Q. 5
8“ 5L
e M as a"
HINTS ()N THE CARE AND USE or OPTICAL
INSTRUMENTS AT SEA. ' '
PREPARED AT THE UNITED STATES NAVAL OBSERVATORY.
Optical instruments, such as binoculars, ships’ telescopes, and sextants, are often
returned to the Naval Observatory for survey and repair on account, of ignorance
of the proper care in handling, or of the proper methods of cleaning ‘the lenses and
reassembling the parts. 7 _ .
1. Cleaning Zenses.——Employ a small tuft of absorbent cotton wet with dlstilled
water for removing dirt or spray deposits from the lenses, using spiral or eccentric
motions. If the lenses have become greasy from any cause, a few drops of pure alcohol
per ounce of water will assist in removing it. Do not employ undiluted alcohol to
clean lenses in their mounts, as it affects the lacquer finish of the metal. A fresh tuft
of dry absorbent cotton should then be used to dry the glass surfaces thoroughly. Any
remaining lint can be easily brushed off with the cotton if the surface is dry. Do
not use a handkerchief, chamois, or piece of silk to clean lenses. These usually contain
hard articles which produce scratches. Clean cheesecloth may be substituted for
the algsorbent cotton if necessary, but is not as good as the former.
Should the lenses be unmounted they ought always to be handled by the edges. In
case of mounted lenses they should always be handled by the cell. Never put the
fingers on lenses. - _ Y
2. Should it be necessary to remove the lensesfrom their mounts for cleaning,tcare
should be taken to reassemble them accurately. This applies especially to the
objectives or larger lenses of these instruments. These object glasses, as they are also
called, are generally composed of two lenses of different varieties of glass, flint and
crown, in which case they are termed “achromatic ”—or without color. Sometimes
these two lenses are cemented together with “Canada balsam,” in which case the
line of separation can be'detected by the examination of the edges of the combined
lenses. A cemented lens should never be cleaned with pure alcohol, benzol, or like
preparations, as they attack the cementing material. Likewise a cemented lens
should never be suddenly subjected to extreme changes of temperature, as damage is
likely to occur.
In the reassembling of an uncemented objective extreme care should be taken that the
separate lenses composing it are replaced in their proper positions and that the proper
surfaces are together. For binoculars and telescopes a convex surface is generally
found on the exterior of the cell of the object glass. Marks are nearly always applied to
the edges of uncemented objectives by the makers, indicating the correct order of sur-'
faces and the right position in which the lenses are centered with respect to each other.
Upon taking a ‘lens out of its cell these marks should be looked for and identified.
The marks generally used are V, A, or II, and sometimes a single straight line I. The
" A" markings should be used by the makers, as they indicate the proper relation of
lens surfaces—i. e., those which should be in contact or facing each other. They also
indicate the best positions of the lenses for centering. If the image in a telescope is
blurred at the best focus the cause is often the reversal of the crown or positive lens
of the objective. This can be determinedfif there is any doubt, or if the makers’
marks do not indicate the fact, by removing the lenses and trying them out with the
crown lens’ surfaces reversed.
, Sometimes a metallicring is found in ‘the cell with the objective lenses. This ring
is a separator ring and should always be located between the two lenses of the object
glass. Sometimes this ring is not in its proper lace, and the definition of the instru-
ment consequently is not good. The ring is o ten beveled to fit a proximatel the
curves of the lens surfaces with which it comes in contact. This act can be etere
mined by inspection. A good test for a binocular or ship’s telescope is to direct it
upon a bright star at night. (a) Observe if there are any stray wings of light sur-
36954-18
2 CARE AND USE OF OPTICAL INSTRUMENTS AI‘ SEA.
rounding the image. If so, the material of the lenses generally is defective or the
lenses are not pro erly centered. (b) After focusing on the star, run the image out
of focusboth insi e and outside the focal plane, slightly. If the appearance of the
expanded star disk is practically the same in each of these cases, the adjustments are
correct. (0) If, on the other hand, these expanded disks show a ring with dark
center, or an irregular disk with a hazy outline, the objective is probably wrongly
adjusted. These errors may be due to causes previously mentioned, and can be
determined by investigation. Sometimes, however, a lens may be subject to undue
gressure from being too tightly clamped in its cell. This will sometimes produce
ares and distorted images of a star. An objective lens should have a slight rattle
in its cell if shaken rapidl
' vThe eyepieca'a—In the ordinary binocular the eyepieces consist generally of simple ~
double' concave lenses of moderate diameter. These are mounted in parallel draw
tubes 1n suitable cells, which are easily removable for cleaning the lenses. Some-
times these eye lenses are of triple construction, with the lenses cemented together.
In either case there is little liability of getting them out of adjustment, especially
if the‘ screw ring clamp is set down so that they do not shift appreciably in their cells.
With'the. prism binocular the eyepieces are ‘of a different form, consisting of two
‘lane-convex lenses mounted at either end of a brass tube in suitable cells. These
enses‘ ‘may easily be removed for cleaning. The prisms and their adjustments, how-
ever, should ‘never be tampered with. The prism surfaces may begcleaned by the
absorbent-cotton method. They should not be removed for this purpose from their
mounts, as a slight error of prism adjustment will ruin the performance of a rism
binocular. When a lens is transported from a cold to a warmer place, especiallly if
the former temperature is near or below freezing, moisture is deposited upon its surfaces
ifjit is unprotected. A lens or instrument in a moist condition accumulates dirt very
rapidly,‘ and requires cleaning more often. 7
This dirt film generally takes on an a pearance of haze or fog on the lens surfaces.
If 'of appreciable density it absorbs, re ects, and diffuses light, thus rendering the
instrument "less efficient. An instrument after use should be returned to its case to
protect it from dirt and moisture. If ‘it has been wet with spray, the exposed lens
surfaces should be cleaned by the absorbent-cotton process before it is used again. It
is ' best to‘ do this at once before the spray has evaporated.
‘ Some attention in the use of binoculars and telescopes is desirable to get the best
results; In the daytime and upon moonlight nights the extension shade tubes, if
provided, should be used. Considerable stray light is reflected from the lens surfaces
of anfinstrument. . This interferes with the best vision by reducing contrast. _
On a dark night faint objects can often be seen by oblique vision when they ‘are
unperceptible when looked at directly. This is on account of the fact that the best vision
point of the eye is not the most sensitive to light. To get this oblique vision the eye is
first directed at the object through the instrument and then turned slightly outwards.
In this case it may be necessary to use the instrument as a monocular to get the desired
effect. ‘This method of oblique vision is often employed by astronomers in observing
faint objects, which can not easily be seen directly.
‘Care of instruments—‘In the case of binoculars and other forms of deck glasses do
not subject them to ill usage. A bent draw tube or other part of the instrument if
damaged by 'a blow or other accident will render the glass to some extent inelficient.
It is not enough to look through instruments of this class and get the maximum light
transmission, tbut we must obtain a well—defined imageof objects as well.
' Do not ‘allow the lenses to be scratched. Optical glass, though seemingly hard,
scratches easily if rubbed promiscuously with all sorts of cleaning cloths and material.
Dofnot take an instrument from a warm to a cold place, or vice versa, and expect
tof'get the best definition from it immediately. For the best results instruments
should have the same temperature throughout.
'Focusing.—-—To see an object well the instruments should be accurately focused.
‘This is effected by changing the positions of the eye lenses, in or out, to obtain the
sharpest possible image. Sometimes a slight change of focal adjustment after long use
will rest the eyes. With the exception of the spyglass type of instrument,’ focusing
is generally performed by turning a knurled head of an adjusting screw. Sometimes
.afscale'is attached to either or both of the eye ieces of a binocular to obtain sharp
definition with both eyes. Eyes often differ in t e same individual slightly. Shou
$424.41, ’
hat-L .
.IB
I
Ilj’b)
CARE AND USE OF OPTICAL INSTRUMENTS AT SEA. 3
* this be the case, each eyepiece should be ad'usted toga good focus on a distant object.
_ instruments than in low-E
Then by closing each eye alternately it can e determined if the definition is good in
each case. Slight corrections for the focus of an individual’s eyes can be made in the
ordinary binocular by unscrewing either the eyepiece or objective, or both, slightly,
to obtain the proper ad'ustment. This, of course, is good only for the person making
the adjustment. The cone for distant objects, with all classes of optical instruments
for a given person is practically uniform. As an object approaches the focus becomes
lengthened out or longer. This change of focus is more apparent in high-powered
owered ones. -, .
- It is generally best to ave a mark on the barrel of the sliding tube containing the
eyepiece of a telescope for reference, so that an approximate focus can be obtained at
once’. This mark can easily be made with the blade of a knife on the tube. In this
casethe instrument should first be accurately focused on a distant object. Care should
be taken that the adjustment is not changed while the mark is being made.
Sensitiveness of the eya—We see objects by two methods: 1. By contrast of light and
shade if the dimensions of objects are sensible. 2. By differences of color, and shad-
ings of the same, if the objects have apparent size. Consequently an object may be
too small to see, or, conversely, it may be too large to see, if its contrasts or colors are
not suitable for vision. These conditions are accentuated upon a dark night, where
there is little range of scale value or contrast 'in the available light.
In observing such faint objects at night the eyes should be thoroughly protected
from all extraneous light. Even a faint glow of reflected light will cut down the light
sensitiveness of the eye, or the retina, considerably.
By remaining in darkness f '
or some time the eyes become considerably more acute.
This is partially due to the increased aperture of the iris diaphragm of the eye, which
does not directly affect telescopic vision. The retina, however, increases its sen-
sitiveness in faint light also. This is what counts in seeing faint objects with an optical
instrument.
It is a well-known fact that the eye can be trained in the use of o tical instruments.
,An experienced person with the same instrument and under sim' ar conditions, can
see much more than one without training. The former looks for detail and faint con-
trasts which would escape a novice, and does this, moreover, automatically.
Day and night glassea—By day we generally have a superabundance of light for our
visual reqfiiirements, but at night all light from objects must be conserved. _
The lig t collective power of a telescope depends upon its aperture, or rather upon
the square of this aperture. Consequently the light-gathering powers of two similar
instruments are proportioned to the squares of their diameters. The effective aperture
of the pupil of the eye upon a dark night is about one-fourth of an inch. From the prin-
ciples of optics, a magnifying power of 4 diameters to the inch of the objective gives
the maximum brilliancy to' the vision. The close approach to this condition is, of
course, most favorable in night glasses. In the Gallilean, or opera-glass type of binoc-
ular, this aperture ratio is often much exceeded. > Large object glasses are necessary
' in this type of instrument to obtain a large field of view. In the day glass this ratio
of magnifying power of 1 to 4 can be considerably exceeded; that is to sa ', the magni-
fication available on the night glass can be at least doubled without undu y weakening
the brilliancy of the visual image. But doubling the magnifying power of an instru-
'ment' decreases the illumination under the same lighting conditions to one-fourth of
its previous brilliancy. It will thus be seen that a point is soon approached where
increased magnification must cease. The increase of magnifying power also reduces
the angular field of view, so that the area under observation at once becomes smaller.
It also produces a less steady image in a hand instrument. All the tremois and motions
of its su port are magnified in the instrument to the same extent as the object, thus, in
effect,'b urring the definition of the telescope. Extremely high powers are therefore
to be discouraged except for special purposes.
In active naval service it is important to understand these simple facts thoroughly.
It is as necessary to keep our vision clear, and our optical instruments in good condi-
tion as to be prepared in other ways. These hints have been prepared at the Naval
Observatory for general information on the care and use of optical instruments at sea.
The should be read by all those who have occasion to use or care for such apparatus
on s ipboard. ' ‘ '
WASHINGTON : GOVERNMENT PRINTING OFFICE 2 1918