Abstract:
An automatic microtome is disclosed which comprises a base, a knife-holder mounted on the base for supporting a microtome knife, a specimen holder mounted for longitudinal movement relative to the base towards and away from the knife-holder and also mounted for transverse movement relative to the knife holder for displacing a specimen held by the specimen holder relative to a knife mounted in the knife holder for cutting sections from such specimen. The knife holder is mounted directly on the base without provision for adjustment of the position of the knife holder on the base such as to vary the location of the knife edge in relation to said edge, and the specimen holder is designed to provide enhanced stability over a wide range of longitudinal positions the specimen holder. Thus, the specimen holder comprises a slide mounted for longitudinal movement in a bearing arrangement including first and second bearing elements at longitudinally spaced positions along the slide, and limiting transverse movement of the slide in one direction and a third bearing element at a longitudinal position intermediate the first-mentioned longitudinally spaced positions and biased towards the slide so as to urge the latter into engagement with said first and second bearing elements. The knife holder and base have cooperating arcuate bearing surfaces permitting angular movement of the knife holder relative to the base about the axis of curvature of the bearing surfaces with concomitant sliding movement of one of said arcuate surfaces over the other. A locating device is adjustably secured to the knife holder for cooperation with a datum provided on the base to ensure that with the locating device fixed to the knife holder the angular position of the knife holder on the base at which the locating device cooperates with the datum can readily be established.

Description:
RELATED APPLICATIONS 
     This is a continuation in part of Ser. No. 09/239,103 filed Jan. 28, 1999 abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     THIS INVENTION relates to microtomes, that is to say to devices used for cutting thin sections from specimens for, for example, microscopic examination. The invention relates, more particularly, to microtomes which are to some degree mechanised or automatic in operation. 
     2. Description of Related Art 
     Mechanised or automatic microtomes are known which comprise a rigid base, a knife holder secured to said base, a microtome knife secured in the knife holder and a specimen support, on which a specimen to be sectioned is mounted, the specimen support being mounted for movement relative to the base so as to cause a specimen mounted on the specimen support to execute a transverse movement with respect to the knife blade, appropriate to the cutting of slices or sections from the specimen. Generally, in such known microtomes, the specimen support is also mounted for advancing and retracting movement relative to the knife for the cutting of successive sections or slices from the specimen and for withdrawing the specimen holder from the region of the knife for mounting a fresh specimen and so on. 
     In known microtomes of this character, the microtome knife is supported in a knife holder which is adjustable in position to vary the proximity of the knife to the specimen holder. This known arrangement has, however, the disadvantage that the knife is not always supported as rigidly as it might be, with consequent deficiencies in the operation of the microtome. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention in one of its aspects to provide an improved microtome in which the firmness of mounting of the microtome knife can more readily be assured than in the past. 
     According to this aspect of the invention there is provided a microtome comprising a base, a knife-holder mounted on the base for supporting a microtome knife, a specimen holder mounted for longitudinal movement relative to the base towards and away from the knife-holder and also mounted for transverse movement relative to the knife holder for displacing a specimen held by the specimen holder relative to a knife mounted in the knife holder for cutting sections from such specimen, wherein the knife holder is mounted directly on the base without provision for adjustment of the position of the knife holder on the base such as to vary the location of the knife edge in relation to said edge. 
     In addition to the rigidity secured in accordance with this aspect of the invention, the fixed knife mounting, in the preferred embodiment makes for ease of cleaning the instrument. 
     The invention in another of its aspects provides improved means for ensuring reproducibility of the microtome knife angle with respect to the specimen to be sectioned, between replacements or other adjustments of the knife relative to the remainder of the microtome. It is known that, in operation of mechanised or automatic microtomes, in particular, different angles are required for the various plates or knives used to compensate for different facet angles, embedding mediums and materials to be sectioned. Routine practice requires the setting of only one angle on the knife holder, which angle can be maintained or reproduced when, for example, the knife holder is removed for cleaning or replaced with an alternative holder. In conventional microtomes, it is normal to utilize a knife holder having a base which is removable from the frame or base structure of the microtome and which is replaceable on such structure. Accordingly, in normal practice, the knife holder and its base are removed together from the remainder of the machine and replaced together, so that the knife angle is thereby retained. In a microtome in accordance with the first aspect of the invention, where the knife holder, or at least the knife holder base, is a fixed part of the base or frame of the microtome as a whole, the procedure described above in relation to conventional microtomes is not available and it is an object of the present invention, in another of its aspects, to provide a means of maintaining or reproducing a predetermined angle of the microtome knife when mounted in the microtome, which means is not confined to microtomes having knife holders with bases removable from the remainder of the microtome with the knife holder. 
     In accordance with this further aspect of the invention, accordingly, there is provided a microtome comprising a knife holder and a knife holder support having cooperating arcuate bearing surfaces permitting angular movement of the knife holder relative to the knife holder support about the axis of curvature of said bearing surfaces with concomitant sliding movement of one of said arcuate surfaces over the other, wherein a sensor or locating device is adjustably secured to the component providing one of these bearing surfaces, for cooperation with a datum provided on the component providing the other of these arcuate bearing surfaces, to ensure that with said sensor or locating device fixed to said component providing said one of said bearing surfaces the angular position of the knife holder on the knife holder support at which said sensor or locating device cooperates with said datum can readily be established. 
     The invention in another of its aspects relates to the mounting of the specimen holder in a microtome. In microtomes in which the specimen holder is advanced towards and is retractable from the knife, it is important, for proper sectioning of the specimen, that the specimen support, whilst being adjustable in position relative to the remainder of the microtome structure, be supported rigidly in such a way as to resist effectively any unintended displacements and deflections. This is particularly so in the case of the microtome in accordance with the first-noted aspect of the invention in which the position of the microtome blade is fixed and in which the specimen holder must therefore be capable of being advanced over a greater linear range towards and away from the blade than in microtomes in which there is the possibility of mounting the blade holder nearer to or further from the structure for which the specimen support is supported. 
     According to this aspect of the invention, accordingly, there is provided a microtome comprising a base, a knife-holder mounted on the base for supporting a microtome knife and a specimen holder mounted for longitudinal movement relative to the base towards and away from the knife holder, wherein said specimen holder comprises a slide mounted for longitudinal movement in a bearing arrangement including first and second bearing elements at longitudinally spaced positions along the slide, and limiting transverse movement of the slide in one direction and a third bearing element at a longitudinal position intermediate the first-mentioned longitudinally spaced positions and biased towards the slide so as to urge the latter into engagement with said first and second bearing elements. 
     The features according to this aspect of the invention are of particular utility in relation to a microtome in accordance with the first aspect of the invention, where the fixed knife position requires the specimen to be moved up to the knife rather than vice versa. To allow for a range of block depths or of specimen depths to be sectioned, enough specimen travel must be allowed between specimen and knife position. This implies that the specimen may be required to be sectioned with the specimen holder supporting the specimen at or near to its full extension from the structure from which the specimen holder is adjustably supported, so that the need for stable support of the specimen holder is particularly acute. 
     According to a yet further aspect of the invention, there is provided a microtome comprising a base, a knife fixed with respect to the base, a specimen holder mounted for longitudinal movement relative to the base towards and away from the knife, motor means for advancing and withdrawing the specimen holder automatically to controllable extents, and automatic control means capable of advancing or retracting the specimen holder to a pre-set position upon execution of a command operation. 
     This feature enables the rapid positioning of the specimen from the loading/unloading position to the trimming/sectioning position. In microtomes in accordance with the first-noted aspect of the invention, in particular, the fixed knife position means that the specimen must always move up to and away from the knife, rather than vice versa and the auto-load feature speeds up the processing between blocks or specimens when the correct “memory” position is set and ideally ensures no clashes between block and knife. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are described below by way of example with reference to the accompanying drawings in which: 
     FIG. 1 is a view in vertical longitudinal section of part of a microtome embodying the invention, 
     FIG. 2 is a perspective view of part of the microtome of FIG. 1, 
     FIG. 3 is an exploded detail view showing part of a knife angle setting mechanism, 
     FIGS. 4 and 5 are side elevation views showing the knife holder mounting of the microtome of FIGS. 1 to  3 , FIG. 5 showing a detail partly in section, 
     FIG. 6 is a view to an enlarged scale of the detail in FIG. 5, 
     FIG. 5A is side elevation view, corresponding to FIG. 5, of a variant and likewise showing a detail partly in section, 
     FIG. 6A is view, similar to FIG. 6, of the detail in FIG. 5A to an enlarged scale, 
     FIG. 6B is view in section along the line B—B in FIG. 6A, 
     FIG. 7 is a partial view in section on the line VII—VII in FIG. 1, 
     FIG. 8 is a view in cross section along the line VIII—VIII in FIG. 1, 
     FIG. 9 is an exploded view of part of the microtome of FIGS. 1 to  8 . 
     FIG. 10 is an end view of the part shown in FIG. 9, in an assembled condition, 
     FIG. 11 is a side elevation view of the last-noted part in the assembled condition and 
     FIG. 12 is schematic side elevation view of the microtome. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, a microtome comprises a main frame or base  9  which may, for example, be a metal casting, and upon which the remaining components of the microtome are mounted. At one end of the base  9  is mounted a knife holder assembly indicated generally at  20  and comprising a knife holder base  11  which is fixedly secured to the base  9  and on which is adjustably secured a knife holder block  12 , on which a microtome knife proper, indicated at  22 , is removably secured in manner known per se. As best shown in FIG. 3, the knife holder block  12  affords two, laterally spaced upstanding pillars  12   a  defining therebetween a space within which a specimen can be located and displaced for sectioning by the knife  22 , the cutting edge of which defines, in effect the upper edge of an end wall of this space, which space IS spanned by a specimen section deflector  24  secured to the pillars  12   a.    
     The knife holder base  11  may be integral with the base  9  or may be secured fixedly thereto, for example by bolts (as illustrated). The knife holder base  11  has an upwardly facing concave cylindrical bearing surface which mates with a complementary convex bearing surface forming the underside of the knife holder block  12 , an arrangement permitting angular adjustment of the knife holder block  12  relative to the base  11  about a horizontal transversely extending axis. The centre of curvature of these cooperating part-cylindrical bearing surfaces coincides with the position of the cutting edge of the blade  22  so that angular adjustment of the block  12  with concomitant sliding rotational movement between the convex bearing surface on the block  12  and the concave bearing surface on the base  11  does not significantly alter the position of the cutting edge of the blade  22 . Except when angular adjustment is required, the block  12  is fixedly secured on the base  11  by mean of a retaining element or plunger  10  in the form of an elongate bolt which extends slidably through an aperture in frame  9  and through a bore in the base  11 , through the mouth of a T-section slot  24  which extends within the knife holder block  12  along an arc about the centre of curvature of the convex lower face of the block, the slot  24  lying adjacent said lower face, the narrower mouth of the slot  24 , defining the upright of the “T”, opening onto said lower convex surface. The slot  24  extends from the forward end of the block  12 . The bolt  10  has a neck portion extending through said narrow mouth and an enlarged head  26  within the slot  24 , providing a peripheral flange which bears upon the ledges, defined by the slot  24 , lying on opposite sides of the narrower portion of the slot through which the neck portion of bolt  10  extends. The lower end of the bolt  10  is screw-threadedly engaged in a nut  28  which is pivotally connected to the end of one arm of a lever  8  which is mounted within a recess in the underside of the base  9  and pivots on a transverse pivot pin  32  extending through the base  9 . The lever  8  has another arm, remote from nut  28  and which is acted on by an eccentric  30  on a horizontal transversely extending shaft (not shown) which is rotatable manually or by motor means (not shown) for pivoting the lever  8  about the pivot  32 , in the plane of FIG. 1, anti-clockwise as viewed in the figure to draw the bolt  10  downwardly and thus pull the block  12  down firmly against the base  11 , or clockwise to release the bolt  10  and lift the bolt  10  sufficiently to release lease the downward pressure on the edges of slot  24  and thus permit angular adjustment of the knife holder block  12  relative to the base  11 . The arrangement described enables a range of similar knife holders  12  to be attached to the base and angled to achieve optimum sectioning performance, with the cutting edge of the blade being always at the centre of the radius of curvature of the surfaces of the block  12  and base  11  whereby said cutting edge is always at a fixed position in relation to the remainder of the microtome. 
     In order to allow ready re-establishment of the desired angular position of the block  12  and knife  22 , relative to base  11 , about the axis of curvature of the mating bearing surfaces of the block  12  and base  11 , for example after the knife holder block  12  has been temporarily removed from the base  11  and replaced thereon, an adjustable locating device or “memory” feature is provided, as described below. Thus, located in a further slot  41   a  extending arcuately about the centre of curvature of the cooperating bearing surfaces of the block  12  and base  11  and which slot likewise opens onto the bearing surface of block  12 , is a slider block  41  shown in perspective FIG.  3 . Block  41  has a transverse screw-threaded bore to receive a complementary screw-threaded shank of a bolt  43  which extends through an arcuate slot  43   a  in the side face of the knife holder adjacent the slot  41   a  which receives the block  41 . The last-noted arcuate slot  43   a  opens into the adjoining side wall of the slot  41   a  which receives the block  41 . The arcuate slot  43   a  receiving the bolt  43  is rebated to receive the enlarged head of the bolt  43  which may, as illustrated, be of the Allen type, i.e. of the type having a polygonal—(typically hexagonal) section recess extending axially into the bolt head, to receive a key of complementary cross-section for turning the bolt. Thus, with the bolt  43  slackened, the block  41  can be adjusted to any desired one of a range of positions along the slot which accommodates it and can be secured in that position by tightening the bolt  43 . 
     As best shown in FIG. 6, the block  41  has a blind bore  45  extending upwardly from its lower surface, the bore  45  being screw-threaded and receiving a correspondingly screw-threaded insert  47  in which a steel ball  49  is held captive, together with a biasing spring (not shown) urging the ball  49  downwardly to project downwardly from the block  41  for engagement with the upwardly presented surface of the base  11 . More particularly, the bearing surface of the base  11  has a pit or slot  49   a  therein in which the ball  49  engages in one angular position of the knife holder. The last-noted angular position is, of course, determined by the position of the block  41  along its slot. In order to facilitate adjustment of the knife holder to a desired angle, the adjoining side face of the block  12  is graduated in degrees, as illustrated in FIGS. 4 to  6 , and the adjoining edge face of the base  11  is provided with a reference mark cooperating with these graduations. It will be appreciated that, once the position of the block  41  in its slot has been set, the correct angular position of the block  12  with respect to the base  11  can be found simply by rotating the block  12  on the base  11  until the ball  49  engages in the slot or pit  49   a  in the upper bearing surface of the base  11 . 
     The block  41 , together with the insert  47 , ball  49  and its biasing spring and the bolt  43  together form an adjustable sensor or locating device adjustably secured to the block  12 , whilst the slot or pit  49   a  in the bearing surface of the base  11  constitutes a datum for cooperation with the adjustable locating device and in particular for cooperation with the ball  49 . 
     In a variant, illustrated in FIGS. 5A,  6 A and  6 B, the locating device is adjustably secured to the base  11  and the pit or slot  49   a  is provided on the bearing surface of the knife holder block  12 . Thus, in the variant of FIGS. 5A,  6 A and  61 B, the arcuate slot  41   a  which receives the slider block  41  and which again extends arcuately about the centre of curvature of the cooperating bearing surfaces of the block  12  and base  11 , is formed in the bearing surface of the base  11  and the shank of the bolt  43  extends through arcuate slot  43   a  in the side face of the base  11  in this variant. The arcuate slot  43   a  is again rebated to receive the enlarged head of the bolt  43 . Corresponding parts in FIGS. 5A,  6 A and  6 B corresponding to parts in FIGS. 1 to  6  have corresponding reference numerals. 
     It should be noted that the sectional view of FIG. 6B would equally be a representation of a vertical section through the longitudinal axis of bolt  43  in FIGS. 5 and 6 if the figure were simply inverted (i.e. rotated through 180°) and the references  11  and  12  simply transposed. In both the arrangement of FIGS. 1 to  6  and in the variant of FIGS. 5A,  6 A and  6 B, when the bolt  43  is tightened to fix the locating device  41 ,  47 ,  49 , etc. in a desired position, the effect is to clamp the thinner portion  43   b  (see FIG. 6B) of the wall of the block of the block  12  or base  11  through which the shank of the ball  43  extends between the block  41  and the head of the bolt  43 , or where a washer is provided around the shank of the bolt  43  between the head of the bolt  43  and the wall  43   b , between that washer and the block  41 . The block  41  is thus held in the selected location by friction until such time as the bolt  43  is unscrewed releasing the clamping pressure applied to the wall portion  43   b  and allowing adjustment of the block  41  along its slot  41   a.    
     The bolt  43  retains the block  41  in position in its slot when the knife holder block  12  is removed from the base  11  and thus serves to “memorize” the appropriate angular setting. Thus, when the knife holder block  12  is returned to the base  11 , it is simply necessary for the operative setting up the microtome to place the knife holder block  12  with its bearing surface on that of the base  11  and to rotate the knife holder substantially about the axis of curvature of its bearing surface until the ball  49  meets and drops into the slot  49   a , an occurrence which the operative can readily detect by feel and which additionally provides an added resistance to further rotation of the knife holder block, providing the operative with an unequivocal indication that the correct position has been reached, at which the knife holder block may then be clamped by means of the bolt  10  and lever  8 . The datum  49   a  preferably takes the form of a V-section slot or groove extending longitudinally parallel with the axis of curvature of the bearing surfaces, and thus also parallel with the axis of bolt  43 , so as to accommodate slight movements of the block  41  in directions parallel with said axes during clamping and unclamping of the block by tightening slackening the bolt  43 . 
     Referring again to FIG. 1, the microtome includes a specimen carrier, shown only partly in FIG. 1, the portion shown comprising a straight mounting tube  50  to the forward end of which is secured, in use, a specimen mounting arrangement (not shown but known per se), including a mounting plate for holding a specimen to be sectioned. In use of the microtome, the specimen to be sectioned is mounted on the mounting plate, in manner known per se, by means of a vice (not shown) and an orientation head (not shown), the vice and orientation head being likewise known per se. The specimen tube  50  extends with its longitudinal axis horizontally, through a support or carriage  52  which is movable in a vertical plane, parallel with the cutting edge of the blade  22 , by means not shown, to effect sectioning of a specimen carried by the tube  50 . 
     The carriage  52  includes a generally vertically extending end flange  54 , (constituting the mounting plate referred to above), guided for vertical sliding movement in guideways at the front of a housing, indicated at  56 , secured to the microtome base  9 . The specimen tube  50  has, externally, the form of a smooth cylinder from which surface portions have been removed, a first such surface portion comprising an upper longitudinal portion defined by a flat  57 , horizontally extending, extending longitudinally along the top of the tube, and a second such portion comprising a groove  64 , extending along the bottom of the tube, diametrally opposite the flat  57 , is a groove  64  of truncated inverted V-section. The tube  50  is guided for horizontal sliding movement towards and away from the microtome knife, through a horizontal bore in a stem portion  58  of the carriage  52 , in two sintered cylindrical oil-filled bushes  60  fitted in respective ends of the bore in stem portion  58 , adjacent respective open ends of said bore. 
     In the groove  64  run two wheels  66  which are spaced apart longitudinally of the stem  58 . The wheels  66  are rotatably mounted, for rotation about respective horizontal axes, perpendicular to the longitudinal axis of the tube  50 , in an undercarriage assembly  74  (see also FIG. 9) extending downwardly from a fixed portion of the microtome structure. As shown in FIG. 9, screw-threaded studs  75  extend downward from said fixed portion of the microtome through respective bores in a chassis of the undercarriage assembly  74 , said chassis being supported and urged upwardly by a stack of disc spring washers  77  on each of said studs, retained by a respective nut  79  at the lower end of each said stud. Thus, the undercarriage assembly is urged, by the spring washers, upwardly against the specimen tube  50 , which in turn is forced upwardly against the upper regions of the bores through the bushes  60  and, in particular, against the portions of these bushes lying directly adjacent the longitudinal flat  57  along the top of the specimen tube. As a result, and by virtue of the mutual inclination of the portions of the bushes engaging the specimen tube adjacent said flat  57 , the specimen tube is centred in the bushes so as to resist lateral horizontal displacement therein. The wheels  66  on the undercarriage  74  rotate as the specimen tube  50  advances and retracts. The disc springs  77  allow some movement of the undercarriage  74  to compensate for any misalignment and any wear in the system. The flat  57  along the top of the specimen tube  50  has the effect that the specimen tube  50  is in fact supported on three localized areas—the two areas at the sides of the top flat  57  and the preloaded undercarriage wheels  66  running in the groove  64 . As shown in FIG.  7  and also in FIGS. 1 and 9, two plungers  63  extend vertically, are guided closely in respective vertical bores in the lower wall of the stem  58  and have tapered noses which engage respective side walls of the inverted V-section groove in the underside of the specimen tube  50 . The plungers  63  are urged vertically upwardly by respective springs. These plungers  63  act to prevent rotational movement of the specimen tube about its longitudinal axis and take up any rotational play which the specimen tube  50  might otherwise have in its bearings  60 . 
     The specimen tube  50  can be longitudinally advanced and retracted automatically, for example by means of a feed screw arrangement driven by an electric motor controlled by electronic control means, for making the feed movements between successive cutting strokes for cutting successive thin sections from a specimen and for making the relatively gross movements required to move a specimen from a position withdrawn from the knife at which the specimen is mounted, for example, to a position adjacent the knife in preparation for sectioning. 
     The preferred embodiment of the invention preferably has a block auto-load feature as described below. 
     The block auto-load feature is an automatic system to assist the user in the routine of mounting specimen blocks in the microtome at a safe distance from the knife, bringing the block to the knife position for trimming and/or sectioning, and returning the block to unload from the microtome at a safe distance from the knife, ready to re-load. 
     This auto-load is likely to be of particular use in rotary microtomes. 
     Referring to the schematic diagram forming FIG. 12, the auto-load feature in the preferred embodiment is controlled by two keys (not shown) on a control unit (not shown) controlling a microcontroller or microcomputer (not shown) included in the control means referred to and which may be installed within the microtome casing. One of these two keys is a MEMORY key, used to set a position ( 94  in FIG. 12) of the specimen mounting plate  90  (carried by the specimen tube  50 ) to which all succeeding specimens/blocks of the same depth may be advanced, to be ready for trimming/sectioning. The control system may include a facility for advancing and retracting the specimen tube  50  under manual control, and this may be used to place the specimen tube in a desired “memory” position  94 , suitable for commencement of cutting of a specimen  92  mounted on plate  90 . This position may then be stored by operation of the MEMORY key referred to. Once the memory position  94  is set, operation of an AUTO key allows the specimen to be rapidly advanced to the memory position, (if the current position of the specimen is anywhere behind the memory position), or rapidly retracted from the current specimen position (if the current position of the specimen is at or in front of the memory position), to a load/unload position  96 . The amount retracted behind the memory position, i.e. the location of the load/unload position  96 , may be set by the user. Indicators on the keys advise whether a memory position has been set, and whether the specimen is at or in front of this position or behind the memory position. The control means allows the tube  50 , and with it the plate  90  and specimen  92 , to be retracted further, when required, to a reset position  98 . 
     The following summary will assist in an appreciation of the preferred embodiment of the invention described above with reference to the drawings. The following are the four features of the preferred embodiment considered the most significant. 
     Fixed Knife Position—the specimen must always move to the knife and the distance will vary according to the specimen/block size and must accommodate the largest block depth, this is true of other, known, microtomes but the distance to be moved can be varied by moving the knife towards the specimen and re-clamping. The main advantages of fixing the knife position are rigidity when sectioning and ease of cleaning the instrument. 
     Knife Angle Position Memory—the chosen knife angle for sectioning can be retained when the knife holder is moved or removed and replaced. Different angles are required for the various blades and knives used to compensate for different facet angles, embedding mediums and materials to be sectioned. Routine use requires the setting of only one angle on the knife holder which can be “remembered” when the knife holder is removed for cleaning or replaced with an alternative holder. The fixed knife position feature means that only the knife holder is removed whereas most microtomes have a removable base which means that the base and top are removed together hence retaining the knife angle position. 
     Auto Load Feature—this feature enables the rapid positioning of the specimen from the loading/unloading position to the trimming/sectioning position. The fixed knife position feature means that the specimen must always move to the knife and the autoload feature speeds up the processing between blocks when the correct memory position is set, and should ensure no clashes between block and knife. 
     Specimen Tube Mounting Arrangement—this feature enables the stability of the specimen to be maintained for sectioning over the full travel of the specimen tube. The fixed knife position feature requires the specimen to be moved to the knife. To enable a range of block depths to be sectioned enough specimen travel must be allowed between specimen and knife position. This implies that the specimen is being sectioned very often with the specimen tube near full extension, so that it is necessary for stability to be maintained. 
     The above features are interrelated in that the last three features noted are particularly advantageous in conjunction with the fixed knife position feature, but each such feature can also stand alone. 
     The fixed knife position feature referred to means that the knife holder base is fixedly attached to the structure of the microtome. 
     The knife holder base is the platform for mounting a range of knife holders and its stability is critical to the sectioning performance of the microtome. By bolting down this part to the microtome, for example, the knife holder mounted on it is more stable leading to improved sectioning performance. 
     Other, known, microtomes have a knife holder or knife holder base that can be moved forward and backward relative to the specimen on a track or dovetail to which it is then clamped. 
     The angular adjustability of the knife holder  12  on knife holder base  11  allows a range of knife holders  12  to be attached to the base and angled to achieve optimum sectioning performance, with the cutting point of the blade at the centre of the radius and hence always at a fixed position on the microtome. 
     The knife angle position memory feature enables a chosen knife angle to be set on the knife holder such that when it is removed and replaced it can easily be reset to the same angle. 
     A knife holder, whether it be for disposable blades or solid knives, can be typically set across a wide angle. Those on the preferred embodiment allow 15 degrees of adjustment. This range is available to accommodate different types of blades and knives which can have a range of facet angles for the knife edge. Different facet angles typically require the knife to be set at different angles to the specimen to be sectioned. If the wrong knife angle is set on the knife holder it may have a detrimental effect on the sectioning performance. 
     A microtome user will typically use one blade/knife type for the majority or work. The user will set the knife angle position memory feature to suit this blade/knife type. 
     At the end of a sectioning session there will be much debris from the sectioning process around the knife area. It is usual to remove the knife holder to allow better access for cleaning. Another reason for removing the knife holder may be to fit another type. If there were no position memory feature when the knife holder was replaced the angle would have to be reset by remembering what the correct angle setting is or else by carrying out some sectioning and adjusting the angle to suit. The knife angle position memory feature on the other hand enables the correct knife angle to be easily reset. 
     The preferred embodiment of microtome described with reference to, and shown in, the accompanying drawings, can be used with a range of knife holders which all fit on one common knife holder base on the instrument. The different knife holders hold different types of knives which are used for sectioning different specimen types. 
     On the majority of known microtomes, as well as angular adjustment of the knife holder being possible, the knife holder base can normally be slid forwards and backwards for additional adjustment. This generally means that the knife holder base can be removed, with the knife holder still attached, by sliding it off the end of its adjustment guides. The knife can therefore be removed and replaced without changing the knife angle. This is not possible with the preferred embodiment of the invention since the knife holder base is permanently fixed to the instrument. The knife holder can therefore only be removed by separating it from the knife holder base which, but for the knife angle position memory feature described, would mean that the knife angle setting would be lost. 
     In the preferred embodiment, in order to save the knife angle once the knife holder has been set to the required position and locked in position, the screw  43  in the side of the knife holder is released using an Allen key and the screw and with it the block  41  carrying the spring plunger  47 ,  49 , is slid along its slot until the ball spring plunger  49  is felt to engage in the knife holder base slot, after which the screw  43  is re-tightened. 
     The specimen tube mounting arranged used in the preferred embodiment enables the stability of the specimen to be maintained for sectioning over the full operating travel of the specimen tube. As noted above, in the preferred form of the microtome, the specimen to be sectioned is mounted, by means of a vice and orientation head, on a mounting plate secured to the specimen tube. The movement of the specimen tube backwards and forwards controls the thickness of sections produced by the instrument. The stability of the specimen tube within the instrument is critical to the production of good quality sections. Any large deflections in the specimen tube due to cutting forces can have a detrimental effect on sectioning performance. 
     The mounting method described with reference to the drawings ensures that the tube  50  is still held rigid when it is fully advanced. This is particularly necessary for the preferred embodiment as the knife position is fixed. The specimen must be advanced to the knife and so there is no way of controlling how far the specimen tube is advanced. Other, known, microtomes typically have a movable knife holder so that the knife can be moved to the specimen and hence sectioning can take place with the tube extended a minimum amount.