usually consists of two main parts, viz: a strong outer tube
and an inner frame which supports the delicate telescopic
system, any slight derangement of which would seriously upset
the accuracy of the range-finder. The outer tube is made as
strong and rigid as possible, having regard to the weight which
can be allowed. The inner frame is supported in such a way that
any slight bending of the outer tube will not affect it.

Some form of double end reflector is always used. This at
one time consisted of a pentagonal prism, but large pentag-
onals are very costly; they absorb a good deal of light and are
liable to slight distortion with changes of temperature. There is
therefore a tendency to replace them by a combination of two
silvered plane mirrors inclined to one another at 45, and either
fixed in a mount made of a metal having the same coefficient of
expansicn as glass, or rigidly attached to an upper and lower
glass support, to prevent change in their relative positions.

The central reflectors usually take the form of two or more
prisms balsamed together, and are known as the centre prism
combination. Their object is to deflect the rays received through
the two ends of the range-finder into the eye-piece, and to
present the two images of the target in the field of view immedi-
ately above and below a thin separating line. Their construction
is usually rather complicated. The fine separating line is as a
rule obtained by means of a special separating prism ; or by the
edge of a silvered surface on one of the prisms, so arranged that
the rays forming one image pass through the prism, and those
forming the other image are reflected by the silvered surface.

The centre prism combination is also used for erecting or
inverting the images, and for defining the limits of the fields of
view received from each half of the range-finder. The arrange-
ments of fields of view usually met with are the following:

1. The field of view is divided into two equal halves and the
images in both are erect. When a coincidence has been made, the
separating line cuts straight through the image of the target. This
system is used with naval and coast defence range-finders which
have to deal with targets having long vertical features, e.g. masts
and funnels. It is known as the ' erect " system.

2. The field cf view is divided into two parts by the separating
line. One field (in field range-finders usually the lower) is erect, and
the other one is a replica of it, being inverted up for down but not
right for left. With this system it is much easier to make an accurate
coincidence on small targets, but it has the slight drawback that the
total actual field of view is necessarily considerably smaller than
with the erect system, and a slight elevation or depression of the
range-finder may cause the target to pass the separating line and
disappear. This, however, is not of much importance if the target is
a stationary or slowly moving one. The upper field is often made
smaller than the lower, so that as much ground as possible may be
seen in the field of view. This system is known as the " invert," and
is used in many field range-finders.

3. The whole field is erect with the exception of a small central
rectangle in which the image is inverted. The advantage of this
system is that the field of view is as large as that of a range-finder of
the erect type, except that the part covered by the rectangle is
missing. It is known as the " invert rectangle " and is used in many
field range-finders, especially in foreign armies.

4. The whole field is erect with the exception of a narrow hori-
zontal strip which passes right across its centre. The field in the
strip may be either erect or inverted. This system, known as the
" strip " system, is used in the British height- and range-finder.



In early range-finders the axis of the eye-piece was horizontal
when targets in the horizontal plane were being viewed. It is
now usually inclined downwards at angles of 45, 60, or even
90 for anti-aircraft work, so that the range-taker can observe
from a more comfortable position. Eye-pieces can be focussed
for individual observers, the two images of the target and the
separating line coming to focus simultaneously. Coloured and
neutral tinted glasses are useful in cutting out excessive glare,
haze, etc. In some range-finders, the magnification of the
eye-piece can be altered so as to obtain the best effect under
various atmospheric conditions. Astigmatisers are sometimes
used for drawing out a point of light or small object into long
lines or bands. Coincidence, which without their use would be
almost impossible to effect, can then easily be made. They
consist of two negative cylindrical lenses with horizontal axes,
one being placed at each end of the range-finder between the
pentagonal and the objective.

Range-finders are provided with halving and coincidence
adjustment heads, which, when turned slowly, move the optical
systems mentioned above. Correct adjustment is of course
essential for accurate work.

Accuracy of One-man Range-finders. The accuracy of the
range-finder, other things being the same, depends upon its
base length and magnification; but there are limits to the
magnification which can be conveniently used. It is usually
between 10 diameters (for the smaller range-finders) and 30
(for the largest). Under good conditions, two images can be
aligned across a fine separating line, with an error of only a
few seconds. There is little doubt that this degree of accuracy
can be obtained under the best atmospheric conditions and when
the target is stationary, as the mean result of several observations
being taken as the range. If, however, the atmospheric condi-
tions are bad and the target is moving rapidly, such accuracy
cannot be expected.

The base lengths of range-finders used in the field usually
vary between half a metre and two metres. The Barr and
Stroud range-finder with a base of one metre, which is used for
field artillery by the British and other armies, is typical.

German field coincidence range-finders, such as the Zeiss
and Goerz, are used in a similar way to the Barr and Stroud,
although their construction differs materially in details. Many
of these were introduced to avoid infringements of earlier
patents. The chief features of Zeiss coincidence range-finders
are that they have only one eye-piece in which are seen a small
rectangular inverted field in the centre of a large erect field
and on the left of these a range scale. Coincidence is effected
by revolving a working head which rotates two wedge-shaped
prisms in opposite directions.

Stereoscopic Range-finders. The principle of the stereoscopic
range-finder is entirely different. Stereoscopic rapge-finders
have not found much favour in the British services; but they
have been extensively used by the Germans. Speaking generally,
a stereoscopic range-finder contains the elements of a stereoscopic
telescope rigidly mounted in a tube. It is very similar in outward
appearance to a coincidence range-finder with two eye-pieces.
Objects viewed through a stereoscopic range-finder are seen to
stand out in stereoscopic relief; and it is comparatively easy
for the observer to judge their relative distances. The actual
distance of a target is obtained with the assistance of one or
more marks which are seen in stereoscopic relief in the field of
view. By means of suitable optical arrangements the stereoscopic
relief of either the objects in the field of view or of the mark can
be varied until the target and mark appear to the observer to
be the same distance away from him. The range of the target
can then be read off a range-scale attached. The mark which
is seen in stereoscopic relief, really consists of two similar marks
which are photographed on glass diaphragms mounted in front
of the eye-pieces of the range-finder. If the two marks are on
the optical axes of the two halves of the range-finder, they will
appear to the observer as one mark at an infinite distance. If
the two marks on the diaphragms are made to approach one
another, the resulting single mark will appear to come towards



RANGE-FINDERS AND POSITION-FINDERS



243



the observer. In the later stereoscopic range-finders, marks
in the eye-pieces are adjusted so as to appear to be at a fixed
distance when seen stereoscopically. The view containing the
target appears to approach or recede from the observer when the
working head is turned; and the range is read off from an
external range scale or drum.

Stereoscopic range-finders suffer from the disadvantages
that very few people are able to see stereoscopically with
sufficient exactness to obtain good results and the degree of
accuracy obtained by a range-taker appears to vary from time
to time. Under the same atmospheric conditions, stereoscopic
range-finders do not appear to give such good results as coinci-
dence ones. Owing to the marks on the diaphragms in the
eye-pieces being more distinct than the image of the target,
there is difficulty in deciding when they are at the same apparent
distance. On the other hand, owing to both eyes being used,
the target should be seen more distinctly than through a coinci-
dence range-finder. Ranges of targets with the sky as a back-
ground, e.g. aircraft, masts of ships, and trees on a crest line,
are comparatively easy to take with a stereoscopic range-finder;
but those of comparatively indistinct objects, objects with near
backgrounds and objects in a shadow, e.g. a gun under a tree,
are very much more difficult to take.

Directors. The director is an instrument employed chiefly
for the measurement of azimuth angles and angles of sight.
There are numerous patterns of these instruments. Some are of
simple form ; while others are complicated and are provided with
quick and slow motion movements for their azimuth and altitude
movements and for laying on a gun or target without the setting
of their azimuth scales being altered. Some are used as directors
pure and simple, whilst others are provided with powerful
telescopes for observing fire. British directors are graduated in
degrees and minutes, whereas those of nearly all other powers
are graduated in milliemes. Instruments graduated in milliemes
can be made much more compact than those graduated in degrees
and minutes, as the main azimuth plate has only 64 divisions
instead of 360. The azimuth scales of some German directors
are graduated in i6ths of a degree and i6ths of a degree are
also sometimes used for angle-of-sight scales.

The German director for field artillery is a good example of
modern types of this instrument. Powerful telescopes like the
stereoscopic or scissors telescope are often provided with fittings
which enable them to be used as directors as well as for observing
fire. Azimuth angles can be measured by means of an azimuth
scale, and angles of sight by means of a small bubble and
scale. In one pattern of German stereoscopic telescope, the
angle of sight is measured by means of a device which is com-
pactly pla'ced between the two arms of the telescope, above its
elevating gear.

The Field Plotter is an instrument used for obtaining the
gun range when the range-finder is not near the gun. It consists
of two similar nickel-plated steel plates, having azimuth scales
and a scale of yards (upon which the distance of the gun from
the range-finder can be set) engraved upon them, and two
range arms which are pivoted at the centres of the azimuth
scales and connected by means of a slider. The two plates
are slid along one another until the distance between the gun
and the range-finder is opposite the reader. The slider is then
moved until an arrow upon it reads on the arm the distance from
the range-finder to the target. The arm is then moved until
another arrow reads the angle, recorded by the director, between
the target and the gun. The plotter is then turned over, and
the range from the gun to the target and the angle at the gun
between the target and the director are read off. When fire has
been opened, it is necessary to' observe the results, so that
corrections for both range and direction may be made. Also if
time fuzes are used, the angular height above the target at
which they burst the shell must be observed and corrected if
necessary. Various instruments, e.g. graticuled binoculars,
stereoscopic telescopes and periscopes, are used for this purpose.

The binoculars used in the British service are of the prismatic
type having a magnification of six diameters and are fitted with



a glass diaphragm in front of the right eye-piece on which are
engraved the vertical lines 5 apart; their heights above the
horizontal line being alternately 5 and i. The central line
is longer, extending 15 above, and 5 below the horizontal
line. When observing fire, the point of intersection of the
centre vertical line with the horizontal line is laid on the target ;
and, when the burst of the shell is observed, its angular distance
to the right or left of the target and above it can be ascertained
by noting its position with reference to the lines on the
diaphragm. Graticules used by other continental powers vary
very much in design and often consist of a very large number of
short lines at definite distances from the axis of the telescope.

The stereoscopic or scissors telescope was very largely used in
the World War, especially by the Germans. The British pattern
has a magnification of 10 diameters and a field of view of 4.
The two arms can be used either vertically for observation from
below cover, or in line with one another for observation from
behind vertical cover.

FOR COAST DEFENCE

Coast defence range-finding instruments are usually either
range-finders or position-finders. A range-finder measures
ranges from itself to the target and may also record the bearing
of the target. A position-finder measures ranges and bearings
to the target from the point for which it has been adjusted. This
would usually be a gun which may be many hundred yards
away from the position-finder. The position-finder sometimes
also records the range and bearing of the target from its own
position. Coast defence range-finding instruments may be
divided into classes, depending upon the nature of their bases,
as follows:

1. Depression instruments, having vertical bases (either range-
finders or position-finders) the accuracy of which depends upon
their height above the sea-level. They measure ranges with equal
accuracy in all directions; and, if sufficient height is available, they
are most reliable instruments. It is usually considered that, to
obtajn ranges with an error not exceeding I %, loo ft. of height is
required for every 5,000 yd. of range. All that the range-taker has
to do is to keep a cross wire in the telescope of his instrument laid on
the water line of the target.

2. Instruments having a fixed horizontal base (usually position-
finders). Each usually consists of two instruments installed at the
ends of a base which may be several thousand yards in length, the
length required depending upon the range and arc of fire of the gun
for which the instrument has been installed. The instrument at one
end of the base is known as a transmitter and usually consists of a
telescope mounted above an azimuth circle. It measures the bearing
of the target and transmits it to the instrument at the other end of
the base which is known as the receiving instrument. The bearings
may be transmitted by telephone; or, as the telescope is traversed,
electric impulses may cause some portion of the receiving instrument,
e.g. a metal arm or a dial, to move automatically. The receiving
instrument is usually an accurate, solidly constructed plotter, made
to a certain scale, e.g. 500 yd. to an inch. In its simplest form it would
consist of a base plate upon which two straight arms were pivoted,
the relative positions of the pivots being, to the scale of the instru-
ment, the same as those of the transmitter and receiving instrument.
The first arm would be kept set to the azimuth angles received from
the transmitter, and the second arm would be kept in line with the
target by means of a telescope attached to it. The point where the
two arms intersect would represent the relative position of the target.
The arms being graduated in ranges and moving over azimuth
scales, the range and bearing of the target from either end of the
base could be read off. By means of a third arm, pivoted to the base
plate in the position representing the position of the gun, the range
and bearing of the target from the gun could be ascertained if the
arm were brought above or below the point of intersection of the two
other arms. Horizontal-base range-finding instruments are often
designed to allow of several alternative bases being used, the pivots
of the arms in the receiving instrument being set for the base which
is most suitable for the conditions prevailing at the time. Thus for
very long ranges a very long base would be used ; but, if the visibility
were bad, a much shorter one might be preferable. Horizontal-base
instruments have certain disadvantages. For a definite range, their
accuracy varies with the position of the target. It is greatest when
the target is opposite the centre of the base, decreases as the target
moves to the right or left, and disappears altogether when the target
is in line with the base. It is somewhat difficult for the receiving
instrument to indicate to the transmitter the target whose range is
to be found. There are difficulties in passing azimuth angles from
the transmitter to the receiving instrument. If they are telephoned,
delay and errors are involved, and owing to the fact that they may






244



RANGE-FINDERS AND POSITION-FINDERS



alter at greatly varying rates there is usually some uncertainty as to
the most suitable intervals in which they should be called out. If
they are electrically transmitted, so as to move some part of the
receiving instrument, the mechanism must be capable of moving in
small steps of, say, one minute, in order to obtain the required
accuracy, and at the same time it must be capable of moving quickly
through a large angle when getting on to a target.

3. One-man Range-finders used for coast defences are constructed
on the same general principles as those used in the field ; but, as their
weight and size must not be limited to the same degree, many
modifications to make them as accurate and convenient as possible
are introduced. Their base lengths are usually between 9 ft. and 10
metres, but even longer bases have been considered. They are
practically always of the coincidence type with both fields erect.

The Mark III Depression Range-finder (fig. 2) will be taken as a
type of a depression instrument. In this range-finder the range-
finding triangle is reproduced in the instrument on a small scale.
Figure I shows diagrammatically how this is effected by means of




FIG. i.



two arms. AB represents the height of the axis of the telescope above
the surface of the sea BC. Ab represents the distance between the
pivot of the telescope arm Ac and the range arm be, and this to the
scale of the instrument, represents the height AB. When the instru-
ment is level, the arm cb is horizontal, and therefore parallel to the




FIG. 2. Depression Range-finder Mark III.

surface of the sea. cb is subdivided and graduated in equal divisions
to the scale of the instrument. Ac, which has a telescope mounted
on it, is pivoted at A and can be directed on to the water-line of a
target at C. The triangles ABC and Abe are similar, and the length
of be therefore represents to the scale of the instrument, which is
1,000 yd. to an in., the actual range BC. Similarly bd will represent
the range BD of a target at D. If the instrument is required for
use at any other height than AB, the telescope arm pivot A would
be raised to a height above cb corresponding to the new height of
the range-finder above the sea. In the foregoing, the surface of the
sea has been considered as a plane surface, whereas it is really the
surface of a sphere with its centre at the centre of the earth. Allow-
ance for this curvature could be made by making the arm be the
arc of a circle instead of a straight line. It is, however, found to be
more convenient to curve the telescope arm Ac in the opposite
direction, the effect being the same. Corrections for mean refrac-
tion are made in the same way as those for the curvature of the
earth, but in the opposite direction; the arm Ac being curved to
allow for the combined effect of curvature and refraction.

The Small German Position-finder may be taken as a typical
position-finder. It was used during the World War for coast artillery
on low sites. Very much larger instruments based pn the same
general principles were also used. Air communication was by
telephone. Other nations often use automatic electrical transmission
for moving parts of the receiving instrument when the transmitter
telescope is traversed, and for actuating range and bearing dials on
the gun mountings when the gun arm is traversed.

The Barr and Stroud Range-finder having a base-length of 30 ft.
is typical of the one-man range-finder used for coast defences.



None of the range-finders or position-finders mentioned above is
of any use if it cannot see the target ; the position of a -moving target
can then only be ascertained by aerial observation.

Naval Range-finders, which must necessarily be of comparatively
small size, are nearly always of the one-man type. They are gen-
erally similar to those used in coast defences, but in order to obtain
better illumination their optical parts are often made larger. Their
mountings are designed to enable the range-finders being kept on the
target in a sea way. They are nearly always of the coincidence type
with both images erect; but some stereoscopic range-finders have
been used by the Germans. It is thought by some that a range can
be more rapidly taken from a ship having considerable motion with
a stereoscopic range-finder than with a coincidence range-finder;
as with the latter it is difficult to keep the separating line across
the target. The coincidence range-finder will, however, give more
accurate results. Range-finders with a base-length of 10 metres are
the largest which can conveniently be used on board ship. Base
lengths vary with the size of the ships on which the range-finders
are employed and the uses for which they are intended ; one of less
than a metre being employed for navigational purposes. Anti-
aircraft range and height-finders used on ships must be provided
with some form of pendulum to keep certain of the parts horizontal,
as the mountings cannot be kept level. Certain small ranee-finders
have been made which can measure the range to a ship if the height
of its mast or funnel is known. This height is used as the base of the
range-finding triangle, the angle of parallax being measured by the
instrument and read off it as a range. Such a base is obviously
longer than that of a range-finder which could be carried on a
ship. This type, however, has not been generally adopted.

FOR AIR DEFENCE

Range-finders and height-finders used in connexion with
anti-aircraft guns have a much more difficult task to perform
than the range-finding instruments used in the field or in coast
defences. In the latter cases the target, if it moves at all, will
move in one plane only and its speed will not approach that of
aircraft. As the result of experience, the range of an aerial
target which may alter by several thousands of yards in a minute
is not taken into account; but its height, which will not vary to
the same extent, is used as the basis of laying 8 and is measured
by a height-finder. The rate of burning of the fuze has also
to be considered, as it will not be the same for a definite range
if the target is at different heights.

When laying a gun on aircraft, the setting of the sight is
therefore based on the height and angle of sight of the target
and on the setting of the fuze. At the beginning of the World
War, no height-finders were available, so existing one-man
range-finders, e.g. the Barr and Stroud, were used, and long base
height-finders which could be rapidly made were introduced.




FIG. 3.

One-man range-finders were usually of two metres base length, and
coincidences were made on the target in the usual way. In order to
convert the ranges into heights, a slide-rule attached to the range-
finder was employed. One scale of this slide-rule was automatically
set to the angle of sight of the target, by means of a cam, as the