Abstract:
A method and apparatus relate to an optical sight that includes a first section, and a second section having optics thereon and having spaced first and second portions. One aspect of the method and apparatus involves: adjustably positioning the first portion relative to the first section; and yieldably urging movement of the second portion toward a target position relative to the first section while yieldably resisting movement of the second portion away from the target position, movement of the first portion by the adjusting structure effecting movement of the second portion relative to the first section.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to optical sights and, more particularly, to optical sights that have a movable component. 
     BACKGROUND 
     Over the years, various techniques and devices have been developed to help a person accurately aim a firearm such as a rifle or a target pistol. One common approach is to mount on the barrel of the firearm a sight or scope, through which a person views the intended target in association with a reticle, often with a degree of magnification. In this regard, firearm sights sometimes include a tumbler having optics thereon, the tumbler being movable between two different positions in which the sight provides different levels of magnification of an image viewed by the person using the sight. Although existing firearm sights of this type have been generally adequate for their indented purposes, they have not been satisfactory in all respects. 
     As one aspect of this, it is common to provide one or more adjustments for the position of the movable tumbler, to effect calibration that aligns the optics on the tumbler with other optics within the sight. For example, there may be a tilt adjustment and/or an elevation adjustment for the tumbler. Existing arrangements for adjusting the position of a movable tumbler tend to involve a significant number of parts, as a result of which they are more expensive and less compact than would be desirable. 
     As another aspect, a manually-operable mechanism is normally provided to move the tumbler between its operational positions. In pre-existing mechanisms, inadvertent manual contact or a physical shock could cause the mechanism to effect an unintended release that permits the tumbler to move away from a selected position, sometimes to an intermediate position that is not a valid operational position of the tumbler and that effectively renders the sight optically non-operational. In a combat situation, it can be potentially dangerous for a soldier using the weapon if an unexpected movement of the tumbler either changes the magnification to a setting that is not currently useful, or renders the sight optically non-operational. A further consideration is that pre-existing mechanisms tend to be physically larger than desirable, and are integrated into the sight in a manner making it difficult to assemble the sight during production, and/or to disassemble and reassemble the sight if repairs are needed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic perspective view showing an apparatus that includes a conventional mounting rail typically provided on a weapon such as a firearm, and an optical sight that is supported on the mounting rail and that embodies aspects of the invention. 
         FIG. 2  is a diagrammatic exploded perspective view of the apparatus of  FIG. 1 . 
         FIG. 3  is a diagrammatic, partially-exploded perspective view of a cartridge that is a component of the optical sight of  FIG. 1 . 
         FIG. 4  is a diagrammatic central sectional top view of the cartridge of  FIG. 3 . 
         FIG. 5  is a diagrammatic exploded perspective view of a flexure assembly that is a component of the cartridge of  FIG. 3 . 
         FIG. 6  is a diagrammatic central sectional side view of a tumbler assembly that is a component of the cartridge of  FIG. 3 . 
         FIG. 7  is a diagrammatic bottom view of the cartridge of  FIG. 3 . 
         FIG. 8  is a diagrammatic central sectional top view of the cartridge that is similar to  FIG. 4 , but that shows a different operational position of the tumbler assembly. 
         FIG. 9  is a diagrammatic perspective bottom view showing in a larger scale and from a different angle a magnification control module that is a component of the optical sight of  FIGS. 1 and 2 . 
         FIG. 10  is a diagrammatic exploded perspective bottom view of the magnification control module of  FIG. 9 . 
         FIG. 11  is a diagrammatic exploded perspective top view of the magnification control module of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagrammatic perspective view showing an apparatus  10  that includes a conventional mounting rail  12  typically provided on a weapon such as a firearm, and an optical sight  11  that is supported on the mounting rail  12 , and that embodies aspects of the invention.  FIG. 2  is a diagrammatic exploded perspective view of the apparatus  10  of  FIG. 1 . The drawings and the following description do not illustrate and describe in detail all aspects of the optical sight  11 . Instead, the sight  11  is depicted and described to the extent necessary to facilitate a clear understanding of relevant aspects of the present invention. 
     With reference to  FIGS. 1 and 2 , a not-illustrated weapon has a conventional mounting rail  12  fixedly secured thereon. The rail  12  has a plurality of transversely extending slots in its upper side. The optical sight  11  includes a base  16 , and the underside of the base has one or more transverse ribs that are not visible in  FIG. 1 , but that each engage a respective one of the slots in the rail  12 . To attach the base  16  to the rail  12 , the base is moved downwardly toward the rail until the ribs on the base each engage a respective slot in the rail. Then the base  16  is releasably and fixedly secured to the rail  12  in this position by actuating a known locking mechanism that is part of the sight  11  but that is not visible in the drawings. 
     The optical sight  11  includes an optics unit  17  that is supported on the base  16  for pivotal movement about a transverse horizontal axis  22  by two coaxial pivot bolts  18  and  19 . The optics unit  17  includes a tubular outer housing  23  that has a central longitudinal opening therethrough, and that has spaced portions at one end cooperating with the pivot bolts  18  and  19  to effect the pivotal support of the optics unit  17  on the base  16 . 
     At an end of the base  16  remote from the pivot bolts  18  and  19 , an elevation adjusting mechanism  24  is mounted on top of the base  16 . The elevation adjusting mechanism  24  is conventional, and therefore described here only briefly. The elevation adjusting mechanism  24  has a horizontal plate  25  near its upper end, and has a knurled thumbwheel  26  that is disposed between the plate  25  and the base  16 , and that is supported for rotation about a vertical axis. In response to manual rotation of the thumbwheel  26 , the mechanism  24  causes the plate  25  to move vertically up or down with respect to the base  16 , in a known manner. 
     The plate  25  is fixedly secured to the underside of the outer housing  23  by four screws  27 . Thus, in response to manual rotation of the thumbwheel  26 , the plate  25  moves up or down, thereby pivoting the outer housing  23  and the optics unit  17  about the pivot bolts  18  and  19  with respect to the base  16 . This pivotal movement adjusts the orientation of the optical axis  29  of the optics unit  17  with respect to the base  16  and thus with respect to the boresight of the not-illustrated weapon having the rail  12  upon which the optical sight  11  is mounted. A person using the sight  11  to aim the weapon looks through the sight along the optical axis  29 , in a direction from right to left in  FIGS. 1 and 2 . 
     A manually-operable knob  36  is supported on one side of the outer housing  23  of the optics unit  17  for rotation about a horizontal axis. The knob  36  controls some electrical circuitry provided within the optics unit  17 , for example to control the intensity of illumination provided to a reticle. The details of this electrical circuitry are not necessary to an understanding of the present invention, and the electrical circuitry is therefore not shown and described here in detail. 
     A magnification control module  41  is fixedly and detachably secured to the underside of the outer housing  23  by four screws  42 . The module  41  has a movable lever  43  that can be manually operated. Manual movement of the lever  43  changes the degree of magnification provided by optics within the sight  11 , in a manner discussed in more detail later. In the disclosed embodiment, reciprocal movement of the lever  43  changes the degree of magnification between 1× and 4×. However, it would alternatively be possible to use other degrees of magnification, or to adjust an optical characteristic other than magnification. The structure and operation of the magnification control module  41  are discussed in more detail later. 
     An eyepiece optics unit  46  is removably mounted within the outer housing  23  at a rear end thereof nearest the elevation control mechanism  24 . The eyepiece optics unit  46  includes a known configuration of eyepiece lenses, one of which is visible at  47  in  FIGS. 1 and 2 . 
     As best seen in  FIG. 2 , the optics unit  17  includes a cartridge  51  that is removably installed within the outer housing  23 .  FIG. 3  is a diagrammatic, partially-exploded perspective view of the cartridge  51 .  FIG. 4  is a diagrammatic central sectional top view of the cartridge  51 . The cartridge  51  includes a cartridge frame  53 , and an object optics unit  54  that is detachably coupled to one end of the frame  53  by a commercially-available adhesive. As best seen in  FIG. 4 , the object optics unit  54  includes an object lens doublet  55  and  56 . An intermediate portion of the frame  53  supports a further lens  59 . The optical axis  29  extends through each of the lenses  55 ,  56  and  59 . 
     Three prisms  61 ,  62  and  63  are fixedly supported on the cartridge frame  53 , at an end thereof remote from the object optics unit  54 . Radiation traveling along the optical axis  29  is successively reflected at five prism surfaces  71 ,  72 ,  73 ,  74  and  75  that each have thereon a reflective coating that is not separately illustrated. A subassembly  78  is supported adjacent the prism surface  75 , and superimposes an illuminated reticle onto radiation that is passing through the prism  63  of the optical sight along the optical axis  29 . In the disclosed embodiment, the prisms  61 - 63  and the subassembly  78  have a configuration that is known in the art, and that is disclosed in U.S. Patent Application Publication No. 2005/0200965. The prisms  61 - 63  and subassembly  78  are therefore not described in further detail here. Alternatively, however, it would be possible to use some other suitable optical configuration in place of the prisms  61 - 63  and the subassembly  78 . 
     With reference to  FIG. 3 , the cartridge frame  53  has a horizontal top wall with an approximately square hole  84  extending vertically therethrough. A flexure assembly  85  is disposed within the hole  84 , and is fixedly secured there by four screws  86  that engage respective threaded openings provided in the cartridge frame  53 . 
       FIG. 5  is a diagrammatic exploded perspective view of the flexure assembly  85 . The flexure assembly  85  includes a flexure  91  that is a platelike piece of resilient material. In the disclosed embodiment, the flexure  91  is stamped from a plate of spring steel, or from a plate of stainless steel. However, it could alternatively be made from any other suitable material. The flexure  91  has a central portion or base  92  that has an approximately square perimeter, and that has a cylindrical opening  93  extending vertically through the center thereof. The flexure  91  also includes four U-shaped flexible arms  101 - 104  that are each fixedly coupled at one end to a respective corner of the square base  92 , and that each effectively function as a leaf spring. The arms  101 - 104  are all identical, and therefore only the arm  101  is described here in detail. 
     More specifically, the U-shaped arm  101  has a bight  111 , and two legs  112  and  113  that extend outwardly from opposite ends of the bight. The legs  112  and  113  extend approximately parallel to each other, and each extend approximately parallel to the nearest edge of the square base  92 . The leg  112  is fixedly coupled at its outer end to one corner of the square base  92 . The outer end of the other leg  113  has a slot  114 , and one of the screws  86  ( FIG. 3 ) extends through the slot  114  in order to fixedly secure the outer end of leg  113  to the cartridge frame  53 . Although the peripheral edge of the base  92  defines approximately a square in the disclosed embodiment, it could alternatively be a different regular polygon with a larger or smaller number of sides, and with a correspondingly larger or smaller number of U-shaped arms each coupled to a respective corner. As still another alternative, the base  92  could have a peripheral edge with a shape other than that of a regular polygon, and/or the arms  101 - 104  could each have a different shape, provided the arms have an appropriate degree of flexibility. 
     The flexure assembly  85  also includes a ring  117  with an annular flange  118  projecting upwardly from a top surface  119  thereof. A cylindrical opening  121  extends vertically through the center of the ring  117 . The annular flange  118  has an outside diameter approximately equal to the inside diameter of the opening  93  in the flexure  91 . The opening  93  receives the annular flange  118  with a friction fit, such that the top surface  119  of the ring  117  engages a bottom surface of the base  92  of the flexure  91 . After the annular flange  118  has been press-fit into the opening  93 , several angularly-spaced crimps  122  are created in the upper, outer edge of the annular flange  118 , in order to prevent separation of the flexure  91  and ring  117 . 
     The flexure assembly  85  also includes a ball bearing  126 . Although not shown in detail in the drawings, the ball bearing  126  has a standard internal construction that includes two concentric annular races with a plurality of spherical balls between them. The ball bearing  126  has an outside diameter that is approximately equal to the inside diameter of the cylindrical opening  121  through the ring  117 , and the ball bearing  126  is snugly press-fit within the opening  121 . The ring  117  thus serves as a holder for the ball bearing  126 . The ball bearing  126  has a radially outwardly projecting annular flange  128  at the lower end thereof, which engages a bottom surface of the ring  117  and prevents the ball bearing  126  from moving upwardly relative to the ring  117 . The ball bearing  126  has a cylindrical opening  127  extending vertically therethrough. 
     Referring again to  FIG. 3 , the cartridge frame  53  has a bottom wall with an approximately square opening  136  extending vertically therethrough, in alignment with the hole  84 . An approximately square adjusting plate  141  is disposed within the opening  136 . In a direction parallel to optical axis  29 , the plate  141  has a length that is somewhat shorter than the length of the opening  136  in that direction, so that the position of the plate within the opening can be adjusted in directions parallel to the optical axis. The plate  141  has a width that is slightly smaller than the width of the opening  136 , so that the plate is held against any significant movement within the opening  136  in directions transverse to the optical axis  29 . The plate  141  has a threaded cylindrical opening  142  extending vertically through the center thereof. 
     The four corners of the plate  141  have respective slots  146 - 149  that each extend parallel to the optical axis  29 . Four screws  152  each extend through a respective one of the slots  146 - 149 , and each engage a respective threaded opening provided in the cartridge frame  53 . When the screws  152  are not fully tightened, the plate  141  is capable of limited sliding movement within the opening  136  in directions parallel through the optical axis  29 , while the screws  152  slide within the slots  146 - 149 . The screws  152  can be tightened in order to fixedly secure the plate  141  in any selected position with respect to the frame  53 . 
     As shown in  FIG. 3 , the cartridge  51  includes a movable tumbler assembly  161 .  FIG. 6  is a diagrammatic central sectional side view of the tumbler assembly  161 . With reference  FIGS. 3 and 6 , the tumbler assembly  161  includes a generally cylindrical housing  163  that is approximately concentric to a horizontal axis  164 . A roughly cylindrical opening  166  extends lengthwise through the housing  163 , approximately concentric to the axis  164 . An opening  167  of roughly frustoconical shape extends transversely through the housing  163 , approximately concentric to a further horizontal axis  168  that is perpendicular to and intersects the horizontal axis  164 . 
     A cylindrical projection  171  extends upwardly from the top of the housing  163 , and a further cylindrical projection  172  extends downwardly from the bottom of the housing. The cylindrical projections  171  and  172  are each coaxial with a vertical axis  173  that extends through the intersection of the horizontal axes  167  and  168 . An opening or recess  174  with a square cross section extends upwardly into the cylindrical projection  172  from the bottom surface thereof. At its upper end, the recess  174  communicates with the openings  166  and  177  that extend through the housing  163 . The vertical axis  173  extends approximately centrally through the recess  174 . 
     With reference to  FIG. 6 , a lens doublet  181 - 182  is supported at one end of the housing  163  within the opening  166 , and a further lens doublet  183 - 184  is supported at the opposite end of the housing  163  within the opening  166 . The lenses  181 - 184  have a common optical axis that is coincident with the axis  164 . A cap  187  is held with a snap fit on the housing  163  at the end thereof nearest the lens doublet  181 - 182 , and has a circular opening  188  extending centrally and axially therethrough. 
     Still referring to  FIG. 6 , the tumbler assembly  161  includes a ball bearing  192  that, in the disclosed embodiment, is identical to the ball bearing  126  ( FIG. 5 ). The ball bearing  192  has a radially-outwardly projecting annular flange  193  at its upper end. The ball bearing  192  has a central cylindrical opening  194  extending vertically therethrough, and the projection  172  of the tumbler housing  163  is press fit within the opening  194  through the ball bearing. 
     The tumbler assembly  161  further includes an adjusting ring  201  that has a central cylindrical opening  202  extending vertically therethrough. The ball bearing  192  is press fit within the opening  202 , and the flange  193  prevents downward movement of the ball bearing relative to the adjusting ring  201 . With reference to  FIG. 3 , the adjusting ring  201  is rotatably received within the opening  142  in the adjusting plate  141 . The adjusting ring  201  has a radially-outwardly facing annular side surface  203  with threads thereon that engage the threads in the threaded opening  142 .  FIG. 7  is a diagrammatic bottom view of the cartridge  51 . As best seen in  FIG. 7 , the adjusting ring  201  has two vertical holes  207  and  208  therein on diametrically opposite sides thereof. The holes  207  and  208  each extend vertically upwardly from the bottom of the ring  201 . A not-illustrated tool has two spaced prongs that can each be inserted into a respective one of the holes  207  and  208 , and the tool can then be used to rotate the adjusting ring  201  relative to the adjusting plate  141 . Due to the cooperating threads on the ring  201  and plate  141 , rotation of the ring within the plate causes the ring to move vertically upwardly or downwardly with respect to the plate, thereby adjusting the vertical position of the entire tumbler assembly  161  within the cartridge frame  53 . 
     The cylindrical projection  171  on top of the tumbler housing  163  is press fit within the cylindrical opening  127  ( FIG. 5 ) in the ball bearing  126  of the flexure assembly  85 . When the adjusting ring  201  is rotated within the adjusting plate  141  in order to cause the tumbler assembly  161  to move upwardly or downwardly, the flexible arms  101 - 104  of the flexure  91  resiliently flex to accommodate this movement, while also yieldably urging the tumbler assembly  161  to move back toward a neutral or target position in which the arms of the flexure are not tensioned. 
     As also discussed above, if the screws  152  are loosened slightly, the adjusting plate  141  ( FIGS. 3 and 7 ) is capable of a limited amount of movement relative to the cartridge frame  53  in directions parallel to the optical axis  29 . This movement of the adjusting plate  141  moves the cylindrical projection  172  at the bottom of the tumbler assembly  161  a small distance forwardly or rearwardly with respect to the cartridge frame  53 , while the projection  171  at the top of the tumbler housing stays in approximately the same place. Thus, the entire tumbler assembly  161  can be tilted about a transverse horizontal axis  216  ( FIGS. 3 and 6 ) that extends through the projection  171 . The arms  101 - 104  of the flexure  91  resiliently flex to the extent necessary to accommodate this pivotal movement about axis  216 , while resiliently urging the tumbler assembly  161  to pivot back to the neutral position in which the flexure  91  is not tensioned. 
     Assisted by the ball bearings  126  and  192  ( FIGS. 5 and 6 ), the tumbler housing  163  with the lenses  181 - 184  therein can pivot about the vertical axis  173  ( FIG. 6 ) through a range of approximately 90° with respect to the cartridge frame  53 , the flexure assembly  85 , and the adjusting plate  141 .  FIG. 4  shows one operational position of the tumbler assembly  161  relative to the cartridge frame  53 . The cartridge frame  53  has a side wall with a threaded opening  221  therethrough, and a limit setscrew  223  is disposed in the opening  221 . A stop plate  224  made of plastic or some other suitable material is fixedly adhesively secured to a side surface of the tumbler housing  163 , and engages the inner end of the setscrew  222  in the operational position of the tumbler assembly  161  shown in  FIG. 4 . 
     When the tumbler assembly  161  is in the operational position of  FIG. 4 , the optical axis  164  ( FIGS. 3 and 6 ) of the lenses  181 - 184  in the tumbler assembly  161  should be coincident with the optical axis  29  of the sight. In order to achieve this alignment, the adjusting ring  201  ( FIGS. 3 and 6 ) can be rotated in order to move the entire tumbler assembly  161  up and down, and the adjusting plate  141  can be moved forward or backward to adjust the tilt of the tumbler assembly  161  about axis  216 . Further, the setscrew  222  ( FIG. 4 ) can be rotated to adjust the pivotal position of the tumbler assembly  161  about the vertical axis  173 . In response to rotation of the ring  201  or movement of the adjusting plate  141 , the flexure  91  resiliently flexes to the extent necessary to accommodate the adjustment. The flexure  91  is extremely compact, but resiliently accommodates several different degrees of freedom in the adjusting movement of the tumbler assembly  161 . These adjustments of the tumbler assembly  161  are typically made at the factory during assembly and test of the sight  11 , and should not normally need to be adjusted again later in the field. 
       FIG. 8  is a diagrammatic central sectional top view of the cartridge  51  that is similar to  FIG. 4 , but that shows a different operational position of the tumbler assembly  161 . More specifically, in  FIG. 8 , the tumbler assembly  161  has been rotated approximately 90° clockwise from the position shown in  FIG. 4 , until a surface  231  on the tumbler housing  163  engages a limit surface  232  on the cartridge frame  53 . In this operational position of the tumbler assembly  161 , the optical axis  29  extends through the frustoconical opening  167  in the tumbler housing  163 , and the lenses  181 - 184  in the housing  163  are all spaced radially from the optical axis  29 , so that radiation traveling along the optical axis  29  does not pass through any of the lenses  181 - 184 . 
     In the operational position of the tumbler assembly  161  that is shown in  FIG. 4 , the lenses  181 - 184  of the tumbler assembly cause a 1× magnification of radiation traveling along the optical axis  29 . In contrast, in the operational position shown in  FIG. 8 , radiation does not pass through the lenses  181 - 184 , and thus the tumbler assembly exerts no influence on the magnification imparted to the radiation. In this position of the tumbler assembly  163 , the sight  11  provides an effective magnification of 4×. 
     Referring again to  FIG. 2 , the magnification control module  41  will now be described in more detail. The magnification control module  41  includes an upwardly projecting protrusion  251  of square cross section that is rotatable about a vertical axis. The protrusion  251  is slidably removably received in the square recess  174  ( FIG. 6 ) provided in the cylindrical projection  172  of the tumbler housing  163 . The square cross-sectional size of the protrusion  251  is slightly smaller than the square cross-sectional size of the recess  174 , in order to prevent significant relative rotation therebetween, while accommodating the small amount of forward or rearward movement of the projection  172  on the tumbler housing  163  that can result from forward or rearward adjustment of the adjusting plate  141  ( FIG. 3 ). When the lever  43  is manually moved, it pivots the protrusion  251  in a manner described in more detail later. Due to the cooperating square cross sections of the protrusion  251  and the recess  174 , pivotal movement of the protrusion  251  causes the tumbler assembly  161  to pivot between the two operational positions that are respectively shown in  FIGS. 4 and 8 . 
       FIG. 9  is a diagrammatic perspective bottom view of the magnification control module  41  of  FIG. 2 , showing this module in an enlarged scale, and from a different angle.  FIG. 10  is a diagrammatic exploded perspective bottom view of the module  41 , and  FIG. 11  is a diagrammatic exploded perspective top view of the module  41 . With reference to  FIGS. 9-11 , the module  41  includes a cover  256  of approximately square shape, with an opening  257  extending vertically therethrough near each corner. The screws  42  that then secure the module  41  to the outer housing  23  each extend through a respective one of the openings  257 . A shallow and approximately square recess  258  opens downwardly into the cover  256  from the top side thereof. Four vertical posts  261  are provided at spaced locations within the recess  258 , and a threaded opening  262  extends vertically downwardly into each of the posts  261 . 
     A cylindrical opening  266  extends completely through the cover  256  at the center thereof, and thus communicates at its upper end with the recess  258 . The cylindrical opening  266  has two portions of different diameter, with an axially-upwardly facing annular shoulder therebetween. A threaded vent opening  267  also extends completely vertically through the cover  256 , at a location spaced radially outwardly from the opening  266 . A projection  271  extends downwardly from the underside of the cover  256 , and a blind cylindrical opening  272  extends vertically upwardly into the projection  271 . A transverse cylindrical opening  273  extends horizontally into the cover  256  from a side surface thereof, and communicates with the upper end of the blind opening  272 . An arcuate flange  276  projects downwardly from the underside of the cover  256 , and is approximately concentric to the opening  266 . The flange  276  has opposite end surfaces  277  and  278  that are inclined so they converge upwardly. A resiliently flexible O-ring  282  is provided between the cover  256  and the outer housing  23  ( FIG. 2 ), in order to facilitate a fluid seal therebetween. The O-ring  282  is made from rubber or plastic, or from any other suitable material. 
     A spring holder  284  has a platelike sector-shaped main portion  285 , a cylindrical projection  287  that extends downwardly from the main portion  285 , a square post  288  that extends downwardly from the lower end of the projection  287 , and a threaded hole  289  that extends upwardly into the post  288  from the lower end thereof. A bushing  291  and a resiliently flexibly O-ring  292  are each disposed within the opening  266  through the cover  256 , with the O-ring against the annular shoulder, and the bushing just above the O-ring. The cylindrical projection  287  on the spring holder  284  extends through the bushing  291  and the O-ring  292 , and is rotatable with respect to the cover  256  about a vertical pivot axis  293  that is coaxial with the opening  266  and the cylindrical projection  287 . The O-ring  292  facilitates a fluid seal. The spring holder  284  has a cylindrical recess  296  in the upper side of the main portion  285 , concentric to the pivot axis  293 . The spring holder  284  also has an arcuate recess or grove  297  in the upper side of the main portion  285 , concentric to the axis  293  but spaced radially outwardly from the recess  296 . 
     A lever mount  301  has an upwardly-open square hole  302  that slidably and non-rotatably receives the square post  288  of the spring holder  284 . A screw  303  extends through an opening in the lever mount  301  and engages the threaded hole  289  in the square post  288 , in order to fixedly secure the lever mount  301  to the spring holder  284 . The lever mount  301  has an upwardly-open cylindrical recess  306  therein at a location spaced radially outwardly from the square hole  302 . 
     The lever  43  has a bifurcated inner end with two spaced portions  308  and  309  that are disposed on opposite sides of the lever mount  306 . The spaced portions  308  and  309  each have a cylindrical horizontal hole therethrough with a bushing  311  or  312  disposed therein. Screws  313  and  314  extend through the respective bushings  311  and  312 , and engage threaded openings provided in opposite sides of the lever mount  301 , thereby supporting the lever  403  for limited pivotal movement on the lever mount  301 . The lever  43  has a downwardly open cylindrical recess  316  ( FIG. 10 ). A helical coil spring  317  has its lower end disposed in the recess  306 , and has its upper end disposed in the recess  316 . The spring  317  urges the lever  43  to pivot upwardly about the lever mount  301 . The cover  256  has two downwardly-projecting posts  318  and  319  that are positioned to engage the lever  43  in order to limit pivotal movement of the lever about the vertical axis  293 . 
     A bushing  323  is disposed within the cylindrical recess  296  in the spring holder  284 . Two coil springs  321  and  322  are each disposed within the arcuate recess  297  in the spring holder  384 , in an end-to-end relationship. 
     A drive part  324  includes a sector-shaped plate  326 , and a cylindrical projection  327  that extends downwardly from the underside of the plate  326 , coaxial with the pivot axis  293 . The cylindrical projection  327  is rotatably received within the bushing  323 . The plate  326  has, at a location spaced radially outwardly from the projection  327 , a downwardly projecting tab  328  that is disposed within the arcuate recess  297  of the spring holder  284 , between the adjacent ends of the two coil springs  321  and  322 . The plate  326  has a small opening  329  extending vertically therethrough near a radially outer edge. The drive part  324  has a cylindrical projection  331  that extends upwardly from the plate  326 , coaxial with the projection  327  and the pivot axis  293 . The drive part also includes the previously-mentioned square protrusion  251 , which projects upwardly from the cylindrical projection  331 . 
     A coil spring  336  has one end  337  that is bent to extend downwardly, and an opposite end  338  that is bent to extend upwardly. The end  337  is received in the small opening  329  in the plate  326  of the drive part  324 . An X-shaped retainer  341  has a cylindrical opening  342  extending vertically through the center thereof. A bushing  343  is disposed within the opening  342  with a press fit, and the cylindrical projection  331  on the drive part  324  is rotatably received within the bushing  343 . The retainer  341  has a short cylindrical projection  344  on the underside thereof, and the coil spring  336  encircles the projection  344 . The retainer  341  has a small vertical opening  346  near the projection  344 , and the end portion  338  of the coil spring  336  is received within the opening  346 . The retainer  341  has four legs that project radially outwardly in different directions, and the outer end of each leg has an opening  348  extending vertically therethrough. Four screws  349  each extend through a respective one of the openings  348 , and engage the threaded opening  262  provided in a respective one of the posts  261  of the cover  256 . 
     The coil springs  321  and  322  resiliently urge the tab  328  toward the center of the arcuate slot  297 , and are sufficiently strong to keep the tab centered within the slot, except under certain specific conditions that are described later. The coil spring  336  continuously urges the drive part  324  and thus the spring holder  284  and lever  43  toward an operational position in which the tumbler assembly  161  is in the 4× position shown in  FIG. 8 . Alternatively, however, it would be possible for the coil spring  336  to urge rotation of the drive part  324  and thus the spring holder  284  and lever  43  in the opposite direction, toward an operational position in which the tumbler assembly  161  is in the 1× position shown in  FIG. 4 . 
     Approximately halfway between its ends, the lever  43  has inclined surfaces  356  and  357  that are on opposite sides thereof, and that can respectively engage the inclined surfaces  277  and  278  provided on the arcuate flange  276  of the cover  256 . A helical spring  361  has its upper end disposed within the blind opening  272 . A cylindrical pin  362  is disposed in the opening  273 , and cooperates with the upper end of the spring  361  in order to retain the spring in the opening  272 . The lower end of the spring  361  is fixedly coupled to the base  16  ( FIGS. 1-2 ) by another pin that is not visible in the drawings. The spring  361  is an expansion spring that resiliently pulls the module  41  downwardly toward the base  16 , and thus resiliently urges the optics unit  17  to pivot downwardly about the pivot axis  22  relative to the base  16 . 
     A screw  366  extends through an annular gasket  367 , and engages the threaded opening  267  provided through the cover  256 . After the optics unit  17  has been assembled, the opening  267  can be used to fill the interior of the optics unit  17  with dry air or an inert gas, in order to remove moist air that could potentially cause undesirable condensation on optical parts in cold temperatures. After the interior of the optics unit  17  has been filled with dry air, the screw  366  and gasket  367  are used to seal the opening  267 . 
       FIG. 9  shows the lever  43  in a position corresponding to the 4× position of the tumbler assembly  161  ( FIG. 8 ). To move the tumbler assembly  161  to the 1× position ( FIG. 4 ), the lever  43  is first moved slightly counterclockwise in  FIG. 9  about the axis  293 , in order to separate the inclined surfaces  277  and  357 . Then the lever  43  is pulled downwardly a short distance against the urging of the spring  317 , and is pivoted clockwise in  FIG. 9  about the axis  293 . During this clockwise movement, the coil springs  321  and  322  will keep the tab  328  centered within the arcuate recess  297 , so that there is no relative rotational movement between the spring holder  284  and the drive part  324 . The lever  43  thus rotates the spring holder  284 , which in turn through the springs  321 - 322  and tab  328  rotate the drive part  324 , and the square protrusion  251  on the drive part rotates the tumbler assembly  161  from the 4× position of  FIG. 8  toward the 1× position of  FIG. 4 . This movement occurs against the urging of the coil spring  336 , and serves to increase the tension in the coil spring  336 . 
     Before the lever  43  passes the right end of the arcuate flange  276  in  FIG. 9 , the stop plate  224  of the tumbler assembly of  161  engages the setscrew  222  ( FIG. 4 ), thereby stopping pivotal movement of the tumbler assembly in the desired position, and preventing the tumbler assembly from moving past this position. Since the drive part  324  is directly coupled to the tumbler assembly  161 , the drive part is also held against further movement. As the lever  43  is then manually moved slightly further in the clockwise direction in  FIG. 9 , the spring holder  284  moves even though the drive part  324  cannot move, and so the coil spring  321  is compressed between the tab  328  and an end of the arcuate recess  297 . Once the lever  43  is beyond the end of the flange  276 , the lever is manually moved upwardly, assisted by the coil spring  317 , and then is released. The urging of the coil spring  321  moves the lever slightly counterclockwise in  FIG. 9  so that the inclined surfaces  357  and  278  come into engagement. The lever  43  and spring holder  284  are still about 3° to 5° past the position in which the drive part  324  and tumbler assembly  161  stopped moving, and so the coil spring  321  is still somewhat compressed, and continues to urge the lever  43  to pivot counterclockwise, thereby urging the inclined surfaces  357  and  278  against each other. 
     The inclination of the surfaces  357  and  278  is such that, when they are urged together by the spring  321 , they resist downward movement of the lever  43 . The coil spring  317  also resists downward movement of the lever  43 . Consequently, the lever  43  is reliably held against movement out of this position, and the optics are reliably maintained in the selected position. If for some reason the lever  43  is inadvertently bumped with sufficient force to move it downwardly until the surfaces  357  and  278  becomes disengaged, the coil spring  336  will automatically pivot the lever  43  counterclockwise in  FIG. 9  to its original position, so that the tumbler assembly  161  is returned to its 4× position of  FIG. 8 . Of course, as mentioned earlier, it would alternatively be possible for the coil spring  336  to urge rotation of the drive part  324  and thus the spring holder  284  and lever  43  in the opposite direction, toward an operational position in which the tumbler assembly  161  is in the 1× position shown in  FIG. 4 . 
     Assume for sake of discussion that the lever  43  is not bumped so as to cause an inadvertent release. In order to return the optics from the 1× position to the 4× position, the lever  43  is manually pulled clockwise a short distance about the axis  293  in  FIG. 9  to separate the inclined surfaces  357  and  278 , which slightly further compresses the spring  321 . Then the lever  43  is pulled downwardly a small distance against the urging of the spring  317  until the lever is capable of pivoting counterclockwise without engaging the flange  276 . The lever  43  is then manually pivoted counterclockwise in  FIG. 9  about the axis  293 , assisted by the urging of the coil spring  336 . During this movement, before the lever  43  passes the opposite end of the flange  276 , the surface  231  ( FIG. 8 ) on the tumbler housing  163  will engage the limit surface  232  on the cartridge frame  53 , thereby preventing further pivotal movement of the tumbler assembly  161  past the 4× position shown in  FIG. 8 , which in turn prevents further pivotal movement of the drive part  324 . As the lever  43  is then manually pivoted further in the clockwise direction in  FIG. 9 , the drive part  324  cannot move but the spring holder  284  will move, thereby compressing the coil spring  322  between the tab  328  and an end of the arcuate recess  297 . The lever  43  is then moved upwardly and released, and the coil spring  322  moves the lever  43  a short distance in the clockwise direction in  FIG. 9  until the inclined surfaces  356  and  277  are in engagement. The lever  43  and spring holder  284  are still about 3° to 5° past the position in which the drive part  324  and tumbler assembly  161  stopped moving, and so the coil spring  322  is still somewhat compressed, and continues to urge the lever  43  to pivot clockwise, thereby urging the inclined surfaces  356  and  277  against each other. The inclination of the surfaces  356  and  277  is such that, when they are urged together by the coil spring  322 , they resist downward movement of the lever  43 . The coil spring  317  also yieldably resists downward movement of the lever  42 . 
     Although a selected embodiment has been illustrated and described in detail, it should be understood that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the claims that follow.