of coordinates represents the measured blade width the distance from the tending side end is indicated in the horizontal system of coordinates as millimeters as shown in the chart the measured wear of the doctor blade is uneven increasing toward the driving side end away from the tending side end fig2 a is a side view of the device in accordance with the invention in which a doctor blade 11 doctors a face of a roll 10 and a jet s is introduced from a jet pipe 12 to the forward side of the doctor blade 11 the direction of rotation of the roll is indicated by arrow l 1 three lubricant ducts are passed into the jet pipe 12 i e ducts 13a 1 13a 2 and 13a 3 and each duct comprises regulation means such as regulation valves 14a 1 14a 2 14a 3 for regulating the flow quantity of lubricating medium to the respective duct flow meters and pressure gauges c 1 c 2 c 3 d 1 d 2 d 3 are also positioned between the valves 14a 1 14a 2 14a 3 and the ducts 13a 1 13a 2 13a 3 as such it is possible to monitor and measure the amount of flowing lubricating medium passing into compartments in formed in the interior of the jet pipe 12 along the width thereof i e in a direction transverse to the running direction of the roll through the duct 13a 1 the lubricating medium is passed into the compartment g 1 in the interior of the jet pipe 12 through the duct 13a 2 the lubricating medium is passed into the compartment g 2 in the interior of the jet pipe 12 and.