fig1 to 3 show the schematic structure of a main microscope 1 having an assistant microscope 21 . the assistant microscope 21 is attached to the outside of the main microscope 1 at a fixed angle by means of a mounting fixture 17 ; preferably pivotable around the viewing axis 15 , 15 ′ of the main microscope 1 . the main and assistant microscope 1 , 21 are in particular concerned with stereo microscopes that are used as surgical microscopes , wherein in each case only one of the two stereoscopic beam paths is shown here . in the object plane 10 a normally circular object field 10 ′ is illuminated with incident light illumination by a lighting device that is not shown here . the light of the object from the object field 10 ′ is imaged via the objective 2 of the main microscope 1 inside the binocular tube 3 via a tube lens 30 in the eyepiece intermediate image plane 6 . from there the intermediate image in the intermediate image plane 6 is projected via the eyepiece 4 with eyepiece lens 7 into the eye 5 of the primary observer . the microscope is a microscope with an infinite optical system . here the object plane 10 is at a single focal distance from the objective 2 so that the light coming from the object points passes through the afocal area between tube lens 30 and objective 2 as almost parallel beam bundles and is imaged by the tube lens 30 in its focal plane ( intermediate image plane 6 ). additional optical elements such as in particular a zooming system 32 for setting the zoom may be provided in the afocal area between objective 2 and tube lens 30 . the primary viewing direction 15 corresponds to the axis 15 ′ of the objective 2 . it is perpendicular to the object plane 10 . thus the intermediate image plane 6 is parallel to the principal plane of the objective 2 ( objective plane 2 ′) and to the principal plane of the eyepiece 7 ( eyepiece plane 7 ′), and the entire object field is sharply projected to the eye 5 of the primary observer . for ergonomic reasons , the beam path inside the binocular tube 3 may be deflected by beam - deflecting means 8 in a manner known per se ; the eyepiece plane 7 ′ is then oriented perpendicular to the deflected axis 15 ″. normally , there is an eyepiece aperture in the intermediate image plane 6 , which provides for a defined edge of the viewed image . the assistant microscope 21 has the same structure as the main microscope 1 in accordance with the state of the art ( fig1 ) and also comprises an objective 22 , a binocular tube 23 , a tube lens 20 and an eyepiece 24 having an eyepiece lens 27 . it projects the light of the object from the object plane 10 via the objective 22 via the binocular tube 23 to the eyepiece intermediate image plane 26 . from there the eyepiece lens 27 of the eyepiece 24 projects the light into the eye 25 of the secondary observer . here too , for example , a zooming system 33 may be provided between objective 22 and tube lens 20 . due to the lateral mounting on the main microscope 1 , the assistant microscope 21 is inclined toward the object plane 10 . its viewing direction 16 or more precisely the axis 16 ′ of the objective 22 forms together with the main viewing direction 15 an angle α of typically 10 - 30 °. in this way the object plane 10 is imaged by the objective 22 in an intermediate image plane 26 which is no longer perpendicular to the axis 16 or 16 ′. only the nominal intermediate image plane 34 , which is at about a single focal distance from the eyepiece lens 27 , is sharply projected to the eye by the eyepiece 24 . due to the angled setting , the image in the actual intermediate image plane 26 no longer concurs with this nominal intermediate image plane 34 . therefore the image is rendered significantly more out of focus than is the case for the primary observer and what is more , in addition it is perspectively distorted . as shown in fig2 , this undesired effect can be eliminated in accordance with the invention by viewing the intermediate image plane 26 by means of the eyepiece 24 or more precisely the eyepiece lens 27 via an eyepiece axis 35 , which is perpendicular to the inclined actual intermediate image plane 26 . as a result the full definition across the entire image is achieved again and perspective distortion of the image is eliminated . the position of the intermediate image plane and thus the angle β can be approximately determined as follows : the microscope has an infinite optical system where the image generated by the objective 22 is created at infinity . the beams coming from one object point in the focal plane ( focal distance w ) exit the objective as a virtually parallel bundle , which is merged into a real pixel by the tube lens 20 ( or the tube system ) aligned parallel to the objective 22 in its focal plane ( focal distance f ). the intermediate image is projected into the eye by the eyepiece 24 . the objective 22 and tube lens 20 of the assistant microscope 21 act together like an objective having a finite focal distance with principal planes , which are approximately defined by the objective plane 22 ′ and tube lens plane 20 ′. the line of intersection s 1 of object plane 10 and objective plane 22 ′ is at a distance x from the axis 16 . according to the scheimplfug principle the line of intersection s 2 of the tube lens plane 20 ′ and 20 of the inclined intermediate image plane 26 has the same distance x from the axis 16 . the inclination of the viewing axis 16 relative to the viewing direction 15 of the primary observer is defined by the angle α , which is predetermined by the mounting of the assistant microscope . it applies x = w / tan ( α )= f / tan ( β ), thus β = arctan (( f / w )* tan ( α )). preferably the binocular tube 23 for the insertion of in particular standard eyepieces has cylindrical sleeves 36 , the cylinder axis of which are inclined by the angle β toward the objective axis 16 ′. as a result , when inserting the eyepieces 24 into the sleeves 36 they are already oriented so that their eyepiece axis 35 provides the correct angle of inclination to project the intermediate image 26 into the eye 25 of the secondary observer without distortion . preferably the intermediate image is created inside the binocular tube 23 in order to cut off as little as possible of the light coming from the edge areas of the object field 10 ′ and with this achieve an image that is as vignette - free as possible . a further embodiment is shown in fig3 . here , the tube lens 20 or more precisely its plane 20 ′ is already inclined by an angle γ relative to the objective 22 or more precisely the objective plane 22 ′. due to the inclined tube lens 20 , the axis 16 of the assistant microscope 21 is deviated in contrast to the case of fig2 ; the new axis given the reference number 16 ″. in this way the position of the intermediate image plane 26 is modified relative to the case with a tube lens 20 that is aligned parallel to the objective 22 . consequently , the scheimplfug principle can now be approximately expressed as follows : the distance x of the line of intersection s 1 between the objective plane 22 ′ and the object plane 10 of the initial optical axis 16 corresponds to the distance x of the line of intersection s 2 between the inclined tube lens plane 20 ′ and the intermediate image plane 26 of the deviated axis 16 ″. thus the angle β between the eyepiece axis 35 and the initial axis 16 , 16 ′ is smaller than in the case of fig2 . this variant has advantages with regard to an imaging of the object field 10 ′ which is as vignette - free as possible thanks to the inclined eyepiece 24 . the embodiments which have been described herein can analogously also be used with microscopes having a finite optical system , i . e . without tube lens . in the case of both finite and infinite optical systems , the position of the actual intermediate image plane 26 is preferably calculated taking all optical components in the beam path into account and the eyepiece 24 is arranged in such a way that the eyepiece axis 35 is aligned perpendicular to this actual intermediate image plane . in the case of a simple optical system , the position of the intermediate image plane as displayed in a simplified fashion in the above embodiments can be established in a geometric - optical approach on the basis of the scheimpflug principle . in the case of more complex optical systems , the beam path is simulated in a manner known per se . the beam path of the microscope can be unfolded in a manner known per se using beam - deflecting means . for example , beam - deflecting means can be provided , by means of which the beam path is deflected inside the tube perpendicular to the initial viewing direction . this is , for example , used to accommodate a zooming system and / or other optical components with the optical axis perpendicular to the objective axis in a space - saving manner , for example , as disclosed in de - b 102 55 961 ( corresponds to u . s . pat . no . 6 , 982 , 825 b2 ). likewise , flexible optical waveguides may be arranged within the beam path between tube lens and eyepiece , in order to provide a moveable tube with adjustable alignment of the eyepieces without changing the effective viewing angle , for example , as described in de 105 03 463 . it should be pointed out here , that the optical waveguides are arranged in such a way that the observer looks vertically down on the inclined intermediate image plane . furthermore , the beam path can be deflected in the eyepiece side end part of the tube in a manner known per se using mirrors or the like . here too , it should be pointed out that the eyepiece is arranged in such a way that its axis is perpendicular to the inclined intermediate image plane now imaged by the mirror . the effective viewing angle is not changed by the above - described beam deflection , i . e . the observer continues to look “ from above ” or “ from the side ” onto the object and the intermediate image plane , even if the eyepiece is tilted in relation to the tube and / or the beam path is directed through the laterally disposed zooming system . as mentioned above , the above - described angles α and β are to be defined without said deflection using opto - mechanical deflection elements . 15 ″ viewing axis of the main microscope deviated by a deflection element 16 ″ viewing axis of the assistant microscope deviated by the inclined tube lens