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Патент USA US2131738

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2,131,738 _
Patented Oct. 4, 1938l
Archer vHoyt, Aspinwall, Pa.,Company,
Gul! Rc
search & Development
Pa., a corporation of Delaware
Application March 30, 193e, serai n». 11.131
11 Claims. v (CL 88-29
Another object is the provision of such a sys
tem utilizing multiple reflections to secure ampli
'I'his invention relates to optical systems; and
it comprises optical means for indicating angular
'deilection of an angularly movable apparatus
part, comprising- a pair of plane, partially trans
fication and in w ‘ch the exact order of redec
tion used is apparent to the observer.
Another object îsthe provision of such a'sys
5"~\parent mirrors arranged substantially parallel to
tem adapted for measurement of very small an
’each other, one mirror being fixed and one at
gular deflections.
Another object is to provide an optical system
ment of said part the mirror attached thereto - which is very sensitive but which is rugged, easy 10
7 is tilted with respect to the fixed mirror, a source to adjust. and capable of being operated by un
- _ftached to said apparatus part, so that upon move
of light, means for directing a collimated beam
skilled persons.
of iightfrom the source throughthe mirrors sub
These objects are achieved by the provision of
- stantlally at right angles to the plane of the mir
an optical indicating system which includes, es- '
rors, and means for bringing the collimated beam
to a focus at a focal piane after its passage
15 through the mirrors, so that upon movement of
sentially, a projection lens combination and two
partially transparent mirrors at ornear the cen» 1
ter of the’combìnation, one of the mirrors being
fixed and the other moving. The mirrors may
be combined with the lenses or may be separate
the apparatus part. inter-reñections occur be
tween the mirrors and a series of images of the
source' is formed at the focal plane; all as more ‘ elements. The moving mirror is attached to
some angularly-defiectable part of the measur- 20
fully hereinafter set forth and ás claimed.
Many physical measuring instruments indicate
ing apparatus to which the system is applied. ' A '
source of light and a screen are set up in such
manner that the s_ource is imaged on the screen
the measurement as an angular deflection. For
example, there has Ibeen developed a gravimeter..
an apparatus for measuring the force of gravity
at any given point on the surface of the earth.
by the lens combination. Then upon~ angular
deflection of the moving element, a series of mul
tiple reflection images appears on the's'creen, each
of which images indicates the actual angular
deflection multiplied by a different factor. The
principles of operation will be clear in the ex 30
which indicates changes in gravity as an angular
deflection of a portion of the apparatus.
In instruments where the angular deñection
is large or where great sensitivity is not required
the deflection may be indicated by a simple needle
or pointer attached to the moving part. In more
delicate instruments it is usual to employ in lieu
of a metal pointer, a so-calied light beam pointer.
That is, a mirror is attached to the moving part.
and arranged to reiiect a beam of light from a
tended descrlption to follow.
The new system will be described in three
typical adaptations: to a gravimeter, an analyti
cal balance and a barograph. Two principal
modifications of the system itself will be de
scribed, to illustrate the invention. Examples of
iixed source to a iixed screen. Upon any twist
35 ing of the moving part, a spot of light moves over specific embodiments of the invention are illus
trated in the accomp
‘ng drawings, in which
the screen.
arrangement presents ad
Fig. l is a simplified view, partly in elevation
vantages over material pointers in that the point
and partly in vertical section, of one embodiment.
er is merely a beam of light which has no mass
of the invention incorporated in a gravimeter;
or friction; and also that the angular deflection
Fig. 2 is a diagrammatic view of the optical
40 indication is, through optical lever eiîect. ampli
system oi Fig. 1:
fied by a factor of 2. However. this single re
Fig. 3 is an optical diagram illustrating the
flection arrangement. while an improvement operation of the optical system of Figs. l and 2;
upon mechanical pointers. has limitations. To
Fig. 4 shows an advantageous form of slit:
45 secure great sensitivity it has been found neces
Fig. 5 shows what is seen in the eyepiece when
the slit of Fig. 4 is used;
Fig. 6 is an optical diagram of a modification
of the system shown in Fig. 2:
Figs. 'i and 8 are diagrammatic views in two
sary to employ light-paths six feet long or more.
which makes for a bulky and cumbersome appa
One object of the invention is to provide an
optical indicating system which utilizes a light
50 beam as a pointer but which gives an enhanced , different directions. of a modified form of optical
amplification of the angular deflection.>
Another object is the provision of an optical
is of great compact
indicating system which
Figs. 9. l0 and il show modified forms of lens
I ': _
, ‘
thereof, except the prisms, which are omitted _ ~
Fig. l2 shows an analytical balance incorporat
ing the optical system;`
» _.
, Fig. 13 is a fragmentary sectional view of a
portion of Fig. 12;
since their only function is to change direction
'of the .light beams. The image of the illament
of bulb 38 is focused between lenses 3i and
3l’ and thus the slit is- illuminated substantially
Fig. 14 shows a barographvincorporating they uniformly. The illuminated slit serves as the
optical system;
ei’iective source of light for the system. The
n Fig. 15 shows a modi?cation giving echelon
lenses, slit and scalev are so arranged that the
slit is imaged on the scale, as shown. In all
Fig. 16 shows the image array with the system
of 1518.15.
` '
In the: showings, in which like reference char
acters indicate like 'parts throughout.
shows one embodiment incorporated in a loaded
spring gravimeter. The gravimeter, of which
the showing is simpliiied, comprises a casing 2l
with top 2l and base 22. A helical coil spring
23 is provided,A being suspended from lthe cas
ing top by a post 2l and angularly-adjustable
plug 25, and having attached to its lower end
20 a post 26 carrying an annular weight 2l. The
spring is oía type adjusted to twist imder the
embodiments of _the invention it_ is provided that
the light beam in the inter-lensl space is col
limated; i. e., the rays are parallel. In the ar~
rangement of Fig. 2 this condition is achieved
by placing the scale and the slit at the principal
foci oi' the lens combination. When the lenses
are parallel the slitis imaged. onthe scale at
BU. This is the normal or zero position of the
system. Upon angular deñection of the post 2l
the lenses are put out of parallel. Inter-reflec
tions between the plane mirror faces occur, the
phenomenon being diagrammed in Fig. 3. Each
inter-reilected bundle of rays gives rise to an
influence of variations in weight of .the annulus * image on the screen, as indicated at 8|, 62, 62,
21, and hence to» impart a twisting movement
to post 26, reñecting changes in the earth's
GI and 65. A large number of reñection images
gravity. The twist is in all cases of very small
magnitude. making special indicating means ad
visable for securing accuracy. The gravimeter,
including the spring,-is described in` detail in
my copending application Serial No. 34,824, filed
are produced; only ñve are shown. The ap
pearance of the screen and scale in the’eye
piece is indicated in Fig. 5, the undeviated direct
image (60) and the higher order images ap- '
pearing as shown.
In all cases the moving lens should be so
Augusti'i,l 1935.
mounted on the post that the nodal axis of the
The optical» _system of the present invention lens coincides, or approximately coincides, with
to indicate angular deñections of post the axis of rotation. In lplano-convex lenses
265 As shown, lthere vis aflixed to post 26 an 'such as those shown, the nodal axis is tangent
annular mount 3|! retaining a. lens-mirror 3i to the plane face, and the'plane face is shown
having 'a convex unsilvered vface 32 and a plane as not- rfar from tangency to the axis of rota
face partly silvered as at 32 forming a mirror. tation. This adjustment is not especially impor
A similar lens-mirror 3l’ having corresponding tant when the angles measured are small.
faces 22 and 33 is mounted on the base (22) by
means of an annular mount 3B. The lenses vare
ordinarily made of glass, though they can be
made of Iother ytransparent material if desired.
' 'I'he plane faces 33 of ’the lenses are parallel, in
the normal or Azero position of the apparatus.
If necessary knob. 25 vis adjusted to lbring the
_faces into parallelism. The plane faces are ar
ranged closely adjacent. vIn top 2i is attached
’a housing 31 containing a light bulb 38, a con
densing lens 39 and a slit element Il (Fig. 4)
having a slit 4I. The slit member can be of the
adjustable type if desired and can be of 'shape
other than _that shown. yAlso mounted in the
top is an eyepiece l2 including a lens 43 and a
transparent screen member 4I (Fig. 5) having
a scale I5. In Figs. 2 and 6, 7 and 8 the eye
piece is indicated diagrammatically at l2, the
appropriate image ray lines for one of the images
The axis'of rotation being tangent or nearly
so to the nodal axis of the lens, then for small
angles the direct image is undeviated upon angu
lar displacement of post 26 and serves as a ref
erence point. But each oi' the images due to
inter-reflections represents the angular displace- "
ment of post 26, angularly amplified by a factor
depending on the lnumber of inter-reflections re
sponsible for that image. Thus the first inter
reilection noted at 66 on Fig. 3 gives rise to a
ray 61 which produces image 6| and which makes
an angle 2A equal to twice the angle A through
which post 26 moves; The second inter-redec
tion noted at 68 gives rise to a ray ßs'which pro
duces image 62 and which makes an angle IA.
Higher orders of inter-reñections give rise to ray:
making an angle which is equal to _the angle oi'
deiìection of post 2B multiplied by twice the num
ber of reñectlons. That is, the 11th image from
the. undeiiected image moves through an angle
211A when post 26 moves through an angle A. VIn
may comprise a micrometer eyepiece with cross
Fig. 3 all the angles are shown exaggerated for the
hairs, or ocular observing- means may be re
placed by recording means such as a piece of sake of clarity.
The observer may select any of the images.
photographic material (described post). The For
example. he may select the 6th image, and
term screen is used herein to include the focal
plane of the eyepiece and any sort of index or note that the scale reading is 15 millimeters from
receiving surface, upon which the indicating the zero image to the 6th image. Scale readings
are suiiìcient ior most purposes, but if desired the
images are ioc
actual angular deiiectlon of the post can be cal
In lieu ot a slit. a narrow linear lamp fila
culated by trigonometry; in which casel a scale
ment or a point source can be used. as de
value oi 15/12 or 1.25 is taken as corresponding
scribed post. Two 45 degree prisms 50 hav
the actual angular deflection. To ilnd vthis
70 ing reflecting hypotenuse faces 5I are mounted angle (in radians) one divides_l.25 by the lens
on the base in supportsv 52. The function of the
prisms is to direct light from the slit through local length in millimeters.
Since one ray is undeviated, and since the
the. lenses and back up to the eyepiece.
(thev undeilected image) being indicated at 45.
In lieu of a screen and scale. the observing means
The operation of the optical system is best
75 seen in Fig. 2, which shows the several elements
higher -order images extend from it in spaced
series, there is no ambiguity as to which order o! 'Il ‘ I
Fig. 4 shows the slit as of greater width at one
end than the other. While ordinary linear slits
can be used, theform illustrated is advantageous
in that the enlarged end provides some stray light
which makes the scale or crosshairs readily vis- l
reflection the observer is utilizing. He merely
counts from the- undeflected image. This is an
important feature of my system. In making use
of multiple reflections for indicating, it is desir
able that there should be no ambiguity as to
which order of reilection the observed image be
The embodiment of the invention described is
. While the system described is, in principle, op- I the simplest and is in many adaptations the' most
erable 'evenwhen the plane faces-of the lenses useful. 'I‘he system is readily adaptable to all
sorts of measuring instruments, the movable lens 19 _
being in all cases añixed to an angularly movable
are unsilvered, since plain glass surfaces reflect
light to a certain extent (about 4 per cent of the
totalv incident light at normal incidence), in prac
tice best results are secured when the plane
faces are made of higher reflecting power as by
part of the apparatus. Certain modifications o'f
the apparatus have utility in particular installa
tions. Some of these are exemplified in Figs.
15 applying a very thin partially transmitting mir
The embodiment just described, and all other
roi- film of aluminum, silver yor other metal
thereo. The transmission and reflection factors _ embodiments of the invention, have two partly
. have a certain optimum value depending upon transparent mirror faces and a projecting lens
which order of reflected image is to be used. combination. The minimum number of optical
The following are the considerations by which components, therefore, is four; two mirror facings 20
the silvering to be applied to the plane faces vis and two lenses. These parts can be _provided as
individual optical parts or as_combined optical'
Reference to Fig. 3 shows that to get n reflec
_ parts, i. e., the mirror facing being on the lens,
integral number, e. 4g., 1. 2, 3, etc.) , there must be
n other reflections taken from a fixed mirror;
also, that in the case of all images due to inter
subject to a few simple restrictions. It is best to
have the reflecting faces close together and the 25
light being multiply reflected back and forth
between the mirrors should be essentially “par~
reilection, transmission through two and only two
mirror surfaces must occur. Thus to get maxi
positive meniscus, or plano-convex; that is. of
tions from a moving mirror (where n means any
30 mum intensity in the 11th reflection. the mirror
-surfaces should be highly reflecting and only
slightly transparent because there are ' 2n re
ñections but inail cases only 2 transmissions.
The reflection and transmission characteristics
35 of the opposed plane faces may be different, but
allel light".
The lenses can be double convex,v
any converging type. The lenses can be simple 3°
or highly corrected. The mirrors should be high
quality optical flats and partly transparent. The
mirror ñlms can be mounted on glass, quartz or
other suitable backing. Since every air-glass in
terface imposesa 4 per cent (approximately) loss 35
since experience shows that it is best to have _the ' of light intensity, there is some advantage in
reflection and transmission factors of the two -combining the functions of the optical parts.
While it is not necessary to take great pains to
mirror-faces equal, only this case need be dis
conserve light intensity, 'a general simplification
cussed in detail.
The intens'ty of the nth reflection In, depends results in that fewer mechanical mountings are 40
on 2n reñections but on only 2 transmissions. required if the lenses are made plano-convex and
If the transmission of each mirror surface is t, the plane side is used for the mirror backing as
well as just for the other side of a lens. Carried
then the reflection factor is (1_-t) andthe in
to the limit, the system then consists of two plano~
tensity can be represented as
convex lens-mirror umts requiring only two 4I
where L is the incident" light intensity. By
known calculation methods it is determined that
this function has a maximum value for n, ex
pressed ß follows:
Thus in the case of the nth reflection the max
imum possible light intensity is obtained when
the transmission factor is 1:1/ (n+1). For ex
ample. usingl the 6th image, the optimum transmission factor is 1/7. Using ~the 10th image it is
l/ll; in other words the transmission is 9 per
Experience shows that on the whole the best
practical results with the ‘system are obtained
when about the 10th order reflection image is
used, the angular ampliñcation in this case be
ing l0 times that obtainable if only a single reflec
tion were used. In this case a transmission of 9
per cent is, asstated. optimum, but results almost
as good i image intensity not less than 90 per cent
ot optimum) can be abtained with transmission
within the range .12.5 and 6.3 per cent.
I regard aluminum alloy films as best for the
' _
mirrors. by reason of their excellent optical prop- _
' ertles and their durability, lbut other materials,
such as silver. can be used. The _term “silvered"
is used herein to include all suitable reflecting
.75 films, metallic or non~metallic.
mounts (Figs. 1-3). This system is ideal if one
lens mirror is fixed and the other mounted on a
moving part with a well deñned axis. If, however,
small translational displacements of the lens’mir
ror occur, there will result small changes in the 50
optic axis of the system. For use on portable in
struments, such as a gravimeter which has to be
leveled for each new station, the system shown in
Fig. 6 presents advantages.
Fig. 6 shows a system similar to that of Fig. 2 55
but having two fixed plano-convex lenses |3i and
l3i', one of which (advantageously the one on the
eyepiece side) has a partially silvered plane mir
ror face 33, and a parallel plane-faced glass ele
ment 'l0 is arranged therebetween, element 10 00
being affixed to an angularly movable part (2B) of
the measuring apparatus. The face 33’ of ele
ment 'IB adjacent face 33 is silvered. Element 10 '
is thus an angularly movable mirror. This arrangement gives results similar tothose obtained ' 65
with the device of Fig. 2.'
'I'he embodiment of Fig._6. I~regard as being on '
the whole the most useful'system. .The disassoci
ation of the moving mirror fromv the lens on the
source side of the system results in a simpliflca- 70
tion oi' the manipulations involved in the initial
adjustment of the system. The system is espe»
clally useful in gravimeters of the type indicated
in Fig. l. Since the optical part attached to the ‘
moving system is essentially a plane-parallel 'Il
focus4 than lens Il, and lens l8|' has a slightly
longer focus than lens 9|, to image the source
plecenof glass withy a mirror surface on the veye
‘piece side, there occurs n'o shift of the optic axis
The planeiaces 32 of lenses 9| and `
Aot the system of the moving clement which swings v 9|", and faces 33' of lenses Il and 8|', are par-
due to- seismic disturbances. or is 'slightly mis
tlally-silvered to provide plane mirrors- of senilQv .
alined due to errors in leveling. In the Fig. -6
_circular shape onthe opposed faces as shown.A` '»
system, errors due to a lens not rotating about ‘
Fig. l0 shows ,an arrangement of Afour fixed
its nodal point are avoided. This system pos- ' lenses, lui., m, m anc- m, and Aaimerait» `
' sessesl all the advantages of 'a system with four element consisting 'of a parallelog‘ram-‘shaped
completely disassociat'ed optical elements with the
glass block or double prism :|18 with partially
added advantage ot one less mechanical mount ' transmitting plane mirror faceslívlitl and. Iii'
ing. since the mounting for the'ñxed mirror is 'opposed'v parallel' to lenses `lll'nnd V|2|, and '_
likewise the mounting 4for the lens on the eye
beveled,unsilver`ed"faces lill and |4|. The opera~
tion of this device is' similar to that Vof Fig. 6'.
piece side of the system.
The lenses of Figs. 1 to 3 and 6 can be of the The reason .for beveling faces' |49 andl _Hi lis
same focal length or diiîerent.' It different. the to make them not parallel with respect to faces
image of the slit will be magniñed or minified 33 of lenses I|| and |2I; if they .were parallel,
depending on the ratio of focal lengths. This is spurious and disturbing reflection images might
appear. The angle of Abevel‘is slight, only a
sometimes advantageous.
While the systems described are sumciently few degrees oiî of parallel being necessary to
sensitive, even for very delicate apparatus, it is achieve the results. The bevel is exaggerated in
20 possible to 'secure twice the sensitivity by making
the showing for the sake of clarity. In this. .3
use of a modiiicati'on, which is essentially a double
arrangement a double lamp arrangement as in
system so constructed that two equal .and opposite Fig. 1 is employed. and the aspect of the reiiec»
series of images are produced upon angular de
tion images is asin Fig. 8. The modiñcation of
flection of the apparatus. Readings are taken" Fig. l0 has the advantage that all four lenses can
vation) and Fig. 8 (plan). Two plano-convex
be of the same focal length if desired. The
prìsmatic element (|10) is disposed so- that the
prisms bend the rays from the iilament, in a
lens combinations 1| and 1|', are ilxed to the
direction along the length of the ñlament; thus
between the 11th image of each series.
One such modification is shown in Fig. 'l (ele
angularly movable apparatus element in the aline
Iment shown, and these are opposed to two fixedlenses |3| and |3|' as shown. Plane faces 33'-of
lenses 1| and 1|', and piane faces 33 of lenses
no diiiiculties 'are introduced due to dispersion
bythe prisms. The image is not widened-across
its width. but across its length, which is of no
|3| and |3|', are silvered to serve as mirrors.
This modification is shown as having line-ilia
ment lamps 18 and 18' _in lieu of the lamp-lens
slit combination of Figs. l, 2 and 6. The vari
ous elements are arranged so that the primary
filament images of the two systems coincide on
thel screen, as at 60; To achieve this result in this
modification, lens 13|’ is 'of slightly shorter focallength than lens 1|, and lens 1|' is of slightly
|3|', and a parallelogram discoid element 21|.
In some cases the bevel can be dispensed with en
tirely, element 21|| taking the form of a plane
discoid similar tn_that of the device of Fig. 6,
. longer focal length than lens |3I. If desired lens
|3I' can be of the same focal length as lens 1|,
provided lamp 18' is set back to bring it at the
principal focus of lens III'.
Fig. 11 shows an arrangement analogous to
Fig. 10 but having‘only two fixed lenses, |3| and
UponÍangular deilection of the apparatus ele
but silvered in halLcircles on opposite sides.
rather than over the whole face of the disc on
only one side as _in Fig. 6. In such modiñcation
it is advantageous to have the moving element
slightly- canted, i. e., oil' parallel, in the zero 45
position. The right naif only or lens Isl. and .
the left half only of lens 13|', are silvered, as
at 33, to provide mirrors. This system, which
' ment, a'double series of images is produced as _ utilizes a single lens-slit combination as in Fig. 2,
shown in Fig. 8. The series on the right of the gives a double series of images as in Fig. 8.
reference point (60) is denoted by 6|, 62,`etc.,
These double-series systems are useful in in
and that on the left is denoted by 6|', 62', etc.
struments i'or measuring directly the tidal forces
Measurement is conveniently made between the of the sun and moon. in which instruments ex~
10th image of each series. The sensitivity of this ` tremely great sensitivity is essential.
device is just twice that of the devices of Figs. 2
>and 6 vhaving similar dimensions, because, when
post 26 is rotated. the two sets of images move in
opposite directions with respect to the central
image. This causes the images to move through
twice the distance in the field of view.
The moving element (lenses 1| and 1I') can
be made by simply mounting two such lenses in
a mount. or it can be made by cementing two thin
plano-convex lenses to a fiat piece of glass.
Figs. 9, 10 and 1_1 show optional forms of lens
combinations >which achieve similar results.
Fig. 9 shows a combination of four half-lenses.
the moving lens comprising oppositely disposed
half-lenses 8| and 8|' as shown. and two iixed
lenses 9| and 8|' being provided. This arrange
ment gives results similar to those achieved with
70 the device oi Figs. 7 and 8, but only one light
source is necessary. In using this embodiment a
lamp-lens-slit assemblage as in Fig. 2 can be
used, and the appearance of the image array
is as in Fig. 8. Lens Il’ has a slightly shorter
Figs. l5 and 16 show a useful modiilcation of
the invention, which may be used with all em
bodiments but which is illustra-ted'in connection
with the system of Figs. l and 2. By mounting
the movable mirror lens element (3|) on post
26 in such manner that the plane of themirror
(33) makes an angle to the axis oi' rotation of
the post, the multiple images produced on angular
rotation of the post are no longer allned, but ap-4
pear in the eyepiecein echelon arrangement as
indicated at R in Fig. 16. When the post swings
back to central position. the images appear over
lapped in a line. as indicated at C in Fig. 16.
As the post swings past center to the left. the ' '
images again appear as an echelon as indicated at
L. Thus when the post is swinging. the appear
ance in the eyepiece is as of a movable brush
or pointer swinging about a pivot at 60. With
this arrangement it is very easy to observe when
the images are ailned (at C) hence this modi
ilcation is useful in instruments based on the 75
null principle. As the alined vimages partly over
lap. there is less loss in light for any particular
measurement at this point, the appropriate factor
being applied.
In all embpdimentsof the invention the mirror
‘ faces of the various elements are advantageously
. It should 'be noted that vthe optical system pro~.
vides indicating or recording means which im.
pose no resistance upon the moving apparatus
element. Indeed, the 'system has no mechanical
'disposed close together, and ordinarily the faces »connection whatever with the apparatus mecha.
_are parallel in the zero position.
' In an optical -system of the present invention l
The systems are shown as'adapted for measuring deflections in one dimension only, but those of sensitivity equal to that of an ordinary lamp-l4
mirror-scale system, the light pathv is only 1l»
'10 ofv Figs. Zand 6 can be >readily 4_adapted for indi
.cating deflections in two dimensions, by sub- 1 as long, where n is the order of reflection utilized.
Vstitutin'g for the slit an illuminated pinhole For example, in the system of Fig. 1 utilizing thev
aperture or other point source of light, andy 10th reflection, a total light path of 2 Àfeet is
mounting the movable optical element on an equivalent to an ordinary system having the lamp
and scale spaced 20 feet from the mirror. In a
15 apparatus element subject to angular motion in system as in Fig. 2 with a source-screen distance
more than» one plane. Such a modification is
of 22 inches and using a 10:1: ocular, it was`easy
of particular utility in certain types of seismo
Figs. l2 and 13 illustrate the adaptation of the
system of Fig. 6 to an analytical balance. The
to measure angular deflections as small as 0.1
second of arc. In a system of similar dimensions
but using only a single reflection, no angleßless
. balance comprises the usual elements; a base 2N,
upright 2li, knife edge support 2|2, beam 213,
knife edge 2H and pans 215.v The moving plate
10 'of the optical system is aillxed to the beam,
in alinernent with the knife edge, in a mount 2l!
as shown'.- The under side -of plate 1li is silvered.
' Fixed lenses I3i and i3l' are attached to the up
than 1 second can be measured with the same ' '
Using a straight line scale, deflections noted
on the scale are practically a linear function of
the angular deflection when the angular deflec
tion is not more than a few degrees; the sca-le
defiection being proportional to the tangent of
the angular displacement. For larger ranges of
right, as shown, in mounts 2|! and 220. The up
per side of lens |31' is silvered'vas described. A deflections, a tangent correction can be applied
straight filament lamp 18 is provìdedas in Fig. '1. I arithmetically or, the scale can be curved about
On the base are mounted an inclined Iscale-screen the center of the lens combination. ÍFor most '
22|, of ground glass or Celluloid. and an inclined installations the tangent correction can be neg-'
mirror 222 arranged to reflect the measuring lected, as the total angular aperture o! the eye
beam on to the scale (Fig. 13). The operation-of piece is less than 1 degree of arc.
’.l'o secure best results, in all cases the plane
thisv device is similar to that of Fig. 6. ‘ The bal-.
mirror surfaces should be finished optically fiat
to la high degree of precision. In >ordinary lens
ance point is indicated by the series _of images col
lapsing and. coinciding. The optical system
and mirror practice, surfaces flat to within M4,
forms an extremely'sensitive balance indicator
for weighing apparatus.`
- where )i is the wavelength of light with which the
Fig. 14 shows a recording barograph in which
the optical system of the invention is incorpo
lens’ or mirror is used, are considered perfect;
that is, when the maximum difference in level be
rated. A light-weight ñoat 240, consisting of a _ tween the “hills and valleys" of the surface is
very thin blown glass shell filled with air under vwithin M4. it is considered unnecessary to work
for further flatness. However, in mirrors for the
suitable pressure and hermetically sealed, is sus
pended inside a cylindrical shield 2li (to protect
45 the shell from the effect of air currents and to pro
vide air damping for the moving system) from a
helical ribbon coil spring 242 arranged in a man
ner analogous to the gravlmeter spring of "Fig, 1.
A small air gap 241 is left between the float and
the shield' as shown. The spring is shown as
having 'a blmetalllc .temperature compensating
section 243. Changes in atmospheric pressure
cause the float to rise or fall, changing the length
65 of the spring accordingly. The change in length
is accompanied by twisting of the spring. The
angular deñection is not great, but the optical
indicating means are capable of accurately meas
uring it. The light source and lens combination
60 is similar to that of Fig. i2, but in lieu of a scale
there is shown a recording device comprising a
cylindrical lens system 245 adapted. in coopera
tion with lenses i3! and I3I',.to forma point
image of the lamp filament upon a moving tape
65 2&5 of sensitized film or paper. In operation. the
tape is moved at uniform speed by suitable mech
present apparatus. the surfaces should be finished
to a higher degree of accuracy, since the same
area of the surface is used several times, and
error is cumulative. For best results the surfaces
should be fiat to within x/r?. where n is the
order of reflection used. For example. using -1l)
reflections. the mirrors ideally should be fiat to
or about 0.00004,mm. for yellow sodium light (D
line). However, many of the advantages of the
invention are secured when the precision of work~
manship is much lower than this. In practice
)./10 flats are used. This degree oi'r accuracy is
advisable onlyl in the case of the surfaces between ,
which multiple reflections occur. For other sur
faces. such as the convex lens surfaces, conven
tional degrees of precision are suiiicient.
Except as otherwise indicated, the usual con
siderations applying to optical systems in gen
eral apply here. For example, the ratio of ap
anism (not shown). Variations in atmospheric- erture to local length of the lenses determines
pressure are accompanied by twistings of 'the the brightness of the reflection images; sensitiv 'N
plate 10. A family ,of wavy curves is delineated- ity is higher the higher the magnification of the
70 on the moving tape. any one of which can be ocular used. up to a 'certain limit: and the longer
the light path (i. e.. the longer the focal length
used as a basis of measurement. Should the
of the lens combination) the higher is the sen
curve used for measurement go oil the tape due to sitivity. The lenses can be corrected for aberra
a great variation in atmospheric pressure, one tions in known ways; though this is rarely nec»
of the other curves can be selected for use in
6 ‘
essary.- The light path can be bent as by the
rors each closely opposed to and facing one of*
said vmovable mirrors, and converging lens means - Y
prisms of Fig. l in known ways to llt the system ' on each side of said opposed pairs of mirrors. the
lens means being so arranged as to collimate light
- to 'different types oi apparatus.4
The apparatus in all its embodiments isreasy from the source, pass it as a beam through the 5>
l to adjust.,l even by persons of limited technical
skill. It is thus much more generally useful than
said'pairs of mirrors and focus said beam at a.v ,
focal plane after passage through vthe mirrors,
interferometer indicatingA systems and similar ' whereby 'upondeñection of said movable mech
_devices which require a considerable degree of Xanism part, multiple reflections of co
knowledge and skill for satisfactoryl operation.v - light occur between the fixed mirrorsand the >lo' y
10 Moreover the systems embodied ,in properly con
„ 'structed apparatus _are rugged and do not tend
to‘get out of adjustment. They are thus ideal
for use in portable precision apparatus su'ch as
What I claim is:
to a double series of images of the source o! light.
, 6. An optical system adapted for indicating
angular deñection of an angularly movable ap-
paratus part, comprising two plano-oonvex 15.
movable' mirrors, giving‘rise at said focal plane
1. 1n measuring apparatus having an> angu
` larly deflectable member, optical means for giv
Aing a magnified indication of such deflection.
_ comprising a pair of plane, partially transparent
lenses, a partially transparent mirror nlm on the
plane face of each lens, one of, said lenses being
fixed and one being attached to said movable ap
paratus part, the lenses being arranged with
said plane faces facing each other, a source of 2o
light and a screen, the source and the screen be»
other, one mirror being fixed and one beingfat
ing arranged so that light from the source, is
'cached to said deilectable memberv so that upon collimated by one of said lenses, is passed be- '
deñection of the deñectable member the mirror tween said mirrors as a substantially collimated
, ' attached thereto is moved out of parallelism with beam and is focused by the other lens on the 25
z5 respect to the fixed mirror, a source of light. screen.
means for directing a collimated beam of light
'1. An optical
system adapted for indicating
from said source through said mirrors in a di
angular deflection of an angularly movable mech
20 mirrors arranged substantially parallel to each
rection substantially at right angles to lche plane
_ of the mirrors, and means for bringing said col
80 limated beam to a focus at a focal plane after
its passage through the mirrors, so that upon
anism part, comprising two fixed plano-convex
lenses the plane facesof which are disposed fac- 3o.
ing each other, a partially transparent mirroion one of said plane faces, a movable optical ele
deflection of the deñectable membenvinier-re
ment positioned between said faces and having
iiectlons occur between the mirrors and a series two plane faces each of which is opposed par
of images of the source is formedat the focalA allel to said lens plane faces, a second partially 35
8l plane.
2. In- measuring apparatus having an
ly defiectable member, optical means for- giving
transparent mirror on one of said plane faces
of the optical element adjacent the ñrst mirror.
said movable element being attached to said
a magnified indication of such deilection, corn mechanism part, concentrated small-width
prising a pair of plane, partially transparent mir
means for emitting light and a screen, the light 40
40 _rors arranged closely adjacent and substantially emitting means and the screen being so arranged
parallel to each other, one mirror being ñxe'dand that light from the light»emitting means passe#
one being attached to said deñectable member for through the lenses and the movable element and
motion therewith, so that upon deflection of the is focused on the screen.
l .
ldeiiectable member the mirror attached thereto
8. The optical system of claim 1 wherein the _
45 is'moved out of parallelism with respect vto the mirrors are very thin'aluná'num alloy films o! 4"
ñxed mirror, a source of light, and a pair of con
such' thickness as to transmit a small fraction
verging lenses, one lens being on one side of the of the light impinging thereon.
mirrors and at a distance from thesourœ sub.
9. The apparatus of claim 1 wherein the source
stantlally equal to the principal focus oi' the lens, ' of light is an illuminated slit member, the slit
w so that the lens directs a collimated beam of light being narrow at one end and wide at the other 50
through the mirrors, the second lens being on to provide stray light.
the other side of the mirrors and adapted to
10. In measuring apparatus having a member
bring the collimated beam to a focus at a focal angularly deñectable about an axis. optical means
plane, so that upon deilection of the movable for giving a magnified indication of such de
55 mirror, multiple reilections occur between the ñection, comprising a pair of plane, partially 55.
mirrors and a series of images of the source ap
transparent mirrors arranged closely adjacent
pears at the focal plane.
each other, one being parallel to the axis and the
3. The optical system of claim 2 wherein the other inclined at a small angle with 'respect to
stationary mirror is carried on one of the lenses, the axis, one mirror being attached to said de
so and the other lens is attached to the deilectable flectable member and the other being ñxed, n 60.
apparatus member, and the other _mirror is car
small source of light, means for directing a col
ried on said attached lens.
limated beam of light from the source through
4. The optical system oi claim 2 wherein both said mirrors and means for focusing the beam,
lenses are stationary. and the stationary mirror after its passage through the mirrors, upon a
¿5 is carried on one of the lenses.
focal plane, so that upon deflection o! the de- 65
5. An optical system adapted to give a magni
ñed optical indicationy of angular deflection of a
mechanism part which is deilectable about an
axis. comprising narrow light-emitting means,
70 four partially transparent plane mirrors, two'of
said mirorrs being attached to the movable mech
anism part, facing on opposite sides of the said
axis and displaced from each other along the
direction oi the axis, two similar stationary mit»
flectable member an echelon series of images of
the source is formed atthe i'ocal plane.
ll. The optical system of claiml wherein the
transparency of each mirror is between 12.5 and
6.3 per cent, to secure an optimum image for 7°
the image corresponding to ten interrefiections.
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