are moved in the direction of arrows r and s respectively such that the fibre fringes 28 a 28 b are taken up and clamped as shown in fig5 h the conveyor elements 46 and 47 shown in fig5 h are subsequently moved slowly in the direction of arrows t and u respectively with the result that the fibre fringes 28 a 28 b are aligned straight and substantially horizontally or parallel to the axis of the take up device 34 as shown in fig5 i and 5 k the take up device 34 with the aligned fibre fringes 28 a 28 b is moved along the guide 52 fig4 into the fibrograph 23 the take up device 34 reaches the level of the intermediate space between the light transmitter 54 and the light receiver 55 see fig4 c within the sensor 53 the sensor 53 is subsequently displaced back and forth in the direction of arrows l m fig4 over the take up device 34 as this happens the light transmitter irradiates the fibre fringes 28 a 28 b the light rays passing through are received by the light receiver 55 converted into electrical signals and fed in known manner to an evaluating and display device in this way the fibre lengths and fibre length distribution in the fibre fringes 28 a 28 b are ascertained by means of the fibrograph 23 which reproduces the analysis in the form of a fibrogram fibre fringe curve.