31800"

This gives the correct method of determining the distance of the eye from the ring under the conditions given. This distance varies inversely as the speed of the enemy machine; and for enemy speed of greater than 100 miles per hour, the distance AB should be decreased and vice versa. In actual practice, however, the sights are placed at a certain distance from the eye, which is based on an average value of, the enemy speed, and for any speed greater or less than this, the gun is fired when the enemy machine is just inside or outside the ring. The bead is fixed at such a height in relation to the center of the ring sight that the elevation is correct for 200 to 300 yards.

We have thus far considered only the case of an enemy machine flying at right angles to the line of sight. For any other direction of enemy flight, the part to be hit, usually the forward part of the fuselage, should be placed within the ring at a point determined by the enemy direction and varies from a point on the ring for the condition considered above to a point at the center for an enemy flying directly toward or away from the gunner. The intermediate positions may be determined from the appearance of the enemy machine, that is, from the foreshortening of the body.

It is of greatest importance that in every case the enemy airplane is placed so that the head of the airplane is pointing accurately toward the center of the ring.



AUXILIARY RING AND POST SIGHT

This is a ring and post sight to be used in conjunction with, or instead of, the unit sight on fixed guns. The set consists of a post mounted in a convenient position and a ling, the diameter of which is so calculated that when placed at a fixed distance from the pilot's eye, it will correspond to the graticulation on the lens of the unit sight and afford correction in the sighting of the gun.

Figure 22 shows an auxiliary sight as mounted on a De H 4.

WIND-VANE SIGHTS

The wind-vane sight is used as a front sight in connection with a ring sight, on machine guns mounted on flexible mounts.

The purpose of this sight is to correct for the speed of the gunner's machine and when used with the ring sight, which corrects for the speed of the enemy machine, a very good combination is obtained.

Description:
The bead sight on post B is off-set from the main pivot A, which is fixed perpendicular to the axis of the gun. The vertical and horizontal vanes C, keep the bead " B " into the wind at all times, no matter what position of the gun. The distance from pivot A to post B was calculated the following manner:

In figure 32 let A = The position of the rear sight.

AB = A convenient distance, on the gun for the wind vane sight from the ring sight.

AC = Distance, traveled by the bullet in 1 second. C - The target.

CD = The distance the gunner's machine will travel in 1 second.

It will be seen from figure 33 that if A is traveling in the direction of arrow with a speed of CD, to hit the, target C, it will be necessary to aim gun along AD. Therefore the required correction is BE, which is easily found by proving the equilateral triangles ABE and ACD, since:

AC/AB = CD/BE

Wind-vane sights are made for the following speeds:
90 miles per hour
120
150 miles per hour.

The 90-mile-per-hour sight is for speeds between 80 and 100 miles-per-our sight. The 120 mile-per-hour sight is for speeds between 110 and 130 miles per hour, and the 150 mile-per-hour sight is for speeds between 140 and 160 miles per hour.



UNIT SIGHT


The unit sight is an optical sight consisting of a tube, approximately 2 inches in diameter and 25 inches in length, containing a system of lenses so designed as not to magnify or diminish any object which is looked at through it. Within the tube is a glass screen carrying the sighting ring, which is so placed between the lens, that, on looking through the tube at a distant object, the ring is seen with its center on the spot at which the tube is pointing irrespective of where the eye is placed.

This tube, when fixed rigidly, constitutes a sight which offers practically no obstruction to the view and shows instantly the spot at which the gun is pointing without the necessity of aligning the eye upon a front and back sight.

The above diagram shows the graticulation on the lens of the unit sight. The size of the larger ring in proportion is calculated in the same manner as in the auxiliary sight.

The number on the graticulation indicates the approximate speed of the enemy airplane for which the sight is calculated.




SIGHT MOUNTINGS

The mountings for the unit and ring and bead sights for the fixed guns on the DH-4 plane consist of brass sockets secured to the cowl by two small bolts. The sockets are threaded to take the five-sixteenths-inch post of the sight bracket, which is clamped by means of a nut and lock washer.

Care should be taken in adjusting, not to turn the stem by means of the ring, as is very liable to become bent or twisted off . Pipe pliers should be used on the stem above the threaded part.