the leuco dye material of the original images 2a to produce an image as already mentioned the printing master which includes the sheet 3 and the original images 2a is mounted on the drum 5 which is then set into rotation the copy sheets 8 are then fed towards the drum 5 prior to reaching the drum 5 the copy sheets 8 pass through the device 7 which moistens them thereafter the copy sheets 8 pass between the drum 5 and the counter pressure roller 6 which latter urges the copy sheets 8 into engagement with the original images 2a so that an image forming reaction takes place of course the copy sheets 8 are to be positioned in such a manner that it is the layer 10 thereof which faces the drum 5 and is contacted with the original images 2a of the printing master fig3 shows part of a copy sheet designated generally at 8 39 which has just passed between the drum 5 and the counterpressure roller 6 and it may be seen that images 2b 39 have been produced on the layer 10 as the drum 5 continues to rotate and copy sheets 8 continue to be fed to the printing location between the drum 5 and the counter pressure roller 6 there exists the possibility of for example a faulty alignment of one of the copy sheets 8 so that.