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Патент USA US2406807

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Sept? 3, 1946. _
p. |_. coLBATH
Filed :Dec. 17, 1945
4 Sheets-Sheet 1
Dan L.- Cnlbath,
1%. W76. ‘WM
H is. Atbor'nes.
Sept. '3, 1946.
Filed Dec. 17, 1943
4 Sheets-Sheet 2
‘L. Colba’oh,
v en
His Attof‘neg.
Sept. 3, 1946.'
p. 1.. coLBATH ,
- Filved Dec; 17, 1943
4 Sheet's-Sheet 3
by :v' 62%
His Abbot-“neg;
Sept. 3,‘ 1946.
‘ D. 1.. COLBATH I
2,406,807 .
Filed Dec. 17, 1943
4 Sheets-Sheét 4 -
' Dan L..Co|bath,
Patented Sept. 3, 194-6
Dan L. Colbath, Schenectady, N. Y., assignor to;
General Electric Company,‘ a corporation of
New York
ApplicationDecember 17, 1943, Serial No. 514,591.
5 Claims;
This invention relates. to sighting apparatus,‘
more particularly to sighting. apparatus which is
especially useful. as. a. stadia range ?nder.
Stadia range. ?nders operate on the principle
that the range of a. distant. target whose actual
(Cl. 88—2>.3~)‘
In accordance with this invention, inv one em
bodiment thereof}. there. is a setof'masking plates.
including twoopaque plates-which have intersect
ing- spiral light-conductingportions. The points
of. intersection of‘ these spiral portions de?ne
stadia. range ?gures in the form of spaced dots of
lightarrangedin concentric circles, the diameters
of which may be varied by rotating the plates rel
ative to each other. One. plate. is rotated in ac
the. observer an imageof a stadia ?gure the size 1O cordance. withlmcwn target. size, while. the. other.
of which. may be adjusted,v to coincide with the
i'srotafued in. accordance with target range so that;
known dimension of the observed target. When
when the proper. stadia circle coincides with the,
coincidenceis obtained the size. of ‘the stadia?g
target,_the target’ range is automatically deter
ure bears a. ?xed relation to. the subtended angle
mined. A. third opaque plate. is superimposed
and target range,,.and‘ hence adjustable means for . upon the other two and. has a series of radial
varying. the sizeoffthe stadia ?gure can. be cali
light-conducting, portions. The. plates. are ar
brated in terms. of. target range.
ranged so that these radial ligntr-conducting por
A stadiarange?nder is known whichcomprises
tionspass thrcughbut one of. the series of points
a transparent. member through which a target is
of‘ intersection of the spiral light-conducting por
observed. A collimating light-projecting system
tionsof the other. two. plates that form. the proper
is provided. which projects an image. of a stad'ia
circular stadiarange ?gure, the remaining. points
?gure so that. it isre?ected from the transparent
of intersection. of these. spiral light-conducting
member into the observer’s line of sight where it
portions beingmaskedout. This third plate. is al
. appearsas a virtual image at an in?nite distance
waysmovedthroughafraction of the angle of di
and effectively coincident with. the observed tar 25 vergence. between the other two plates, as. they are
get. Such an optical system is described and ' rotated‘. to vary the size of. the stadia. ?gure, so
claimed in the .copending. applicationof Charles
that itsradial' linesv always-pass through the points
S. Grimshaw, Serial'No. 443,031, ?led May 15,
of intersection of. thechosen stadla ?gure.
1942, and. which application. is assigned to the as
For a. more complete understanding of this. in
30 vention, reference. should- be. had to the. accom
signee of the present application.
The Grimshaw systemv includes. a plurality of
panyingdrawings in which Fig. 1 isavertical sec
superimposed opaque. stadia. ?gure; generating
tional'v View taken. through optical. sighting ap
plates disposed between. thev sourceof light and
paratus embodying thisinvention; Fig. 2 is a hori
the re?ector and substantially in the focal’ plane
zontal sectionalview taken through the appara
of- the collimating system. These plates have a 35 tus shown. in Fig. 1;,Fig...3gis- a. front elevation of
series of. light-conducting portions arranged to
the apparatusshown in Figs. 1 and 2, parts being
produce a. luminous. stadia ?gure. formed of a
shown.v in section andpartsbroken away so as to
plurality of points of lightappearing at the inter
illustrate certaindetails of construction; Fig. 4 is
sections of. the light-conducting portions of the
an-end. elevationv illustrating parts. of the stadia
plates. The light-conducting portions of the 40 ?gure generating apparatus.‘ used, in this invene
plates have a con?guration such. that the points
tion, parts being broken away and parts shown‘
size is known may be determined. by measurement
of. theangle the target subtends at the eye ofthe
observer.v The. subtended? angle is. usually meas
uredby superimposingupon thev line of sight of
of light are arranged in. av circle, the diameter of
which may be varied by arelative rotative move.
in‘ section so as- to illustratecertain details. of con
struction, and.v the?gure. beingdrawn to a larger
mentof the plates.
‘scale- than Figs. 1, 2,. and 3; Fig. 5 is a sectional
A- plurality of sets of... cooperating masking 45 View taken: through the-line 5—5: of Fig. 4. and
platesare provided which are calibrated to deter
looking in. the:v direction of’ the‘ arrows; Fig. 6 is
mine the ranges of differentsizedtargets; that is,
an expanded‘ view illustrating: the-plates and an
a di?erent set must be used. for each different
associated member, this ?gure; being drawn to a
sized target.
larger scale than the preceding ?gures; Fig. 7 is
This invention contemplates improvements in 50 an enlarged view showing the relationship of the
the Grimshaw apparatus, including an arrange
superimposedplates whenadjusted for maximum
ment. wherein a single set of masking plates. can
target range for agiven known minimum target
be. used todeterminetheranges of targets. of any
size;v Fig. 8 illustrates the. manner in which the‘
size. between the upper. and- lower limits of. target
stadia ?gure gen‘erated‘by the plates shown in
size. for which the. device is designed;
55 Fig. 7 appears to the observer; Fig; 9' is a View
similar to Fig. '7 except that the plates are ad
justed for a shorter range; Fig. 10 is a view simi
lar to Fig. 8 except that the stadia ?gure is shown
adjusted for the shorter range when the plates are
in their positions shown in Fig. 9 ; Fig. 11 is a view
similar to Figs. '7 and 9 except that it illustrates
how the stadia circle is increased for a larger
respectively attached thereto whereby they may
be rotated. Meshing with the gear 33 is a pinion
35 mounted upon a shaft 36 which is journaled
in the casing I‘! and which projects to the ex
terior thereof, and which on its outer projecting
end carries an adjustment knob 31. The gear 34
is driven by a pinion 38 which is mounted on a
shaft as also journaled in the casing I]. This
target size at the same range as in Fig. 9; Fig. 12
shaft is driven by means of a shaft 40 journaled
is a view similar to Figs. 8 and 10 except that it
corresponds to Fig. 11; and Figs. 13 and 14 are 10 in the casing l1 and positioned at right angles
to the shaft 39, as shown. The shafts 39 and 49
diagrammatic views illustrating the principle of
are interconnected by means of ‘bevel gears 4|.
operation of the sighting apparatus.
And the shaft 40 projects from the casing, as
Referring to the drawings, there is illustrated
shown, and on its outer end carries an adjust
a stadia range ?nding apparatus arranged in ac
ment knob 42., The ring 28 which supports the
cordance with this invention and comprising a
masking plate 22 is driven from the rings 26 and
transparent re?ector Ill formed of glass or some
2'! through gears 43 and 44 formed on the two
other suitable material through which a distant
rings respectively and which mesh with a gear 45;
target may be observed by an observer whose eye
this gear 45 is mounted to rotate freely upon a
is positioned at I I. The plane of the transparent
shaft 45a which is attached to the ring 28, as
re?ector Ill is disposed at an angle to the line of
shown in Fig. 5. In the speci?c embodiment of
sight I2 so that an image of a range ?nding stadia
the invention illustrated, the gearing is such that
?gure projected along a line I3 to a collimating
the ring 28 and hence the masking plate 22 always
re?ector l4, positioned above the re?ector I0, and
moves through one-half of the angle of diver
from there back along this line 53 to the member
I0 is re?ected from the surface of the re?ector 25 gence between the rings 26 and 21, and hence be
tween the masking plates 2I and 23.
Ill into the eye of the observer where it appears
Mounted within the end plate 39 are a pair of
as a virtual image superimposed upon'the line of
sight I2.
The reflector m is adjustably mounted upon av
pair of brackets l5 supported by a‘ framework I8.
This framework also supports the collimating re
?ector Ill, as shown. In addition, it supports a
suitable light ?lter Ica pivotally mounted upon a
bracket I?b for movement to a position in the
light-collecting lenses 45b which function to col
lect the light from the lamp I8 and to project it
onto the masking plates. These lenses, as shown,
are secured in the end plate 30 by means of a
screw plug 450.
The three opaque masking plates 2!, 22 and 23
are provided with series of light-conducting por
line of sight I2, as shown in full lines in Fig. 1,
tions, which portions in the embodiment of the
to a position removed therefrom as indicated in
dotted lines in this ?gure.
invention illustrated in the drawings, are in the
form of slots cut through the plates. The plate
2% is provided with a series of radially-arranged
The unit including the re?ector IS, the colli
spiral slots 66, the plate 22 with a series of straight
mating reflector it, the frame l6 and the ?lter
Ilia, is mounted upon a casing I‘l arranged at 40 radial slots Ill, and the plate 23 with a series of
radially-arranged spiral slots 48 having the same
right angles to the unit, as shown. Mounted
shape as slots 46 but being conjugate to or re
within the right-hand end of the casing IT, as
versely arranged with reference to them. Each
viewed in Figs. 1 and 2, is a source of light I8
plate has the same number of light-conducting
which preferably will be an electric incandescent
lamp; and at the other end of the casing there 45 slots in its series, and in each, corresponding slots
are spaced alike; and moreover, the slots in the
is a reflector IQ for directing the light rays from
three series are positioned radially with reference
the lamp up along the line I3 through the trans
to circular slots 49, 58 and 5| provided in the
parent reflector Ill to the collimating re?ector I6.
three plates 2!, 22 and 23 respectively and located
Interposed between the lamp l8 and the re?ec
in their axis of rotation.
tor I9 is a stadia figure generating apparatus 26
In order to secure the desired degree of sensi
which is arranged to generate the stadia ?gure
tivity in adjustment of the diameter of a circle
at the focal plane of the collimating re?ector l4
whereby the projected image thereof appears to
formed by the intersecting points of the spiral
the observer to be at an in?nite distance.
This apparatus comprises a set of three opaque
disks or masking plates 2I, 22 and 23, and an
index plate 24, which members are shown more
slots and also for the purpose of securing sharp
intersecting points so that the light-conducting
areas will be small and well de?ned, the slots 4t‘
and 58 have a relatively small radius of curvature;
clearly in Figs. 5, 6, 7, 9 and 11. The assembly
of these plates is mounted within a supporting
ring 25 (Figs. 4 and 5). The plates 2! and 23
have a shallow pan-like shape and are mounted
in the ends’ of the supporting ring members 26
and 2? respectively, while the third plate 221s
also it is desirable to use a relatively large num
ber of slots 66 and 48 so as to provide a circular
stadia ?gure in which the dots are relatively close
mounted in a ring member 28, all as shown more
clearly in Fig. 5. The ring 26 isrmounted to rotate
in an end plate 29; the ring 21 is mounted to
rotate in the opposite end plate member 38; while
the ring 28 is mounted to rotate on the rings 26
and 21. The index plate 24 is secured to the inner
end of the end plate 30.
Because a largenumber of slots having
a small radius of curvature are used, a given
spiral slot 46 or 48 in one of the plates 2I or 23
will intersect a number of the spiral slots in the
other plate at a number of points spaced at dif
ferent radial distances from the axis 0 of the
plates, ,as shown- in Figs. '7, 9 and 11. For exam
ple, the slot 48a (Fig. 9) of the plate 23 intersects
slots 46a, 46b, 48c and 46d of the plate 2| at the
points A, B, C and D at radial distances OA, OB,
OC and OD‘ from the central axis 0.
The re
' The main supportingring 25 and the two end
plates 29 and 39 are secured together as a unit
maining spiral slots 48 of plate 23 intersect other
and this unit is securedto an inturned flange 31
mounted in the casing H by means of screws 32.
The rings 26 'and‘2‘l have spur gears 33 and 34'
therefore neglecting the effect of the third plate
22, there would be formed in the observer’s ?eld
spiral slots 46 of plate 2| in a similar way, and
of views dots lofi'li'ghtl fermingi a-tseri‘es of icon'c‘ener
that} there I will be'fproj'ected into; the "line-20f P sight?
tric iO'i'I‘01éS3aJ’0lH-1d':the centrall‘idot- of- light »'-wh'ichi*
off‘theeob'server' ?gures whichl-indicatelthe target:
si'ielmatchediiagainst' anlimage'o?lthe index‘ 53? all-"1
central 5openings»4-9,l--5?3 and 5115'” It will alsow-be?
asishow-n inlFiéss8il9‘and 10}
apparent‘that-asthe-pIatesZl- a-ndi 23 arerrotat’ed'
relatively to each other these radii? wii-l'kontihu»
ous-ly' change’; For example; the" circle-of; dots
with'aadi'u'sOC' of-lF‘igi 9;7 which" forms the ‘stadila
?gure sli0w~1rin~Fig1 1'0; wi1l~be reduced’ in ‘diam-f
etier t0 gene-rate'ethestadial-circleshown in Fig‘. 8'3
having5 a*\radius-OG"‘b'y4 moving the plates-from
their-positi'onsof-Fig: 9' tol'tHeir positions of? Ffge ‘75'
Iri- order to-avoid possibleecon-fusion offth'e'obe'
The‘ theory orop'eration: maybe ' better under: ‘
stood by referencetb-Fi'gsa Band ‘14:1 The stadiai
ringlof ‘dots'iB (Fig.‘13)‘, which"spans’theiouterztipsf
of the target, has radius p. The knownwihgspan
isD, ' Therefore:
server ‘as to which ~ circle ism vbe ‘used! fer <stadia
measurement it: is desired-that? all! ‘but the-“properv 15'
oneeofith'e ‘series-of? concentric" circles "be masked
onty‘lthap‘l-ate 2-2 does-this: Itiwill bB‘ObSGI‘VGd‘bYi
reference‘td?g: 9 that theeradial‘slot/41d- of~plate
27211'pas‘ses ‘through - the ~peint'1of'l‘interse-cti'orr G"of~'=
spiral bIOtslBdar-rdl?c, andithat likewise uitslco "-e 205
panionate radial - slots- pass through? the‘ other’
points! of? intersection of» ‘the spiral I lines <46"»and£
41? vlwhich-l de?nei the) stadi'a; circle‘ ofiFiga 10,7 but
' thatlall? other-points =01’ 7intersection between; the
New referringytoiFig.‘ 1-4; theisl-otsrd'?ii4?-'anduMi:
slotsll?» andfd'giwil-l be masked out‘; tthu-s, the-'pointsi
intersect zat=point R which lies in afcircleeofrradiusr
.Aszthe curvature of the slots ' Miranda 48 are:
equaLrthe point of'intersection P“Wi'11:00C1l1’.:ail'0ng:
an'd'the slots-45a; 45b and‘ 'li’?dwilPbe-mask‘ed "out;
a line ‘bisecting-_;tlrretv angle ‘¢r¢}:~¢z<which,makesithe
angle (91:02‘v (the:~line:O'—X¢ being: an arbitrary‘
' When-thap-lateg 2|?v and; 23* areangularlyradi 30' reference dine) .‘ Asaslotr?‘ofe diskx22~ always .bi
sectssthe. angle zqir?imewhich ~isr the: angle :of sch-=
justedwith relation to‘ each other, say‘ byr‘rotating‘
vergence betweenvdisks?2:’andr23athem '
pIateIZJI‘ counterclockwise from its position of Fig.‘
9'1 to<~its ‘position of .‘Fi’g. »'7,'v the'rpl‘ate '- 22~by>~¢vilrtue
and-‘so also will all other-unwanted points-of»? in;
of'thefact- that "it moves-through-lone-half the
angle of divergence between the p1ates-2-1-1andT233
willé'move-its-radial line ‘4 Ta to always pass-through
tlie~point of intersection (T, C’f betwee-n‘ithe-E spiral.
slots 48w and546'c1; and again-all- the points ofiinv-a
tersection between thespiral-slots-ll? and¥48iiwill
be‘- masked’ out‘ except‘ those; lying; in» the‘ circle
wh'oseeradius is/OC". In this way; only-loneicircle:
ofi dots:v appears in. the observers a?eld‘ of iview-and
its-diameter‘ may be varied byrotating the?twov
plates iii-land 23 -re1atively~1to=each otheri:
Thearrangement of plates-'shown'in Fig. 11 corr‘-' 45
responds-tea settingv forv a-‘larger targetsize; the‘
dimensions of" this target being"v twice that: satin _
Figs. 7'"- and-I91 It infill-be‘ observed that*infFig.'.11I.
the slots ABIinterseCtthe slots Ab'iand 41 at points‘
lrin‘gwon a circle whoselradiusisiOC”.
» ‘As pointed outab'ove; in-the;em-bodimentiof-the
invention-1. illustrated ' in the drawings;: the spiral?
slets Miami; 48 have'lidénticallyitlie-same curva-'-»
Moreover, ‘ they: are; logarithmic-‘spirals;
The-‘plate 23'ih‘av-ing-1 the lSlOts' 48i isvv adiustedi'byl
knob‘ 42 in accordance‘witli the ~logarithm1range ;
while ‘the? plate -2 'I having slots 46 adjusted.‘ by
knob' 3T-liin ‘accordance? with the’ logarithm" of.itar‘get ‘size. ‘ T6 assist the nb‘server in making. the; 1at=
t'erv‘adju'stment, the plate'ilwis calibrated in terms‘
of- target size in feet,» for example',:1wing' span-in
feet,. andlis providedlwith scale slots‘i 52ito indie
cateetheecalibration; Th'e?sc'ale. 52 'is' matched?
against a: ?i'z‘ed reference-index 53iin1the form ‘of
aj~triangular slot'formed 'inithe» ?xed p'lateil?;
The '- plate1~24i is providedwith raI-?'ange: 54 which
masks" out-the major portion 10f fthe scale .52; only
theiindi'cia-I-of -' this scale.‘ adj acent~the index; 5'3
being‘ exposed; this ‘range = of‘. the; scaleis exposed
byla-n‘otch‘ 55 fer-med in the ?ange-54; This-notch?
55'v aligns with elongatedislotsr'56- an'd'iST for-med;
in the plates 22‘and-l2'3 respectively, a'n'd'Lthe plate‘
2| isprovided With an elongated slot 58 which is
alignedlwith the ?xed‘index 53.
‘ which isthe desired relationship.
50 to log’of"
rOtatedJanamOunt equal
and‘ disli 23* is.rotatedfuntifpoints, P'occur atla
radius p which spans the target,._then rotatibn of‘
disk 23 is equal'to the log,K'1R;..
In. using this apparatus,- the observer ?rst‘de
termines by recognition‘of the target'tlie proper‘
set'of the plate "2 I“ to be used in the rangedet-er
min'ati'on; this;( as pointedfout previouslxg. isef
fected‘b'y; setting the knob 31.. The observer
then viewsthe target through the transparent;
member. l0 and. sees re?ected in the line of ‘sight
an‘image of a" luminous stadi’a‘ ?gure-compris
ingriaiplura'lity of vvpoints arranged in the. form of‘
.a .circle and a'luminousdotat the center of vthe
circle,_ as pointed out‘ above. This observed
imageapp'ears to be at an in?nite distance. and‘
sincei-t is superimposed upon the ‘line of .sight
by the. re?ector I4 on’ the. transparent member
l0;;it appears effectively coincidentlwithithe tar.
get'.. Thediameter of‘thi's circle may. be varied
by'rrotiatingithe knobv 42 which rotatestherange
plate2’3‘." The knob‘ 42"‘is' adjustedby 'tlie’obi' ,
In view of the foregoing, it will’ be understood 75 server until two diametrically opposite points of
the stadia circle are superimposed upon the out
ermost extremities'of a dimension of the target,
such as the wing span of the aircraft shown in
Figs. 8, 10 and 12. This movement'of the range
a luminous stadia ?gure comprising a plurality
of points‘of, light arranged in a circle is produced
at the intersections of the light conducting por
tions of said plates, the diameter of said stadia
plate automatically determines the range of the
?gure being variable by a relative rotation of said
target, and its movement may be utilized in aux
?rst two plates.
3. In an optical sighting device, a transparent
iliary equipment indicated by ‘the numeral 59
through which a target may be observed,
driven by the shaft 39.
a light projecting system for producing a virtual
While in the speci?c embodiment of, the in
vention illustrated in the drawings, the spiral 10 image of a variable sized stadia ?gure in the
lines 46 and 48 of the two plates 2| and 23 have
the same curvature, they may have different
curvatures. In case they do, the masking plate
22 would not be driven at half the speeds of the
line of sight of the observer by re?ecting an
image of said ?gureon said member, said system
comprising a source of light, a collimator, a pair
of superimposed opaque masking plates disposed
plates 2| and 23, but it would be geared to them, 15 between said source of light and said transparent
member and located in approximately-the focal
so as to be driven at some multiple which would
be satisfactory tocause its radial light-conduct
ing portions always to pass through the proper
points of intersection of the light-conducting
portions 46 and 48 of the other plates.
While I have shown a particular embodiment
of my invention, it will be understood, of course,
that I do not wish to \be limited‘ thereto since
many modi?cations may be made, and I there
fore contemplate by the appended claims to cover
any such modi?cations as fall within the true
spirit and scope of my invention.
What I claim as new and desire to secure
Letters Patent of the United States is:
plane of- said collimator, said masking plates
having series of logarithmic light-conducting
curves which are conjugate in pairs and intersect
each other at a plurality of points arranged in
concentric circles, means forrotating one of said
plates in accordance with the logarithm of the
known dimensions of said target, means for rotat
ing the other in accordance with the logarithm of
the ‘target range, a third superimposed opaque
plate having a series of radially extending light
conducting portions which intersect the curved
light-conducting portions of said ?rst two plates
through their points of intersection in, a prede
1. In an optical sighting device, a transparent 30 termined one only of said concentric circles, the
remainder of said circles being masked out by
member through which a target may be observed,
said third plate, and means for driving said third
a light-projecting system for producing a virtual
plate so that its said radial portions always bi
image of a Variable sized stadia ?gure in the line
sect the angle of divergence between said ?rst
of sight of the observer by reflecting an image of
said ?gure on said member, said system compris 35
4.; stadia-generating apparatus comprising a
ing a source of light, a collimator, a pair of
pair of superimposed opaque plates, one of said
superimposed opaque masking plates disposed
plates being provided with a series of spaced
between said source of light and said transparent
logarithmic curved‘ light-conducting portions
member and located in approximately the focal _
plane of said collimator, said masking plates 40 that are calibrated to known target size, and the
having a series of light-conducting portions ar
ranged to intersect and thereby produce a lumi
nous stadia ?gure, the proportions of which may
be varied by relative movement of said masking
plates, means for rotating one of said plates in
accordance with the known dimensions of said
target, and means for, rotating the other of said
plates to vary said proportions of said stadia
?gure to cause said stadia ?gure to coincide with
said target and thereby determine the range of .
said target.
2. In an optical sighting device, a transparent
member through which a ‘target may be observed,
a‘ light-projecting system for producing a. virtual
image of a variable sized stadia ?gure in‘ the line
of sight of the observer by re?ecting an image,‘
of said ?gure on said member, said system com
prising ‘a source of light, a collimator, a pair of
other plate being provided with an equal number
of ‘spacedlogarithmic curved portions that are
calibrated to ‘target range, the latter portions
being conjugate in pairs with the ?rst-named
series of light-conducting portions and intersect
ing, them so as to produce a series of light-con
ducting points, and means for setting said one.
plate'in accordance with known target size and
the other in accordance with target range.
5. stadia-generating
three superimposed opaque plates, two of said
plates having series of identicallyeshaped‘ loga
rithmic spiral light-conducting portions ar
ranged conjugate in pairs so that they intersect
at a' plurality of series of spaced points arranged
in concentric circles, means for rotating one of
said two plates in accordance with the logarithm
of known target size and the other plate in ac
cordance with the logarithm of target range in
superimposed opaque masking plates disposed
between said source of light and said transparent 60 order to‘ vary the diameters of said circles, the
third of, said plates being provided with straight
member and located in approximately the focal
radial - light-conducting‘ portions that pass
plane of said collimator, said masking plates hav
through the points of intersection of said spiral.
ing series of spaced curved light-conducting por- .
light-conducting portions that lie in a prede
tions, respectively, which ‘are conjugate in pairs,
termined one of said ‘concentric circles, and
a third superimposed opaque plate having a series
of radially-extending light-conducting ‘portions,
means for independently rotating each of said
?rst two plates, ‘and means 'for rotatingv said
third plate responsively to rotation of either of
said ?rst two plates so that the third plate always
takes the same predetermined angular, relation
ship to the other two plates, irrespective‘ of ‘the
angular divergence between them, said. various
light-conducting portions being arranged so that
means for operating said third plate through
one-half the angle of divergence between said two
plates so that said radial portions always pass
through said lines of intersection, of said prede
termined circle, irrespective of the adjustments
of said two'plates, the remainder of said circles
being‘m-asked out by said third plate.
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