the invention will now be explained with reference to the above figures wherein like references denote like parts . fig2 is a side elevation showing the principle of an apparatus according to the invention . a copyboard 20 and a glass plate 1 together form an original image plane over which a document to be scanned can be transported along the sensor system . the sensor system includes lenses l and sensors s . each of the sensors s may be constructed with , for example , a ccd array . the sensors s are disposed in the direction perpendicular to the drawing plane and in extension of one another . the document to be scanned ( not shown ) is transported from left to right between two driven roller pairs 21 , 22 and 23 , 24 respectively . a pressure - application plate 25 presses the document against the glass plate 1 . a tubular lamp 26 illuminates the document from the bottom directly . tubular lamp 26 also illuminates the document indirectly via the reflector 27 . pressure - application plate 25 , which is painted white , forms a second reflector that reflects light passing through the document into the optical system . the line 29 indicates the optical path between the scanned part of the document and the sensors s , through the lenses l . this optical path contains an aligning element 7 , which in this exemplary embodiment is formed by a thin rod having a tapering diameter , just beneath the glass plate 1 , in the direction of document transport . the front elevation of the apparatus is similar to the apparatus shown in fig1 a ( with the conventional aligning element 2 being replaced by the inventive aligning element 7 ). therefore , the front elevation of the invention will be explained with reference to fig1 a . in this exemplary embodiment , the sensor system includes two sensors s1 , s2 , each having a lens l1 , l2 . each of the sensors senses a linear area of the document through its lens . these linear areas , which have the width of one pixel , e . g . approximately 60 μm , are hereinafter referred to as the observation area and are indicated by o1 and o2 , respectively . as shown in fig1 a , the observation areas o1 and o2 overlap each other to a small extent . the aligning element 2 ( which is to be replaced by the inventive aligning element 7 ) is disposed at a short distance beneath the glass plate 1 in the overlapping part of the fields of view of each of the two sensors s1 and s2 . the distance between the aligning element 7 and the original image plane is selected such that the aligning element 7 is still imaged sufficiently sharply on the sensors s1 , s2 to enable the edges thereof to be recognized . an inventive aligning element 7 that replaces the conventional aligning element 2 is shown in an enlarged scale in fig3 . the ends 31 and 32 are cylindrical with a substantially constant diameter , and the middle part 33 has a diameter which tapers continuously from the diameter of end 31 to the diameter of end 32 . the aligning element may also be described by revolving a planar shape around an axis extending in the sub - scanning direction . the middle part 33 is used for the adjustment as will be explained hereinafter . real values for the diameters of the ends 32 , 31 are 1 and 4 mm respectively , while the apex angle α of the cone described by the middle part 33 is , for example , 60 degrees . of the parts 31 , 32 and 33 shown in fig3 only the middle part 33 is essential . the other parts need not necessarily be present but are added here for mounting purposes . in the exemplary aligning element 7 shown in fig3 the middle part 33 is purely conical , but the taper could also be curved as shown in fig3 a . more specifically , the aligning element 7 shown in fig3 a has a curved middle part 33 &# 39 ;. other usable shapes for the aligning element 7 are , for example , a flat element or an assembly of wires extending in intersecting directions , preferably situated in a plane parallel to the glass plate 1 . fig3 b and 3c illustrate this assembly of wires . as shown in fig3 b , the aligning element 7 may be constructed with two stretched wires or two thin rods 35 , 36 , so fixed that their projections onto the original image plane extend at an angle to one another . the example shown in fig3 b aligns one such wire or rod element 35 in the subscanning direction and the other element 36 in a direction forming an acute angle β therewith to form a middle part 33 &# 34 ;. another example is shown in fig3 c in which two elements 37 , 38 each extend at an acute angle γ relative to the subscanning direction to thus form a symmetrical middle part 33 &# 39 ;&# 34 ;. the aligning element can also be formed by a pattern of , for example , paint applied to the bottom of the glass plate 1 . the important factor is that the image of the aligning element 7 detected by the sensors s1 , s2 should have two recognizable positions in the main scanning direction , wherein the positions have a previously known relationship with positions in the sub - scanning direction . to construct the scanner apparatus , the sensors s1 , s2 are accurately mounted in position by suitable hardware which are not described in detail here . after the adjustment , the observation areas o1 , o2 of the two sensors s1 , s2 are substantial extensions of one another and overlap in the main scanning direction , in such manner that the aligning element 7 is situated substantially opposite to the middle of the overlapping part of the observation areas o1 , o2 and the middle part 33 of the aligning element 7 is intersected approximately half - way by the observation areas o1 , o2 . impacts or other physical shocks to the apparatus , e . g . on moving it or misalignment caused by disposing the apparatus on an uneven floor , may distort the adjustment of the sensors s1 , s2 to some extent . usually , the observation areas o1 , o2 are then displaced over a short distance , so that they no longer connect - up in the sub - scanning direction and their overlap is different . rotation of the sensors s1 , s2 is also possible , but in practice this is so small that no correction is generally required . fig4 shows the condition after a displacement of the sensors s1 , s2 . fig4 also shows the aligning element 7 and the two observation areas o1 and o2 . the points of intersection r1 , 1 and r1 , 2 of o1 with the edges of the aligning element 7 and the points of intersection r2 , 1 and r2 , 2 of o2 with the edges of the aligning element 7 are also indicated . as a result of the imaging of the observation areas o1 , o2 on the sensors s1 , s2 these intersection points coincide with individual ccd elements of the sensors and , because the ccd elements are distributed uniformly , also coincide with coordinates within the sensors s1 , s2 . the procedure for determining the correction values required to compensate for any deviations in the adjustment of the sensors s1 , s2 is shown in fig5 . a scan is made without any document so that only the aligning element 7 is visible against the white pressure - application plate 25 . the coordinates of the points of intersection r1 , 1 , r1 , 2 , r2 , 1 and r2 , 2 are first determined in step 52 from the digital image data generated by the sensors s1 , s2 . in step 54 , the centerpoints m1 =( r1 , 2 + r1 , 1 )/ 2 and m2 =( r2 , 2 + r2 , 1 )/ 2 are calculated and stored in a memory . from these centerpoints m1 , m2 , the connecting points where the image data of sensor s1 and the image data of sensor s2 are connected to one another during the scanning of documents are calculated by adding , from points m1 and m2 respectively , a number of pixels ( ccd elements ) that are experimentally predetermined , to the middle of the sensor . this is necessary because the aligning element does not lie in the original image plane and the sensors s1 , s2 detect the aligning element 7 from two directions at an angle , so that parallax occurs . in this way , point p ( fig1 a ) is determined , which forms the perpendicular projection of the center of the aligning element 7 onto the original image plane . this point is chosen as the connecting point for the image data from the sensors s1 and s2 . the point p is indicated by p1 and p2 in the image data from the sensors s1 and s2 , respectively . of course , any other point situated in the free field of view of the two sensors can be selected as the connecting point . determination thereof in the image data is similar . in step 56 , the widths w1 = r1 , 2 - r1 , 1 and w2 = r2 , 2 - r2 , 1 of the projection of the aligning element 7 on the sensors s1 and s2 , respectively are calculated . these widths w1 , w2 are converted to a mutual distance a in accordance with the formula where a is half the apex angle α of the conical part of the aligning element 7 . when scanning documents , the image data from the sensor s1 or s2 must be delayed by a time interval corresponding to the transport of the document over the distance a ( the choice of the sensor is determined by the sign of a ). the value a is expressed in units of the distance between the scan lines and converted to two values a1 relating to sensor s1 , and a2 relating to sensor s2 . the values a1 and a2 are also stored in a memory . fig6 is a diagram showing a circuit for compensating deviations in the adjustment of the sensors s1 and s2 , based on the correction factors calculated in accordance with the above description . the sensors s1 and s2 are each connected to an analogue / digital ( a / d ) converter 61 and 62 respectively , and deliver analogue serial image data thereto . the analogue image data are converted to digital serial image data by the a / d converters 61 , 62 . a / d converter 61 is connected to a switch 63 and a / d converter 62 is connected to a switch 64 . during the procedure for determining the correction values as described with reference to fig5 the switches 63 and 64 are in the state shown in fig6 and connect the sensors to an analysis circuit 65 which performs the procedure described above . the analysis circuit 65 is connected to a control circuit 66 , which will be described hereinafter , and delivers thereto the correction values p1 , p2 , a1 and a2 . the control circuit 66 is provided with a memory for the temporary storage of the correction values . after the procedure for determining the correction values has been performed , the switches 63 and 64 are set to their other mode , in which they connect the a / d converters 61 and 62 to a switching unit 67 which connects one of the a / d converters to a combining circuit 71 and the other a / d converter to a bank 68 of series - connected shift registers . the shift registers have a length of one scan line of the sensors s1 , s2 . the individual shift registers are connected to a multiplexer 69 . together with the multiplexer 69 the shift registers form a delay circuit having a delay selectable in units of one scan line spacing . the state of the switching unit 67 determines which of the two sensor signals is delayed . the multiplexer 69 is , in turn , connected to the control circuit 66 and its output is connected to combining circuit 71 , which is also connected to a processing device 72 ( not described in detail ) for the digital image data , e . g . a printing device . during the scanning of a document , the digitized image data from one of the sensors s1 and s2 are delayed by passing them through the shift register bank 68 . the multiplexer 69 passes the image data from one of the shift registers under the control of the control circuit 66 in such manner that the image data being passed relates to the same position in the sub - scanning direction on the document , despite any displacement of the sensors s1 and s2 . the scan lines thus delayed are fed by the multiplexer 69 to the combining circuit 71 , which combines them with the scan lines from the other sensor , received directly from the switching unit 67 . in this way , combining circuit 71 removes from the scan lines of both sensors the image data originating from the end that is overlapped by the other sensor , to the position p1 and p2 respectively , and combines the remaining image data to form a complete image line . the combining circuit 71 is constructed in manner generally known per se so that the scan lines incoming simultaneously from the two sensors are placed in the correct time sequence . the resulting image lines are fed consecutively to a processing circuit . it should be noted here that the term &# 34 ; circuit &# 34 ; does not mean solely hardware , but also can mean a microprocessor loaded with suitable software . in the above - described exemplified embodiment only one aligning element 7 is provided in the overlapping area of the observation areas o1 , o2 associated with the sensors s1 , s2 . of course , it is also possible to provide , just outside the area for scanning , alignment bodies 7 at the free ends of the complete field of view formed by the combination of the observation areas . in this way , for each of the observation areas , position determination in the sub - scanning direction becomes possible for both ends , so that it is possible to determine from this any skewing of the sensors so that it is also possible to correct the same . in the above description , the invention is equipped with two sensors , but the invention can be provided with more than two sensors that have an observation area overlapping the observation area of the adjoining sensors . in that case , an aligning element 7 is provided in each of the overlapping areas . the invention being thus described , it will be obvious that the same may be varied in many ways . such variations are not to be regarded as a departure from the spirit and scope of the invention , and all such modifications as would as would be obvious to one skilled in the art are intended to be included within the scope of the following claims .