Abstract:
A microscope stage ( 14 ) comprising a platform ( 16 ), a specimen holder ( 18 ) resting on the platform ( 16 ) and a positioning device ( 20 ) for moving the specimen holder ( 18 ) in a plane of displacement parallel to the platform ( 16 ) is described. The microscope stage ( 14 ) includes a positioning device ( 20 ) having two displacing devices ( 34, 36 ) which are mechanically decoupled from each other and of which a first displacing device ( 34 ) is designed to move the specimen holder ( 18 ) along a first axis in the plane of displacement, and a second displacing device ( 36 ) is designed to move the specimen holder ( 18 ) along a second axis in the plane of displacement, which second axis runs transversely to the first axis.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of German patent application number 10 2010 061 167.0 filed Dec. 10, 2010, the entire disclosure of which is incorporated by reference herein. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to a microscope stage. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the recent past, light microscopic methods have been developed with which, based on a sequential, stochastic localization of individual point objects, in particular fluorescence molecules, image structures can be imaged that are smaller than the diffraction-dependent resolution limit of conventional light microscopes. Such methods are, for example, described in WO 2006/127692 A2; DE 10 2006 021 317 B3; WO 2007/128434 A1, US 2009/0134342 A1; DE 10 2008 024 568 A1; “Sub-diffraction-limit imaging by stochastic optical reconstruction microscopy (STORM)”, Nature Methods 3, 793-796 ( 2006 ), M. J. Rust, M. Bates, X. Zhuang; “Resolution of Lambda/10 in fluorescence microscopy using fast single molecule photo-switching”, Geisler C. et al, Appl. Phys. A, 88, 223-226 (2007). This new branch of microscopy is also referred to as localization microscopy. The applied methods are known in the literature, for example, under the designations (F)PALM ((Fluorescence) Photoactivation Localization Microscopy), PALMIRA (PALM with Independently Running Acquisition), GSD(IM) (Ground State Depletion (Individual Molecule return) Microscopy) or (F)STORM ((Fluorescence) Stochastic Optical Reconstruction Microscopy). 
         [0004]    The new methods have in common that the structures to be imaged are prepared with markers that have two distinguishable states, namely a “bright” state and a “dark” state. When, for example, fluorescent dyes are used as markers, then the bright state is a state in which they are able to fluoresce and the dark state is a state in which they are not able to fluoresce. For imaging image structures with a resolution that is higher than the conventional resolution limit of the imaging optical system, a small subset of the markers is repeatedly brought into the bright state and thus it is so to speak activated. In this connection, the activated subset is to be chosen such that the average distance of adjacent markers in the bright state is greater than the resolution limit of the imaging optical system. The luminance signals of the activated subset are imaged onto a spatially resolving light detector, e.g. a CCD camera. Thus, of each marker a light spot is detected whose size is determined by the resolution limit of the imaging optical system. 
         [0005]    In this way, a plurality of raw data single frames is captured, in each of which a different activated subset is imaged. Using an image analysis process, then in each raw data single frame the centroids of the light spots are determined which represent those markers that are in the bright state. Thereafter, the centroids of the light spots determined from the raw data single frames are combined to a total representation. The high-resolution image created from this total representation reflects the distribution of the markers. For a representative reproduction of the structure to be imaged sufficient signals have to be detected. Since however the number of markers in the respective activated subset is limited by the minimum average distance which two markers may have in the bright state, a great many raw data single frames have to be captured to completely image the structure. Typically, the number of raw data single frames is in a range between 10,000 and 100,000. 
         [0006]    The time required for capturing one raw data single frame has a lower limit that is predetermined by the maximum image capturing rate of the imaging detector. This results in relatively long total capturing times for a series of raw data single frames required for the total representation. Thus, the total capturing time can take up to several hours. 
         [0007]    Over this long total capturing time, a movement of the specimen to be imaged relative to the imaging optical system may occur. Since for creating a high-resolution total image all raw data single frames are combined after the determination of the centroids, each relative movement between specimen and imaging optical system that occurs during the capturing of two successive raw data singles frames impairs the spatial resolution of the total image. In many cases, this relative movement results from a systematic mechanical movement of the system, also referred to as mechanical drift which is caused, for example, by thermal expansion or shrinkage, by mechanical strains or by the change in the consistency of lubricants used in the mechanical components. 
         [0008]    In the above-described high-resolution methods it is of particular importance to provide for a drift-free positioning of the specimen on a microscope stage. In the prior art, often so-called mechanical stages (X-Y stages) are used for this purposes, which allow to move a specimen holder on a platform in two orthogonal directions (in the following also referred to as X and Y direction) in a plane of displacement (X-Y-plane) that is parallel to the platform. Such a mechanical stage consists of two superposed plates which are mechanically coupled to each other as well as a drive for moving the two plates against each other. Thus, in the case of a mechanical stage the movements of the specimen holder in the direction of the X-axis and the Y-axis are coupled to each other. 
         [0009]    By means of such a microscope stage it is basically possible to position the specimen holder on the microscope stage in an easy and precise manner. However, due to the mechanical coupling of the components forming the mechanical stage a mechanical drift occurring in one of these components as a result of thermal influences or mechanical strains existing in the drive also has an effect on the respective other components. In high-resolution light microscopic methods in which long total capturing times are intended and resolutions in the nanometer range are aimed at, this may result in intolerable image shifts. 
         [0010]    From DE 695 30 095 T2, a planar positioning stage is known having a platform upon which a workpiece to be positioned is placed. The positioning stage further comprises a first and a second actuator means which are adapted to move the workpiece in an actuating plane along a first axis or along a second axis that runs transversely to the first axis. 
         [0011]    In DE 35 14 431 A1 a microscope stage drive is described which comprises two carriages which are movable orthogonally to each other, and coaxially mounted drive members for moving the carriages. The force transmission between the drive members and the carriages takes place via traction means running over deflection rollers. 
         [0012]    From DE 19 38 771 A, a precision drive for two carriages is known which can be guided and displaced orthogonally to each other, of which a first carriage is mounted in a stationary guide and a second carriage is mounted in a guide provided on the first carriage. 
         [0013]    With respect to the prior art, reference is further made to JP 2005 091 866 A, U.S. Pat. No. 5,000,554 A, JP 58-106 514 A and US 2006/0 138 871 A1, from which positioning devices are known with which an object can be moved in a plane of displacement along two axes which are orthogonal to each other. 
       SUMMARY OF THE INVENTION 
       [0014]    It is the object of the invention to specify a microscope stage to be used in high-resolution light microscopy, which microscope stage enables a precise and drift-free positioning of a specimen holder on the microscope stage relative to the imaging optical system. 
         [0015]    Accordingly, the invention provides a complete decoupling of the movements of the specimen holder along the first axis and the second axis. As a result thereof, mutual drift influences of the two displacing devices forming the positioning device are reliably prevented. Thus, it is impossible that a mechanical drift occurring in one of the two displacing devices is transmitted to the other displacing device and thus causes an incorrect positioning of the specimen holder. 
         [0016]    Preferably, the specimen holder rests on the platform with an adhesive force that is greater than a force with which the positioning device acts in its deactivated operating state on the specimen holder. “Deactivated” operating state is understood to mean a state in which the positioning of the specimen holder on the platform is completed and the positioning device is so to speak inoperable. The sufficiently high adhesive force with which the specimen holder rests on the platform can be achieved by a correspondingly high weight of the specimen holder. This weight is to be chosen such that, on the one hand, it still allows for a precise displacement of the specimen holder on the platform by means of the positioning device and, on the other hand, it guarantees that the specimen holder after having been moved into its target position rests on the platform so firmly that an unintentional action of the positioning device caused by a drift does not result in any displacement of the specimen holder on the platform. 
         [0017]    A sufficiently high adhesive force can also be achieved or at least achieved in part by different material properties. For example, it is conceivable to provide for a magnetic interaction between the specimen holder and the platform by which the adhesive force becomes so high that a displacement of the specimen holder by means of the deactivated positioning device is impossible. For example, it is also conceivable to change the surface of the specimen holder and/or of the positioning device to achieve an increase in the adhesive force so that a displacement of the specimen holder by means of the deactivated positioning device is made impossible. 
         [0018]    An advantageous embodiment of the microscope stage provides that the first displacing device comprises a first slider which for displacing the specimen holder can be brought into contact therewith, a first linear guide in which the first slider is guided along the first axis, and a first drive for moving the first slider along the first axis, and in that the second displacing device comprises a second slider which for displacing the specimen holder can be brought into contact therewith, a second linear guide in which the second slider is guided along the second axis, and a second drive for moving the second slider along the second axis. In this embodiment, the mechanical decoupling of the two displacing devices is realized by providing separate sliders, linear guides and drives, which ensure that the specimen holder can be displaced in independent movements along the first and the second axis. The adhesive force with which the specimen holder rests on the platform is greater than the sum of those forces with which the drives via their associated sliders act on the specimen holder in the deactivated operating state of the positioning device. For this, the drives are preferably designed so smooth that the sliders which are coupled to the drives are so to speak pushed off from the specimen holder when they are pressed against the specimen holder as a result of mechanical drift. In particular, it is ensured that the specimen holder has neither a non-positive nor a positive connection to the drives. 
         [0019]    In a further advantageous embodiment, the first slider has a substantially U-shaped form and has a first base portion arranged along the first axis as well as two first leg portions arranged transversely to the first axis. Accordingly, the second slider has a substantially U-shaped form and has a second base portion arranged along the second axis as well as two second leg portions arranged transversely to the second axis. Further, the specimen holder is substantially designed as a rectangular plate which has two first opposite sides which are flanked with clearance by the first leg portions of the first plate, and two second opposite sides which are flanked with clearance by the second leg portions of the second plate. In this embodiment, the specimen holder is arranged each time between the two leg portions of the two U-shaped sliders. In order to minimize the force with which the positioning device acts in its deactivated operating state on the specimen holder, the distance between the two leg portions of the respective slider is so large that the specimen holder is not clamped between the two leg portions. This ensures that in the deactivated operating state of the positioning device at most one of the two leg portions rests against the specimen holder and is so to speak pushed off therefrom when it is pressed against the specimen holder as a result of mechanical drift. 
         [0020]    Preferably, the two sliders are arranged to each other in a crosswise and contact-free manner. For this, one of the two second leg portions of the second slider can have an elongated recess which extends parallel to the first axis, whereas one of the two first leg portions of the first slider passes through the recess parallel to the second axis. Taking into account the paths of displacement of the two sliders, the length of this recess is to be dimensioned such that the sliders are arranged in a contact-free manner over their entire range of movement and are thus decoupled from each other. 
         [0021]    Preferably, the two drives each comprise a rope which is driven so as to run around rollers and a coupling element which is fitted to the rope and by which the respective rope is firmly coupled to the associated slider. This embodiment serves to develop the drives particularly smoothly. This ensures that the forces which arise in the respective drive for example due to thermal deformation relax within the drive without displacing the specimen holder resting on the platform with a relatively high adhesive force when the positioning device is deactivated. 
         [0022]    Preferably, the coupling element is a rope tensioner in which the two ends of the rope are clamped. In this way, the rope tension which has an influence on the smoothness of the respective rope drive can be optimized easily. 
         [0023]    In a further embodiment, the two drives each comprise a manually operated drive wheel and a reduction gear set which is coupled to the drive wheel and the rope and converts the rotary motion of the respective drive wheel into the circulating drive motion of the associated rope. By means of this embodiment, the user is able to position the specimen holder easily on the platform. The reduction gear set increases the precision with which this positioning can take place. 
         [0024]    In an advantageous development, the two linear guides each comprise a guide rail arranged along the associated axis and at least one linear bearing which is coupled to the associated slider and is displaceable on the respective guide rail. For example, linear ball bearings or linear slide bearings, which can be designed particularly smoothly, can be used as linear bearings. 
         [0025]    Preferably, a sufficiently high adhesive force with which the specimen holder rests on the platform is achieved by a correspondingly high weight of the specimen holder. For example, the specimen holder can be made of brass, whereas the two sliders are made of aluminum. This material selection ensures that the specimen holder has a relatively high weight compared to the two sliders and thus a relatively high adhesive force on the platform. 
         [0026]    In a particularly preferred embodiment, an objective is mounted to the platform. Compared to conventional embodiments in which the objective is decoupled from the platform, this development has the advantage that not only the specimen holder but also the objective has a fixed arrangement relative to the platform, as a result whereof the mechanical drift between the specimen holder and the objective is minimized. 
         [0027]    The mechanical drift can be reduced further when the objective is mounted to the platform via a focusing drive. By mounting the focusing drive to the platform in a fixed spatial arrangement relative to the specimen holder resting on the platform, also the focusing of the objective which is required in any case is not prone to drift. 
         [0028]    According to a further aspect of the invention, a microscope having a microscope stage of the above-explained type is provided. 
         [0029]    Preferably, such a microscope comprises an objective revolver for the selective rotation of one of several microscope objectives held on the objective revolver into the beam path. If, in addition to this objective revolver, an objective mounted to the platform is provided, then such a microscope can be used particularly flexibly. For example, it is possible to observe, at first for an overview, a relatively large image section by means of one of the microscope objectives held on the objective revolver and to select a suitable target area within this image section which is then imaged with a particularly high resolution by means of the objective that is held on the platform in a largely drift-free manner. In this case, the objective held on the platform is to be mounted such that it can be removed from the imaging beam path via a suitable swivel device and, if necessary, can again be swiveled into the beam path and can then be fixed on the platform in a position-stable manner. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Exemplary embodiments of the present invention are described in more detail below with reference to the schematic drawings, in which: 
           [0031]      FIG. 1  shows a perspective view of a high-resolution light microscope as an embodiment; 
           [0032]      FIG. 2  shows a perspective bottom view of the microscope stage of the light microscope according to  FIG. 1 ; 
           [0033]      FIG. 3  shows a perspective top view of the microscope stage; 
           [0034]      FIG. 4  shows a perspective, partially sectional top view of the microscope stage, with an X drive and an Y drive being emphasized; 
           [0035]      FIG. 5  shows a top view of the microscope stage, in which a part of the microscope stage is omitted for emphasizing the X-drive and the Y-drive; and 
           [0036]      FIG. 6  shows a top view of the microscope stage in which an X slider and an Y slider are merely schematically illustrated. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    In the following, at first the total structure of a high-resolution light microscope  10  is explained with reference to  FIGS. 1 and 2 . Here, only those components of the light microscope  10  are described which are required for understanding the present invention. 
         [0038]    The light microscope  10  has a microscope stand  12  on which a microscope stage  14  is screwed. The microscope stage  14  has a platform  16  on which a specimen holder  18  rests. On the platform  16 , there is further a positioning device  20  which will be described in more detail later on and which serves to position the specimen holder  18  on the platform  16 . 
         [0039]    As shown in the bottom view of  FIG. 2 , a pivotable focusing drive  22  into which an objective  24  is screwed is mounted to the underside of the platform  16 . By means of a pivoted lever  26 , the focusing drive  22  together with the objective  24  held thereon can be swiveled out and into an imaging beam path of the light microscope  10 . The imaging beam path passes through a through hole  28  which is formed in the platform  16  and above which the specimen holder  18  is arranged. 
         [0040]    The light microscope  10  further has a pair of eyepieces  29  mounted to the microscope stand  12  as well as an objective revolver  30  holding several microscope objectives  32 . In the illustration of  FIG. 1 , the microscope stand  12  is illustrated with the microscope stage  14  being lifted so that the objective revolver  30  which is otherwise covered by the microscope stage  14  is visible. 
         [0041]    Thus, besides the microscope objectives  32  held on the objective revolver  30 , the light microscope  10  in addition comprises the objective  24  mounted to the platform  16 . The objective revolver  30  can be used to observe, at first for an overview, a relatively large image field in that one of the microscope objectives  32  is selectively rotated into the imaging beam path. For this, the objective  24  mounted to the platform  16  is swiveled out of the imaging beam path by means of the pivoted lever  26 . If a certain target area within the observed image field is to be imaged thereafter with a particularly high resolution, then the microscope objective  32  that is currently arranged in the imaging beam path is removed from the imaging beam path and the objective  24  mounted to the platform  16  is swiveled into the imaging beam path. As the objective  24  is arranged directly on the platform  16 , the mechanical drift between the specimen holder  18 , on which the non-illustrated specimen is held, and the objective  24  is relatively low (at least lower than when using one of the objectives  32  held on the objective revolver  30 ) so that in this configuration a high-resolution imaging of the specimen according to one of the methods of localization microscopy described at the beginning is possible. 
         [0042]    In the following, the positioning device  20  is explained in more detail with reference to  FIGS. 3 to 6 . 
         [0043]    The positioning device  20  comprises two displacing devices  34  and  36  which are mechanically decoupled from each other. The displacing device  34  serves to displace the specimen holder  18  in a first direction on the platform  16 , whereas the second displacing device is used to displace the specimen holder  18  in a second direction on the platform  16 , which second direction is orthogonal to the first direction. The two afore-mentioned directions are indicated by the double arrows X and Y, respectively, in  FIGS. 3 to 6 . Accordingly, the first displacing device  34  is referred to in the following as X displacing device and the second displacing device is referred to as Y displacing device. The plane of displacement spanned by the two directions X and Y is referred to in the following as X-Y-plane. 
         [0044]    As can be taken from  FIG. 3 , the X displacing device  34  comprises an X slider  38  which is movable back and forth in X direction. The X slider  38  is formed as a substantially U-shaped plate having a base portion  40  and two parallel leg portions  42  and  44 . The base portion  40  extends in X direction, whereas the two leg portions  42 ,  44  extend in Y direction. Accordingly, the Y displacing device  36  comprises an Y slider  46  which is movable back and forth in Y direction. The Y slider  46  likewise has a base portion  48  and two parallel leg portions  50  and  52 . The base portion  48  extends in Y direction, whereas the two leg portions  50 ,  52  extend in X direction. 
         [0045]    The X slider  38  and the Y slider  46  are arranged to each other such that their leg portions  42 ,  44  and  50 ,  52 , respectively, are so to speak interlaced without contacting each other within their ranges of displacement. For this, the leg portion  50  of the Y slider  46  has an elongated recess  54  which extends in X direction and is passed through by the leg portion  44  of the X slider  38  in Y direction. The recess  54  thus guarantees the interlaced, contact-free arrangement of the two sliders  38  and  46  relative to each other. 
         [0046]    The X displacing device  34  further comprises an X drive  56  and an X linear guide  58 . The X drive  56  and the X linear guide  58  serve to move the X slider  38  in X direction in a linearly guided manner. Accordingly, the Y actuating device  36  comprises an Y drive  60  and an Y linear guide  62  which serve to move the Y slider  46  in the direction of the Y axis in a linearly guided manner. The drives  56 ,  60  and the linear guides  58 ,  62  are explained in more detail in the following with reference to  FIGS. 4 to 6 . In this connection,  FIG. 6  shows the two sliders  38  and  46  in an abstract manner with respect to their form to illustrate the functioning of the inventive arrangement. 
         [0047]    The X drive  56  has an X handwheel  64  which is rotatably mounted to the platform  16  via a bearing  66 . To the X handwheel  64 , a gearwheel  68  is coupled which rotates together with the handwheel  64 . The gearwheel  68  is engaged with a gearwheel  70  which is rotatably mounted to the platform  16  via a bearing  72 . The gearwheels  68  and  70  are dimensioned such that together they form a reduction gear set. 
         [0048]    The rotary motion of the gearwheel  70  is transmitted to an X drive rope  74 . The X drive rope  74  is guided over a drive roller  76  coupled to the gearwheel  70  as well as a deflection roller  78  mounted in the platform  16 . The two ends of the X drive rope  74  are fixed in a rope tensioner  80  by means of fixing screws  82  and  84 . By means of the rope tensioner  80 , the rope tension of the X drive rope  74  can be adjusted as desired. 
         [0049]    The rope tensioner  80  is firmly mounted to the X slider  38 . The rope tensioner  80  thus forms a driving element which converts the circulating motion of the X drive rope  74  caused by the actuation of the X handwheel  84  into a motion of the X slider  38 . 
         [0050]    The X linear guide  58  has an X guide rail  86  extending in X direction and firmly mounted in the platform  16 . The X guide rail  86  forms an axis along which the X slider  38  is guided. For this, two linear bearings  88  and  90  are movably arranged on the X guide rail  86 . The two linear bearings  88  and  90  are firmly coupled to the X slider  38 . They move on the X guide rail  86  when the X slider  38  is moved in X direction by the X drive  56 . 
         [0051]    The Y drive  60  and the Y linear guide  62  are formed in accordance with the X drive  56  and the X linear guide  58 , respectively. Accordingly, the Y drive  60  comprises an Y handwheel  92 , a bearing  94 , gearwheels  96  and  98 , a bearing  100 , an Y drive rope  102 , a drive roller  104 , a deflection roller  106 , a rope tensioner  108  as well as fixing screws  110 ,  112 , whereas the Y linear guide  62  has an Y guide rail  114  as well as two linear bearings  116 ,  118 . The components  92  to  118  of the Y displacing device  36  operate in the same manner as the components  64  to  90  of the X displacing device  34  (apart from the fact that the components  92  to  118  serve to displace the specimen holder in Y direction). 
         [0052]    As can be best seen in  FIGS. 3 to 6 , the specimen holder  18  is substantially formed as a rectangular plate which, in its center, has a circular opening  120  which is aligned with the through hole  28  of the platform  16 . The specimen holder  18  further comprises clamps  122  and  124  which serve to fix the specimen to be observed and not illustrated in the Figures in a female recess  126  of the specimen holder  18 . 
         [0053]    As already mentioned above, the X displacing device  34  and the Y displacing device  36  serve to move the specimen holder  18  on the platform  16  independently in X direction and in Y direction. The functioning of the two displacing devices  34  and  36  is described in detail in the following with reference to  FIG. 6 . 
         [0054]    As illustrated in the view according to  FIG. 6 , the two leg portions  42  and  44  of the X slider  38  are arranged such that they flank two opposite sides of the specimen holder  18  with clearance. This means that the distance between the two leg portions  42  and  44  measured in X direction is greater than the dimension of the specimen holder  18  in X direction. Thus, it is guaranteed that the specimen holder  18  is never simultaneously contacted by both leg portions  42  and  44  and is thus not clamped therebetween. In  FIG. 6 , the clearance which the leg portions  42 ,  44  have relative to the specimen holder  18  is illustrated exaggeratedly large for a better understanding. In practice, it only has to be so large that a clamping of the specimen holder between the two leg portions  42 ,  44  is avoided. 
         [0055]    If the specimen holder  18  is to be moved in X direction, then the X handwheel  64  is rotated, whereupon its rotary motion is converted into a circulating motion of the X drive rope  74  via the reduction gear set formed by the gearwheels  68  and  70 . This circulating motion of the X drive rope  74  is in turn transmitted to the X slider  38  via the X rope tensioner  80 . Here, one of the two leg portions  42  and  44  comes into contact with the side of the specimen holder  18  facing the respective leg portion. This causes that the specimen holder  18  is displaced on the platform  16  in X direction. 
         [0056]    Accordingly, the specimen holder (independent of the movement in X direction) is moved in Y direction via the Y drive  60  and the Y linear guide  62 . 
         [0057]    The specimen holder  18  rests on the platform  16  with an adhesive force that is greater than the sum of all forces which the positioning device  20 , in its deactivated state, exerts on the specimen holder  18  as a result of mechanical drift. In particular, the afore-mentioned adhesive force is greater than the force which one of the leg portions  42 ,  44 ,  50 ,  52  exerts on the specimen holder  18  when it directly bears against the side of the specimen holder  18  that faces the respective leg portion. 
         [0058]    This is achieved, on the one hand, in that the drives  56 ,  60  and the linear guides  58 ,  62  are particularly smooth and, on the other hand, in that the sliders  38  and  46  rest on the platform  16  with a significantly lower adhesive force than the specimen holder  18 . The latter is, for example, achieved in that the specimen holder  18  is made of a relatively heavy material, such as brass, whereas the sliders  38  and  46  are made of a significantly lighter material, such as aluminum. 
         [0059]    By means of these measures it is achieved that the movable parts of the drives  56 ,  60  and of the linear guides  58 ,  62  are moved opposite to the direction in which one of the leg portions  42 ,  44 ,  50 ,  52  presses against the specimen holder  18  when a mechanical drift occurs, i.e. they yield. The leg portion is thus so to speak pushed off from the specimen holder  18 , whereas the drives  56 ,  60  and the linear guides  58 ,  62  do not offer any appreciable resistance to this push-off movement of the leg portion, but yield to this movement. 
         [0060]    The invention is not to be limited to the specific embodiments disclosed, and modifications and other embodiments are intended to be included within the scope of the invention. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           10  light microscope 
           12  microscope stand 
           14  microscope stage 
           16  platform 
           18  specimen holder 
           20  positioning device 
           22  focusing drive 
           24  objective 
           26  pivoted lever 
           28  through hole 
           29  eyepieces 
           30  objective revolver 
           32  microscope objectives 
           34  X displacing device 
           36  Y displacing device 
           38  X slider 
           40  base portion 
           42 ,  44  leg portions 
           46  Y slider 
           48  base portion 
           50 ,  52  leg portions 
           54  recess 
           56  X drive 
           58  X linear guide 
           60  Y drive 
           62  Y linear guide 
           64  X handwheel 
           66  bearing 
           68 ,  70  gearwheels 
           72  bearing 
           74  X drive rope 
           76  drive roller 
           78  deflection roller 
           80  rope tensioner 
           82 ,  84  fixing screws 
           86  X guide rail 
           88 ,  90  linear bearing 
           92  Y handwheel 
           94  bearing 
           96 ,  98  gearwheels 
           100  bearing 
           102  Y drive rope 
           104  drive roller 
           106  deflection roller 
           108  rope tensioner 
           110 ,  112  fixing screws 
           114  Y guide rail 
           116 ,  118  linear bearing 
           120  opening 
           122 ,  124  clamps 
           126  female recess