receiver e experiences a continuous shift between the two positions l1 and l1 39 and thereby scans specifically the threads 1 to 8 of the row of threads to be monitored at the same time the necessary swiveling or rotating movement of the mirror 13 is extremely small compared with the displacement distances of transmitter s and or receiver e required in the case of the arrangements illustrated in fig1 4 and 5 such small movements do not necessarily require a mechanical drive but can also be carried out quasi mechanically for example by means of bi metal bending rods or piezoelectric components of course the exemplary embodiments described provide a person skilled in the art with a further range of possibilities of arranging for a light beam to pass transversely over a row of threads by means of moved light source and or mirror of these such possibilities in which a specifically moved part is not necessary for the movement of the light beam but an already existing movement of the textile machine can be utilized are especially interesting in particular such an arrangement is illustrated in fig7 and 8 here fig7 shows a view in the direction of the axis h fig 1 of a row of threads to be monitored in the region of the so called front roller of a ring spinning frame and fig8 shows a view in the direction of the arrow vili of fig7 according to the representation the threads 1 to 8 are lead over the rotatably driven front roller 14 of the drawing mechanism and lie in a defined plane in the monitoring.