reference will be made first of all to fig1 where a postal envelope of conventional type is shown by way of example , called window envelope . this envelope 10 comprises then a window 12 in which appear the name and address of the addressee , a stamp 14 or a postal franking mark in its upper right - hand corner , as well as cancelling stamps and an indexation code 16 in the lower right - hand corner . this indexation code is formed of a series of fluorescent bars printed on envelope 10 and the arrangement of which corresponds to the coded transcription of the postal code of the address of the addressee . the indexation code may comprise twenty bars 18 ( routing code ) and possibly nineteen additional bars ( distribution code ). when the envelopes 10 travel at high speed , all positioned and oriented in the same way , their indexation code 16 may be read by a device described in the above mentioned french patent , which device controls an automatic sorting machine . when they are sorted , the letters are grouped together by destinations and form more or less high piles in compartments provided at the outlet of the sorting machine . these piles of letters will then be formed into bundles , for example using a plastic material foil , to which a label will be affixed indicating the destination of the bundle . for automatically printing these labels , it is necessary to read automatically the indexation code transferred to envelopes 10 situated on the top of each pile . however , since these piles are motionless , or else are moving at a slow speed on a conveyor belt 20 comprising transverse jolting dividing walls 22 against which the letters bear by one of their edges , and since the upper letters of the piles may assume a variable orientation with respect to the dividing walls 22 , as shown in fig2 it will be understood that it is not possible to use the device described in the prior patent for reading the indexation codes 16 of these envelopes . this problem is solved by the invention by a fine scanning of the surface of envelope 10 which bears the indexation code 16 , such scanning being carried out by means of a raster of successive lines l1 , l2 , l3 , etc ... parallel to a fixed predetermined direction which is perpendicular to bars 18 of code 16 when letter 10 is correctly positioned , for example by bearing against a jolting edge 22 . in practice , depending on the relative orientation of each upper letter , the scanning lines will very often be oblique with respect to the direction of bars 18 of code 16 , as shown in fig3 . more specifically , the invention compensates for the shift of the amplitude peaks of the signals of the photodetector , which shift is due to the obliqueness of the scanning lines with respect to bars 18 . each passage of the impact point of the light beam over a bar 18 during scanning results in the formation of an amplitude peak 24 in the signal produced by the photodetector . the amplitude peaks 24 corresponding to the same bar 18 are shifted from one scanning line to the next , as shown in fig4 . this shift is compensated for in the following way : its existence may first of all be ascertained when , from one scanning line to the next , two amplitude peaks are shifted by a relatively small distance . in fact , for a spacing of 0 . 5 mm between the scanning lines and for an angle of 30 ° between the direction of the scanning lines and the perpendicular to the bars 18 of the code , the shift will be equal to the spacing multiplied by the sine of 30 °, namely about 0 . 25 mm . this very small shift cannot be confused with the distance normally separating the amplitude peaks of two consecutive bars , which is 1 . 66 mm . when such a shift is ascertained it means then necessarily , when it is repeated from one scanning line to the next , an obliqueness of the scanning lines with respect to the direction of the bars . if the position of the fixed reference point is known on each scanning line , it is easy to compensate for the above shift , by calculation , by moving the position of this reference point on the scanning line by a distance equal to the shift but in the opposite direction and thus proceeding from one scanning line to the next . the obliquely aligned amplitude peaks in fig4 are thus aligned on lines perpendicular to the scanning lines , as shown in fig5 to which would correspond the relative arrangement of the bars shown in fig6 with scanning lines which would be strictly perpendicular to the bars . a correlation may be established between the arrangement of the signals in fig5 and the arrangement of the bars in fig6 by forming a cumulative total or summing the amplitudes of the transversely aligned peaks detected on successive scanning lines . when the impact point of the light beam on envelope 10 has a dimension which corresponds substantially to the spacing between the scanning lines , the cumulative total of the amplitudes of the peaks relative to the same bar will correspond substantially to the amplitude of a signal produced by a photodetector receiving the image of a bar in accordance with the technique described in the above mentioned french patent , all other things being equal . in other words , detection of the bars shown in fig6 by the prior device and detection of the bars shown in fig3 by the device of the invention will produce signals of the same type , which may be processed in a similar way . to further strengthen this similarity , the cumulative total of the amplitudes of the aligned peaks corresponding to the same bar may be weighted by the number of scanning lines on which an amplitude peak has been detected for this bar , which will make it possible to take into account the resemblance of the graphic form to a bar , while associating with this bar only two pieces of information ( its position and the amplitude of the detected signal ). reference will now be made to fig7 which shows schematically a reading device in accordance with the invention . this device comprises a light source 28 , such as an argon laser , a quartz or iodine lamp and associated filter or else a semiconductor laser and frequency doubler assembly . the cylindrical light beam 30 produced by this source is reflected , by a mirror 32 , on to a polygonal mirror 34 mounted for rotation about an axis 36 parallel to the surface of the letter 10 to be scanned . the polygonal mirror 34 is in fact an annular mirror with flat facets 38 having an angle to the center of 10 ° for example , which corresponds to a number of facets equal to 36 for mirror 34 , and to a deflection of the beam of 20 °. the light beam 30 is reflected by each facet 38 on to a mirror 40 of very small area which reflects it back on to the envelope 10 to be scanned . mirror 34 is driven in rotation about axis 36 in the direction shown by the arrow so that the light beam is initially reflected as shown at 30 1 , then finally at 30 2 , by travelling over a scanning line l between these two positions . the light beam reflected by the surface of envelope 10 is recovered by a wide angle lens 42 with great depth of field , so as to form an image of the scanning line l on the input window 44 of an anamorphoser 46 , in particular of the optical fiber type , which transforms the image of the scanning line l into a circular surface adapted to the input window of a photodetector 48 . a lens 50 may be inserted between the output of the anamorphoser 46 and the photodetector 48 for image dimension matching . a filter 52 selecting the fluorescence spectrum is moreover placed in front of photodetector 48 . the optical assembly comprising the return mirror 32 , the rotary polygonal mirror 34 , the return mirror 40 , lens 42 , the anamorphoser 46 , the optical assembly 50 , filter 52 and photodetector 48 is carried by a plate mounted for rotation about a transverse pivoting axis 54 which is parallel to the surface of envelope 10 and perpendicular to the above axis 36 . if it is desired to avoid rotating the light source 28 , axis 54 may be substantially aligned with the axis of the light beam 30 at the output of source 28 . in a variant , it is possible to not move the optical assembly but to insert a pivoting mirror 100 of adequate area in the path 30 1 , 30 2 for scanning and recovering the light emitted by fluorescence . it should be further mentioned that mirrors 32 and 40 have no other functional purpose than to make the optical assembly compact but do not form part of the method of the invention . this device of the invention also comprises , for determining on each scanning line a reference point whose fixed position is known , an optical fiber 56 one end of which is placed in the path of light beam 30 1 at the beginning of the scanning line and whose other end is associated , for example by means of a lens 58 , with the input window of a photodetector 60 . the passage of the light beam 30 1 over the end of fiber 56 will produce , via the photodetector 60 , a synchronisation blip which may be considered as a reference point of known position on each scanning line . of course , the optical fiber 56 and possibly lens 56 or photodetector 60 will have to follow the rotational movement about axis 54 . to accommodate the variations in height of the piles of letters , a lens 42 is used for reading , at the level of letter 10 , an impact point of the light beam having a diameter of a few tens of millimeters over a field depth of at least 120 mm . furthermore , the scanning surface travelled over by the impact point of the light beam is 200 × 250 mm 2 . if the spacing between scanning lines is 0 . 5 mm , 400 scanning lines are required each of a length of 250 mm . the corresponding scanning time is between a few tenths of a second and about a second . reference will now be made to fig8 which shows schematically the circuits for processing the signal produced by photodetectors 48 and 60 . the output of photodetector 48 is connected by an amplifier 62 to a low - pass filter 64 whose output is connected both to an a - d converter 66 and to a differentiating circuit 68 associated with a signal front detection circuit 70 whose output is connected to the a - d converter 66 and to the input of a fifo memory ( shift multi - register ) 72 to the other input of which the output of the a - d converter 66 is also connected . the photodetector 60 locating a fixed point at the beginning of the scanning line is connected by a tripping circuit 74 to a line number counter 76 and to the reset input of a counter 78 driven by a clock 80 . the outputs of counters 76 to 78 are connected to inputs of the fifo memory 72 . the latter is associated with a microprocessor 82 connected to a communication interface 84 and controlling the servo circuits 86 and 88 of electric motors 90 and 92 which drive the rotary polygonal mirror 34 and the optical assembly mounted for rotation about axis 54 . the amplitude variations of the signal produced by photodetector 48 are processed by amplifier 62 and the low - pass filter 64 then are digitized by converter 66 before being stored in memory 72 . the differentiating circuit 68 associated with the front detection circuit 70 differentiates the digitized signals corresponding to amplitude peaks of the signal of the photodetector and those corresponding to hollows between which the amplitude peaks are situated and tripping the converter 66 . furthermore , the photodetector 60 counts the scanning lines by means of tripper 74 and counter 76 and resets the counter 78 at the beginning of each scanning line . clock 80 makes it possible to locate on each scanning line the position of the amplitude peaks and hollows stored in memory 72 . the microprocessor 82 drives motors 90 and 92 via servo circuits 86 and 88 then acquires in memory 72 the amplitudes of the peaks and hollows of the signals from photodetector 48 , as well as their position ( number of the scanning lines and position of the peaks and hollows in the scanning lines ). software , recorded in a memory associated with microprocessor 82 , provides the following processing : - calculation of the amplitudes of the significant peaks of the signals from the photodetector 48 ( amplitude of the peak -- half sum of the amplitudes of the adjacent hollows ), - correction of the position of the significant peaks in the case where the scanning lines are oriented obliquely with respect to the direction of the bars of the code , - summing or forming the cumulative total of the information obtained on each scanning line ( sum of the amplitudes of the peaks with concordant position on successive scanning lines ) and association with this information of the position of the corresponding amplitude peak and the number of scanning lines concerned ), - identification of the structure of the indexation code either from groups of three pieces of information ( position , sum of the amplitudes of the peaks and number of scanning lines ) or from groups of two pieces of information ( position of the amplitude peaks and amplitude of these peaks weighted by the number of scanning lines concerned ). one of the advantages of the invention is that the software for carrying out this processing is very little different from the software associated with the method and device described in the above mentioned french patent , so that a man skilled in the art , for implementing the invention , will only have to adapt the known software . the invention may be applied not only to reading and recognizing indexation codes appearing on postal envelopes but also bar codes appearing on labels of objects or different products , on checks and other documents .