FIELD GLASSES AND TELESCOPES.
Reflection--refraction--lenses.
[Illustration: FIG. 20.]
When light falls on a transparent body, part is reflected and part
is refracted. The angle which the ray makes with the normal, or
perpendicular, to the surface at the point of contact is known as the
angle of incidence, and the angles which the reflected and refracted
rays make with the same normal are known respectively as the angle
of reflection and refraction. The reflected ray makes the same angle
with the normal as the incident ray, while the refracted ray, when
passing from a rarer to a denser medium, is bent toward the normal,
and vice versa; the denser the medium into which the ray passes the
greater is the deviation. This law allows us at once to understand the
action of a lens, which may be defined as a transparent medium that
from the curvature of its surface causes the rays of light traversing
it to either converge or diverge. The ordinary lenses have either
spherical surfaces or a combination of spherical and plane surfaces.
This combination will give rise to six classes (fig. 20): (_a_) Double
convex; (_b_) plano convex; (_c_) double concave; (_d_) plano concave;
(_e_) converging, and (_f_) diverging meniscus. Those lenses which are
thicker at the center than at the edges are converging or concentrating
lenses, and those which are thicker at the edges than the center are
diverging.
FOCUS--OPTICAL CENTER.
The focus of a lens is the point where the refracted rays or their
prolongation meet; if the rays themselves intersect after refraction
the focus is real, and if their prolongations meet the focus is
virtual. The line passing through the centers of curvature of the two
surfaces of a lens is called the principal axis and contains a point
known as the optical center, which has the property by virtue of which,
if a ray passes through it, the ray will not be deviated. The optical
center can always be found by drawing two radii parallel to each other,
one from each center of the curvature of the surface until the radii
intersect their respective surfaces, then draw a line joining these two
points. The intersection of this last line with the principal axis will
give the optical center.
IMAGE--CONJUGATE FOCI.
Let AB be the section of a double convex lens and C and D (fig. 21) be
the centers of curvature of the two surfaces. Draw the lines CD´ and DE
from C and D parallel to each other, then join D´ and E by a straight
line. The point O will be the optical center of the lens. Let us take a
point R, on the principal axis as a source of light; the ray RD passes
through the optical center and is not deviated. The ray RK on striking
will be refracted in the direction KG toward the perpendicular to the
surface KD in accordance with the law of refraction, as glass is denser
than air. On emerging at G it is refracted away from the perpendicular
to the surface CG, since it passes from a denser to a rarer medium,
and will intersect the ray RD at the point R´. In a like way the ray
RK´ will be found to intersect the ray RD at the same point, R´, which
is the focus for all rays coming from R. The point R´ is said to be the
image of the object R, and when the two points are considered together
they are called conjugate foci. If the incident beam is composed of
parallel homogeneous light, the rays will all be brought to a focus at
a point on the principal axis, called the principal focus of the lens,
and the distance of this point from the optical center is the principal
focal length, which is always a fixed quantity for any given lens.
[Illustration: FIG. 21.]
LAW OF FOCI.
There is a fixed relation between the principal focal length of a
double convex lens and the position of the image of the object which
may be expressed as follows: 1/_i_ = 1/_f_ - 1/_o_, in which _i_ and
_o_ are the distances of the image and object, respectively, from the
optical center and _f_ the focal length, from which we see that for
all positions of the object from an infinite distance away from the
lens to double the principal focal distance, the image will be on the
other side, between a distance equal to the principal focal length and
double this length. These are the limits of the image and object in the
ordinary cases. If we place this expression in the following form: _i_
= _of_/(_o_ - _f_), and suppose the object to remain the same distance
from various lenses, it will be seen that the image will be closer
to the lens which has the shorter focal length. The principal focal
distance, or, briefly, the focal length of the lens, depends on the
curvature of the surfaces, and the greater the curvature the shorter
the focal length.
FORMATION OF IMAGE.
[Illustration: FIG. 22.]
Let us now see how an image is formed by a convex lens, and suppose
that CD is the section of a double convex lens (fig. 22), O the optical
center, and AB an object at a greater distance from the optical center
than double the focal length. Rays will pass out in all directions
from the object and some will fall on the lens. A ray from A will pass
through the optical center and will not be deviated; others will be
incident at various points, for example, E and G, and if we apply
the law of refraction we will find that AE and AG will intersect each
other and AO at the point A´, provided we do not consider the figure
of the lens, forming one point of the image A´ B´; similarly for rays
from other points of the object, as, for example, B, we can construct
the focus B´, and thus obtain the image A´ B´, which is inverted and
smaller than the object AB. The relative size of the image and object
will be directly as the conjugate foci, and these can be found at once
from the equation of the lens.
SPHERICAL ABERRATION.
If, however, we consider the form of the lens, we will find that all
the rays emerging from one point on the object are not brought to the
same focus, because the rays incident on the edges of the lens are
refracted to a greater extent than those falling on the center, and
will be brought to a focus at a shorter distance from the lens than
those passing through the central part. This confusion or wandering of
the foci from one point is called spherical aberration, or aberration
of form, and is due solely to the geometrical form of the lens.
CHROMATIC ABERRATION.
In what has been said about the visual image we have supposed that the
light was monochromatic, or homogeneous. Let us see what will happen if
the light is polychromatic, say, for example, sunlight, and let a beam
of sunlight be intercepted on a screen after passing through a double
convex lens. It will be observed, as in figure 23, that the violet
rays are brought to a focus nearest the lens, and the red farthest
away, and circles of light will be seen on the screen; this wandering
of the colored rays from a common focus is called chromatic aberration
and depends on the dispersive properties of the material of which the
lens is made. Here is a defect that can not be corrected by a stop,
but as the refractive and dispersive properties of a substance do not
vary together, it is possible to combine two substances, one with high
refractive and low dispersive properties and the other with the reverse
properties. If proper curves are given to them they will correct each
other, thereby producing coincidence of the visible and chromatic foci.
Such a combination gives an achromatic lens, which is usually composed
of a double convex of crown glass cemented to a diverging meniscus of
flint glass, as shown in section in figure 24. This combination is not
absolutely achromatic, but sufficiently so for all general purposes.
[Illustration: FIG. 23.]
[Illustration: FIG. 24.]
TELESCOPES.
The telescope is an optical instrument based on an object glass or
reflector to form a real image of a real and distant object, and of
an ocular to magnify and view the image. Telescopes are classified as
refracting or reflecting according as the object glass is a lens or a
reflector. The object glass must be essentially convex if the telescope
is a refractor, and if a reflector, the object mirror must be concave;
the ocular may be either concave or convex.
There are four types of refractive telescopes used for military
purposes, viz:
1. The astronomical.
2. The terrestrial.
3. The galilean.
4. The prismatic.
Figure 26 is a section of an astronomical telescope. The object glass
(_D_) is a combination consisting of a double convex and a double
concave lens cemented together with Canada balsam. The double concave
lens is added to correct for chromatic aberration. The ocular (_E_) is
a convex-concave lens.
Rays of light from some distant object are converged by the objective
(_D_) and form an inverted image (_ab_) at the _focal_ plane (_F_).
The eye lens (_E_) receives the divergent pencils from _a_ and _b_
and bend them so that they enter the eye as if coming apparently from
the direction of _a´ b´_ where the apparent image is seen. From the
eyepiece (_E_) the rays emerge in a cone of pencils of light smaller
than the pupil of the eye, which enables a telescope of this type to
have a large field of view. The image, however, is inverted and the
astronomical telescope in its original form is therefore not suitable
for military purposes. In a modified form it is much used, as will be
shown in a later paragraph.
[Illustration: _Figure 26_]
Figure 27 is a section of a terrestrial telescope much used for
military purposes. Glasses of this type are quite generally known as
"spyglasses."
As in the case of the astronomical telescope, the first inverted
image _ba_ is formed at the focal plane (_F_), and the first eyeglass
converges these pencils to _L_. Instead of placing the eye at _L_,
as in the astronomical telescope, the pencils are allowed to cross
and fall on a second eyeglass, by which the rays of each pencil are
converged to a point in the second erect image _a´ b´_, which image is
viewed by means of the third and last eyeglass.
[Illustration: _Figure 27_]
Terrestrial telescopes have a comparatively small field of view. The
barrels of this telescope are necessarily long on account of the
additional lenses.
GALILEAN FIELD GLASSES AND TELESCOPES.
Figure 28 is a section of a Galilean telescope which differs from the
astronomical telescope in having a double concave instead of a double
convex, eyepiece or ocular.
In this telescope the rays from an object are converged by the object
glass (_O_) and would normally focus at the focal plane (_C_) and
there form the inverted image _ba_ were it not that the double concave
eyeglass or ocular (_D_) is so located in the barrel of the telescope
as to intercept the pencils before they are focused. This double
concave eyeglass diverges these pencils and forms a magnified erect
image _a´ b´_ apparently at _E_. Due to the diverging action of this
concave eye lens, the cone of pencils entering the eye is larger than
the pupil of the eye, and therefore but a small part of the field
gathered by the object glass is utilized by the eye, which causes
telescopes of this type to have a comparatively small field of view.
[Illustration: _Figure 28_]
PORRO PRISM FIELD GLASSES AND TELESCOPES.
In 1850 a French engineer, Porro, discovered a combination of prisms
which, when inserted between the objective and the eyepiece of an
astronomical telescope, showed the image erect or in its natural
position, while the same telescope without the prisms showed the image
inverted. Practical use of this discovery was not made for many years
after. These prisms served a twofold purpose, viz, showing the image
of the object looked at in its natural position instead of reversed,
and second, the shortening of the telescope by twice turning the ray of
light upon itself. Each tube of the prism field glass contains two of
these double-reflecting prisms. The ray of light passing through the
object glass enters the first prism in such a manner as to be twice
totally reflected, each time at an angle of 90°, thus emerging parallel
to the entering ray, but in the opposite direction. It is thus caught
by the second prism and is similarly reflected and sent on its original
direction without change except in one very important point, viz, the
image of the object observed, which, without the intervention of the
prism, would be upside down, is now erect, and will be magnified by
the simple astronomical eyepiece just as the stars and planets are
magnified in large telescopes.
The field of view of the Porro prism glass is considerably larger than
that of the ordinary field glass. It decreases about 12½ per cent
with each magnifying power, a number 6-power glass giving a linear view
of 118 feet in a thousand, while in a number 10 glass the field is but
70 linear feet. This is explained as follows:
The rays of light emerging from the ocular of the Galilean telescope
are divergent and cover an area much greater than the size of the
pupil of the eye. As all rays falling outside the pupil of the eye are
lost, but a small field of view can be seen, as in looking through an
ordinary cone from the larger end. The prism glasses are constructed
on the opposite principle. The rays of light gathered by the objective
emerge from the eyepiece in a converging pencil of light small enough
to enter the pupil of the eye, thus giving a larger field of view;
theoretically, nine times the area given by the old-style instrument of
the same power. With these advantages, however, the Porro prism glass
has not been found in all respects satisfactory for field service.
With a clear atmosphere and the object which is being viewed well
illuminated, it is distinctly superior to the Galilean field-type glass
in respect to light, power, and definition. The prisms having once
been deranged, however slightly, satisfactory use of the glass can not
be had until the prisms have been readjusted, and until very recently
it was impracticable to have this done elsewhere than at the place of
manufacture of the glass.
[Illustration: FIG. 29.--Porro prism.]
FIELD GLASSES.
The field glass or binocular is a combination of two similar telescopes
and possesses mechanical adjustments capable of focusing the two
telescopes simultaneously or separately, depending upon the type
considered.
Field glasses are divided into two general classes, viz, the Galilean
glasses and the Porro prism glasses.
PROPERTIES OF TELESCOPES AND FIELD GLASSES.
Telescopes and field glasses have four properties, viz, power, light,
field, and definition. These properties are expressed in terms of the
corresponding qualities of the unaided eye.
Eyes are of very different capabilities. Some people have "short" sight
while others have "far" sight. There are normal, excellent, and weak
eyes. In the following discussion the capabilities of the normal eye
are assumed.
For each individual there is a certain distance at which objects may
be most distinctly seen. This is called the "visual distance." With
shortsighted eyes this distance is from 3 to 6 inches; with normal
eyes, from 8 to 14 inches, and with farsighted eyes, from 16 to 28
inches.
The capabilities of the normal unassisted eye may therefore be
expressed as follows: Power, 1; light, 1; field, 45°; definition, 40´´
to 3´.
_Power._--At the "visual distance," all objects seen by the unaided
normal eye appear in their natural size. At less than the "visual
distance" they appear indistinct, blurred, and imperfectly defined; at
greater than the "visual distance" objects are clear and well defined,
but diminish in size, the more so as they are farther removed.
The ability of a lens to magnify the apparent diameter of an object is
termed its power.
The power of a lens is defined as the ratio of the diameter of the
object as seen through the lens to the diameter as viewed by the
unaided eye.
The power is also defined as the ratio of the focal distance of the
object glass to that of the eyepiece.
The power of a field glass can be roughly determined by focusing the
instrument on a wall or a range rod, by looking at the object through
the instrument with one eye and at the same object directly with the
unaided eye. A comparison of the diameter of the two images gives the
ratio.
The power of a telescope or a field glass can more accurately be
measured by means of a dynameter, which is a microscope which can be
fitted over the eyepiece end of the instrument, and which magnifies the
image. The end of the dynameter next to the eyepiece of the instrument
is ruled with a series of lines one-hundredth of an inch apart. On
focusing the dynameter, the image of the emerging pencil appears as a
sharply defined ring of light with the magnified scale of the dynameter
across it.
The number of subdivisions covered by the diameter of the ring of light
is noted. The diameter of the object glass is similarly measured by
means of a pair of dividers and read to the hundredth part of an inch.
The ratio of the diameter of the object glass to that of the image as
seen in the dynameter gives the power of the instrument. This method is
not applicable in the case of the Galilean telescope or the field glass
consisting of two Galilean telescopes, due to the fact that the rays
from the eyepiece of the Galilean telescope are divergent.
Field glasses in which the image appears magnified from one to six
diameters are known as "low-power" glasses. Field glasses which produce
an image magnified over six diameters are termed "high power."
For the mounted man a glass of but 4, or at most 6, powers, can be
used with advantage; on foot, with free hand, instruments of not to
exceed 10 powers can be used. If more than 10 powers are desired, a
holder becomes necessary, and if the holder is intended to be portable
a greater power than 50 is not practicable, as the movement of the air
or the slightest touch of the hand sets up vibrations that render clear
vision impossible.
Field glasses with low magnifying power, which are usually preferred
by ordinary observers, have their chief value in the comparatively
extensive field of view; they should be used to observe extensive
movements, where large tracts of country must be taken in one field of
view or in sweeping the landscape to find the tents of the enemy, their
wagons, etc., or other objects, to be afterwards more closely examined
with the telescope.
They may be used on shipboard or in boats, where the rolling motion
interferes with the use of the telescope; also on horseback or in
hasty examination made on foot or in trees, and generally for all
observations not critical or those to be made under circumstances where
the telescope can not be conveniently handled. The field glass ought to
be held by both hands when in use, and to steady it the arms should be
kept close to the body.
For reading signals at short ranges, say, up to 5 miles, these glasses
are better than the telescope. Flag signals have frequently been read
with glasses of this description at a distance of 10 miles.
_Light._--The illumination of an object when observed with the
unaided eye is impressed upon the retina with a brightness in strict
proportion to that of the object itself. If an object be viewed under
equal illuminating conditions alternately with the naked eye and with
a glass, the brightness of the image seen with the naked eye may be
represented by 1, while that of the image in the glass will generally
differ, being greater or less.
The light of the telescope or field glass is expressed by the number
which shows how many times brighter the object appears through the
instrument than to the naked eye. Light is a function of the dimensions
of the object glass and of the power of the instrument, and is
sometimes determined by dividing the square of the objective aperture
(expressed in millimeters) by the square of the power.
The light of a telescope or field glass can also be determined by means
of the absorption apparatus shown in figure 30 (_a_) (_b_) (_c_).
This absorption apparatus operates on the principle of viewing an
object through a perfectly black liquid, which absorbs all colors
equally, and of increasing the thickness of the liquid layer until
the object becomes invisible. The thickness of the layer of liquid
will then be a measure of the relative brightness or intensity of the
illumination.
[Illustration: FIG. 30(a)
FIG. 30(b)
FIG. 30(c)]
The apparatus consists of two wedge-shaped vessels, made of brass,
with glass windows in the sides. One of these vessels is shown in
perspective in figure 30_a_. The sides _A_ and the one opposite are
of glass. _B_ is tubulure for filling the apparatus, and is stopped
with a cap. The operation of the apparatus is shown diagrammatically
in figures 30_b_ and 30_c_. The edges of the two wedges which
come together are divided into scales of equal parts of convenient
magnitude. Each scale begins with zero; not at the extreme point of
the wedge outside, but at a point, which, allowing for the thickness
of the glass sides, is opposite the point of the wedge of liquid
inside. It will be observed in figures 30_b_ and 30_c_ that the sum
of any two adjacent numbers, on the respective scales, over the whole
overlapping portion of the wedges, is the same. Thus in figure 30_b_ it
is 11, and in figure 30_c_ it is 7. These figures measure the relative
thickness of the liquid layers in the two respective settings of the
apparatus. Suppose the image is just obliterated, when looking with
the unaided eye, at the setting shown in figure 30_b_, and when using
the glass at the setting shown in figure 30_c_. This would mean that
the illuminating power of the glass is seven-elevenths. In using the
apparatus, a focusing cloth, used by all photographers, is useful in
excluding stray light.
_Field._--Maintaining the head and eyes as motionless as possible, the
field of vision of the unaided eye or the range within which objects
can be perceived by the unaided eye varies according to direction.
De Schweinitz gives the following limits: Outward, 90°; outward and
upward, 70°; upward, 50°; upward and inward, 55°; inward, 60°; inward
and downward, 55°; downward, 72°; downward and outward, 85°.
It may be safely said that the field or "visual angle" of the unaided
eye for _distinct_ vision is at least 45° in all directions.
The "visual angle" or "field" of a field glass is always smaller, no
field glass having yet been designed which could equal the field of the
unaided eye.
The field of a telescope or field glass can best be determined by the
use of a transit or other instrument used in measuring horizontal
angles. The glass is placed upon the telescope of the transit in such
a way that the axes of collimation of the transit and the telescope or
field glass are parallel. The extreme limits of the field of view are
marked and the horizontal angle between the markers noted on the limb
of the transit.
_Definition._--One of the chief qualities of the eye is its power of
defining outlines and details distinctly. Relative characteristics in
this respect may be determined in various ways. Thus the distance at
which printed matter can be read, or the details of a distant object
distinguished, will give a fair measure of the defining power of the
eye; but a better method is to express the definition of sight by
angular measurement--that is, by the determination of the smallest
visual angle giving clear results. Experience teaches that this angle
of the normal eye (with good light and favorable color conditions) is
about 40´´, and it is therefore possible to determine the smallest
object which can just be seen, well defined, at an arbitrary distance.
For instance, at a distance of 15 feet an object can be seen which
is one-twentieth of an inch high or broad; at 30 feet distance,
consequently, the object must be twice the size (one-tenth of an inch)
to be seen, and so on relatively, within limits, as distance increases.
But as the distance becomes greater sharpness of vision is impaired
materially by the interposing atmosphere, while it is also affected by
color contrasts and conditions of illumination. It therefore follows
that at considerable distances objects which subtend a visual angle
of 40´´ are no longer clearly defined but become so only as the angle
approaches 60´´, 120´´, 180´´, or more.
The most important and essential quality of a telescope or field glass
is definition, i. e., the sharpness, clearness, and the purity of the
images seen through it. To obtain good definition it is necessary
that spherical and chromatic aberration be overcome, that the polish
of the lenses be as perfect as possible, that the cement possess no
inequalities, and that the lenses be well focused, that there be
no dampness in the interior of the tubes, and, generally, that the
instrument be without optical defect.
Faults in this direction are discovered at once by examination of
definition, whereas in determining the other constants they are less
noticeable. In comparing the definition of any two instruments it is
ordinarily necessary only to scan distant objects and observe to what
extent details may be distinguished.
The following test may also be used: Focus on printed matter at a
distance just beyond that at which perfect clearness is given and
gradually approach until the letters are distinctly defined. The
instrument with which the print can be read at the greatest distance
has the best definition.
To express definition as an absolute measure, use instead of printed
matter, a white sheet of paper upon which a series of heavy lines are
drawn at intervals equivalent to their thickness. Focus upon this and
gradually approach from a point where the impression of a uniform gray
field ceases and the black lines and white intervals begin to appear
distinct and defined.
Let the distance thus found be 20 yards and the thickness of the lines
and intervals between them one-tenth inch. The circumference of a
circle with a radius of 20 yards or 7,200 tenths inches is 14,400 by
3.1416 or 45,240 tenth inches; but a circumference equals 360° or (360
by 60 by 60) 1,296,000´´.
If, therefore, 45,240 tenths inches correspond to 1,296,000´´, then
1 tenth inch equals 1,296,000 divided by 45,240, or 28.6´´. The
definition is therefore 28.6´´, or practically half a minute.
The capabilities of glasses, including telescopes, in a general way,
lie between the following limits:
(1) Power between 2 and 1,000.
(2) Light may be 0.01 to 200 times that of the unaided eye.
(3) Field measures in most favorable case, 10°; in the most
unfavorable, .01°.
(4) Definition varies between 40´´ and 0.1´´.
Thus, as a maximum, an object may be seen by means of a telescope,
magnified 1,000 times, 200 times brighter and 400 times sharper than
with the naked eye.
If these advantages could be fully utilized for military purposes the
use of glasses would be extraordinary, a power of 1,000 practically
effecting the same purpose as the approach of the observed object to
one-thousandth of the distance. A hostile command 10 miles distant
could be seen theoretically as well as if they were only 53 feet away,
and the slightest movement of each single man would become visible.
Of course no such wonderful effect is physically practicable, and the
limiting conditions increase greatly in proportion as either one or the
other of the qualities, power, field, etc., is especially sought.
While astronomers require only that the telescope be made as capable
and perfect as possible in an optical point of view, making all other
conditions subordinate to this one, the military, to whom the glass is
simply an accessory, make other conditions of the first importance. The
glass must have suitable form, small volume, little weight, and that it
may be used without support, mounted or dismounted, and the image must
appear as looked at by the naked eye--that is, not inverted.
The capability of the instrument, however, is thereby much limited;
great powers give plain images only with relatively long tubes; glasses
must be held the steadier the more they magnify; and with increasing
power all vibrations become more troublesome and render minute
observations very difficult or impossible. The additional lenses in
terrestrial telescopes somewhat decrease power and affect also light
and definition. It is clear therefore that expectations of achieving
great power should not be entertained, the function of field glasses
being to bring out and define objects which to the naked eye appear
indistinct and doubtful.
The distinctness with which anything can be seen through the telescope
depends, primarily, upon the number of straight lines of light which
are collected by it from every point of the object.
Telescopes, the object glasses being equal in size, diminish light as
a general rule in proportion as their magnifying power is great. The
most powerful glasses are therefore to be used for minute observations
on the clearest days or when there is a strong light upon the observed
object. When the light is fading or there is a little light upon the
observed object the clearer view will be had with glasses of large
field and low magnifying power.
FIELD GLASSES AND TELESCOPES ISSUED BY THE SIGNAL CORPS.
The Signal Corps issues four standard field glasses, viz, Type A, Type
B, Type C, Type D.
Field glasses issued by the Signal Corps are not supplied for the
personal use of an officer and will not be used in lieu of the
officer's personal field glass prescribed by paragraph 97, General
Orders, 169, War Department, 1907 (Par. 1, G. O. 16, War Dept., 1910).
Under paragraph 1582, Army Regulations, as amended by paragraph I,
General Orders, No. 207, War Department, October 16, 1909, the Signal
Corps will sell field glasses to officers of the army for their
personal use.
Application for the purchase of field glasses should be addressed to
the Chief Signal Officer of the Army, Washington, D. C., inclosing
post-office money order or check on the Treasurer or Assistant
Treasurer of the United States for the amount, payable to the
Disbursing Officer, Signal Corps, and Signal Corps Form No. 240
accomplished in duplicate.
The Government does not pay transportation charges for the shipment of
articles sold to officers. Field glasses are sent from the Signal Corps
General Supply Depot, Fort Wood, New York Harbor, by express, charges
collect, unless purchase request is accompanied by funds so that field
glasses may be sent by registered mail. Forwarding by registered mail
is somewhat cheaper than by express, and the amount of postage required
is 40 cents for Type D glass, 46 cents for Types A and B, and 74 cents
for Type C. Express charges depend upon the distance from New York.
The Signal Corps has purchased many samples of field glasses from
various manufacturers with a view of testing their suitability for the
military service. These samples may be examined by officers of the army
at the signal office in Washington. Among these samples there are many
excellent glasses especially suitable for the military service, but the
higher grades are too expensive for general issue to line organizations
in large quantities. Officers desiring an especially fine field glass
should inspect the samples referred to; these, however, are not for
sale by the Government, but information will be supplied concerning
dealers and cost.
No advice or fixed rule can be stated as to what constitutes the most
suitable characteristics of a field glass. No single field glass can
furnish maximum results under all conditions on account of varying
conditions of the atmosphere.
A high-power glass is unsuitable for use at night, hazy atmosphere,
or for use of a mounted man where the glass can not be rested against
a firm support. A low-power glass with large object lens to permit as
much light as possible is a necessary condition for use at night. The
double power glass which is issued as a part of the visual signaling
outfits was designed for the military service as a compromise for
conflicting conditions.
A brief description of the field glasses issued by the Signal Corps,
together with the cost of the same, is given below.
Type A:
This glass is the current result of the efforts of the Signal Corps
to provide a field glass that will meet the greatest variety of
conditions, and insure efficient service to the greatest number of
military observers. It is really two glasses in one--a _day_ glass of
medium power, and a _night_ glass of low power.
[Illustration: FIG. 31.--Type A. Showing the field glass and case with
sling cord, shoulder straps, belt loops, and compass.]
It is to be clearly understood that while this glass is considered
superior for moderate ranges, it does not replace, under special
conditions, for long ranges, either the porro prism glass or the
telescope.
When held as shown in figure 32 with the tubes drawn out about 1 inch
to secure proper focus, the glass has a power of about 5.6 diameters,
and a field of about 5.4 degrees.
[Illustration: FIG. 32--Signal Corps field glass, Type A.]
If the glass is turned into the position shown in figure 33, the small
plus lenses, just in front of the eye pieces, drop automatically into
position and reduce the power to 3.8 diameters, and increase the field
to 8.3 degrees. This position requires a different adjustment, the
tubes being drawn out about one-third of an inch to get the proper
focus. It will be observed in the illustrations that the rear bar of
the frame is not only lettered to indicate which power is being used,
but the bar itself is shaped with a hump on one side, and hollowed on
the other. When the hump is up, the low power is in use. This is to
facilitate adjustment in the dark.
The action of the small automatic lenses is free and positive. _Neither
the eyepieces nor the sections containing the small lenses should be
unscrewed, except in case of necessity, and then not by unskilled
hands._
[Illustration: FIG. 33.--Signal Corps field glass, Type A.]
The frame, of aluminum and brass, is composite, to give lightness and
strength; and while it is constructed to withstand the rough handling
of field service, no field glass is proof against careless or wanton
treatment. The tubes are covered with tan leather, and a round sling
cord, braided from four strands of pliable tan leather, is fastened by
snaps to eyes in the frame.
The case is of tan calfskin, provided with shoulder strap, and has an
efficient small compass set into the cover. Two loops are sewed to the
back of the case so that it may be worn on a belt.
The glass, complete with case, cord, and straps, weighs 21.5 ounces.
Two of these glasses are issued to each company of infantry and coast
artillery, Philippine Scouts, and Signal Corps, and to each troop of
cavalry for use in instruction in visual signaling. Below is a brief
description of the type A glass.
Magnification, 3½ and 5½ diameters; Galilean type; object
lens, 1½ inches; tan leather finish; tan leather carrying case
with compass; weight of glass, complete, with case, cord, and strap,
25 ounces. At a distance of 1,000 yards the field of view includes
a diameter of 123 yards for the 3½ power, and 73 yards for the
5½ power. Length of glass closed, 4 inches. This glass is issued
as a part of the visual signaling kit to each company of infantry,
coast artillery, and Philippine Scouts, troop of cavalry, machine-gun
platoon, and Signal Corps field company. Price, $12.15.
The latest issue of this glass known as the Type A, model 1910,
includes provision for interpupillary adjustment, the two barrels being
hinged to accommodate the glass to the distance between the pupils of
the eye. The price of the model 1910 glass is $14.75.
Type B:
This field glass is similar in appearance and construction to the
Type A glass, and is issued to the field artillery organizations upon
requisition. The following is a brief description:
Magnification, 4½ and 6½ diameters; Galilean type; object lens,
1¾ inches; interpupillary adjustment; tan leather finish; tan
leather carrying case with compass; weight of glass, complete, with
case, cord, and straps, 26 ounces; length of glass closed, 4½
inches. At a distance of 1,000 yards the field of view includes a
diameter of 90 yards for the 4½ power, and 60 yards for the 6½
power. This glass is issued as a part of the fire-control equipment to
field artillery. Price, $17.50.
Type C:
The type C is a high power glass of the porro prism type and is issued
only to certain organizations of the field artillery, Signal Corps, and
to all machine-gun platoons.
_Description._--Magnification, 10 diameters; prismatic type; object
lens, 1¾ inches; interpupillary adjustment; tan leather finish;
sunshade; tan leather carrying case; weight of glass, complete, with
case, cord, and straps, 46 ounces; length of glass closed, 7¾
inches. At a distance of 1,000 yards the field of view includes a
diameter of 80 yards. This glass is issued to reconnaissance officers
of field artillery. Price, $39.90.
Type D: Purchase has been made for delivery in the near future of a
supply of a new type of high power prismatic field glass for sale and
issue. This new type of glass, to be known as type D, is considerably
smaller than the type C glass, as is shown by figure 34. The glass in a
tan-colored carrying case weighs 15 ounces, the field glass without the
case weighing but 9 ounces. The magnification is 8 powers and the field
of view (with both eyes) 5° 40´. The estimated cost will be $27.
TELESCOPES ISSUED BY THE SIGNAL CORPS.
Type A: This glass complete consists of a 2-inch prism terrestrial
telescope, powers 18 and 24, with alt-azimuth, folding tripod, and
carrying case.
Type B: This telescope is a 19-27 power, 2-draw terrestrial telescope,
in leather carrying case with sling. The leather carrying case also
includes a holder which can be screwed into a tree, post, or other
stationary wooden object.
GENERAL SPECIFICATION NO. 263.
[Revised February 10, 1910.]
SERVICE FIELD GLASSES.
1. _Preliminary._--This specification covers the design and
construction of field glasses, types A and B, each having two powers as
hereinafter specified.
2. _Sample._--The bidder shall furnish with his proposal a sample of
the glass which he will supply, and award will be made after comparison
of the samples with models on file in the office of the Chief Signal
Officer. The maker will be allowed to examine the model glasses
in detail in the office of the Chief Signal Officer of the Army,
Washington, D. C.
3. _Inspection and test._--When the order under this specification is
complete, the contractor will notify the Chief Signal Officer of the
Army, who will cause an inspection to be made. It shall be the duty of
the contractor to remedy any defects pointed out by the inspector, and
the contractor will be held accountable for any imperfections which the
inspector may have overlooked.
[Illustration: FIG. 34.--Field glasses, Types C and D.]
The Chief Signal Officer of the Army reserves the right to inspect
any or all processes of manufacture, and unsatisfactory material will
be marked for rejection by the inspector before, during, or after
assembly, as occasion may arise.
Each glass will be tested for power, field, definition, and light. Any
glass which is not the equal of the sample and model in all respects
will be rejected. The properties above enumerated will be tested as
follows:
(_a_) Power: In testing for power the glass will be placed upon a
firm support about the height of the eye and directed upon a range
rod, accurately divided into divisions of 1 foot, with alternate
divisions colored red and white, respectively. The rod should be placed
approximately 100 feet from the glass in a good light and with strongly
contrasted background.
The rod is observed through the glass with one eye and at the same time
with the other eye unaided. An accurate comparison of the two images by
means of the rod scale determines the magnifying power of the glass.
(_b_) Field: The field will be determined by the use of a transit or
any other instrument adapted to the measurement of horizontal angles.
The glass will be placed upon the telescope of the transit in such
a way that the axes of collimation of the telescope and field glass
barrels are parallel. The extreme limits of the field of view of the
glass are marked in a convenient way and the horizontal angle of view
accurately measured with the transit.
(_c_) Definition: In determining the definition of the glass expressed
in units (seconds) a target will be provided with a number of lines
one-tenth inch thick with one-tenth inch spaces between them drawn on a
piece of heavy white paper.
At a certain distance this target will appear uniformly gray when
viewed through the glass.
The inspector will gradually approach the target, focusing the glass
until he reaches the most distant point from the target where the
uniform field ceases and the black and white intervals appear distinct
and defined.
Assume the distance thus found to be 20 yards and the thickness of the
lines and intervals between them one-tenth inch. The circumference of
a circle with a radius of 20 yards or 7,200 tenths inches is 14,400 by
3.1416, or 45,240 tenths inches; but a circumference equals 360°, or
(360 by 60 by 60) 1,296,000 seconds.
If, therefore, 45,240 tenths inches correspond to 1,296,000 seconds,
then one-tenth inch equals 1,296,000 divided by 45,240, or 28.6
seconds. The definition is therefore 28.6 seconds, or practically half
a minute.
The definition should be as follows:
For 6.5 power glass 30 seconds.
For 5.5 power glass 35 seconds.
For 4.5 power glass 40 seconds.
For 3.5 power glass 55 seconds.
(_d_) Light: The light of a field glass is expressed by a number which
is the ratio of the amount of light which reaches the eye through the
glass to the amount which enters the eye unaided. This comparison
will be reached by means of the absorption apparatus furnished by
the Signal Corps. This apparatus consists of two wedge-shaped vessels
made of brass with glass windows in the sides, and are filled with
a perfectly black liquid. The sky line is first viewed through the
apparatus with the naked eye and the instrument adjusted to limit of
visibility. The reading of the scale is then noted. The sky line is
again observed, using the glass, but in other respects as before, and
a second scale reading obtained. The ratio of these readings measure
the illuminating power of the glass which must conform to the standard
sample.
4. _Service field glass, type A._--(_a_) This glass shall conform in
general to the model, now on file in the office of the Chief Signal
Officer at Washington. The arrangement for changing automatically from
the low power to the high power, and vice versa, by the interposition
of the plus lens at the proper distance in front of the eyepiece, must
be strictly adhered to.
(_b_) The _low power_ shall be approximately 3½ diameters and the
_high power_ shall be approximately 5½ diameters. The figure of
merit given by multiplying the numbers of diameters power by the number
of degrees of field will be considered in the examination of samples,
along with the other properties of light, sharpness of definition, and
general excellence.
(_c_) _The tubes, frame, and metal fittings_ shall be of aluminum or
an aluminum alloy, with the exception that such metal parts as in the
opinion of the maker require greater strength may be made of brass.
Tubes shall be held firmly in the frame, single draw, the draw action
to be through a bearing surface of at least five-eighths of an inch of
best black felt, perfectly fitted so as to preserve perfect alignment.
The exterior metal parts, except where leather covered, must be given
the best and most durable, lusterless black finish. The tubes and
shades will be neatly covered with best quality tanned calfskin, the
leather to be sewed on, and the seams to lie flat next to the focusing
standard.
The interior of all parts to be painted a perfectly dead black.
The sunshades, when drawn out, shall project at least five-eighths of
an inch and not over 1 inch beyond the edge of the cell.
The focusing screw and standard should follow closely that of the
sample, except that the milled focusing disk should have a face as
nearly one-half inch wide as possible and the milling should be sharper.
In addition to the diaphragm upon which the automatic lens is mounted,
there shall be two diaphragms in each tube, so situated and so
proportioned as to cut off all stray light and all internal reflections.
The crossbar supporting the draw tubes should be shaped and engraved
exactly as found in the model.
(_d_) The lenses must be entirely free from mechanical defects, such as
specks, air bubbles, etc.; must be free from interior strain, and must
be ground from the best obtainable glass for the purpose, selected for
general transparency, as colorless as possible, perfectly ground and
polished, and accurately centered.
_The object lenses_ shall be composite, achromatic, and well corrected
for spherical aberration, with a clear aperture of at least 1½
inches, and not exceeding 1-5/8 inches. Bidders will state the number
and shape of the pieces used to make up this lens.
_The compound lenses_ may be either cemented together with Canada
balsam, or left uncemented, as the maker may deem best for durability
and optical performance, but if left uncemented the components shall
have a permanent mark to indicate their proper positions in the cell.
_The eyepieces_ shall consist of a single double concave lens having a
clear aperture of not less than three-eighths of an inch and not more
than one-half of an inch.
(_e_) The sling cord attached to eyes in the frame by means of brass
snaps with black burned finish shall be round and braided from four
strands of pliable tan leather, and shall have a diameter of at least
one-eighth of an inch and not over one-sixth of an inch.
(_f_) _The case and strap_ must be exactly like sample, and of No. 1
stock. Care must be taken to put in only compasses that are in perfect
condition. The strap buckle must be of brass. The glass, when closed,
must not exceed 4 inches in length, and the glass, case, cord, and
strap, complete, must not exceed 25 ounces in weight.
(_g_) The frame shall be constructed with jointed bars for
interpupillary adjustment.
5. _Service field glass, type B._--(_a_) The requirements of part 4,
service field glass, type A, of this specification, shall be followed
in the design and construction of the type B glass in so far as
applicable.
(_b_) Power: The lower power shall be approximately 4½ and the high
6½ diameters.
(_c_) Object lenses: These shall have a clear aperture of at least
1¾ inches diameter.
(_d_) Case: Case and carrying strap shall be furnished as required in
part 4 of this specification.
(_e_) This glass shall be constructed with jointed bars for
interpupillary adjustment.
(_f_) The sunshade, when drawn out, shall project not less than
three-eighths of an inch and not more than 1 inch beyond the edge of
the cell.
6. _Marking._--Glasses furnished under this specification shall be
marked on one barrel with the words "Signal Corps, U. S. Army," and on
the other barrel "Serial No. ----." Serial numbers will be furnished
with the order. If not furnished the contractor at the time the order
is placed, the Disbursing Officer of the Signal Corps should be called
upon for same, and the numbers and other marking placed on the glasses
prior to the delivery of the order.
JAMES ALLEN,
_Brigadier-General,
Chief Signal Officer of the Army_.
SIGNAL OFFICE,
_Electric and Telegraph Division_.
* * * * *
Transcriber's Notes:
There is no figure 25 in this text. The numbering goes from
figure 24 to figure 26.
Page 23, "porportions" changed to "proportions" (in proper proportions)
Page 106, "engineeer" changed to "engineer" (a French engineer)
Page 126, opening bracket added to subtitle ([Revised February 10,
1910.])