the dyeing vessel 10 are closed which prevents a further increase of the temperature since the vessel 28 moves in the direction of decreasing thickness the reference fluid vessel becomes more transparent because the thickness and the concentration are proportional values the potential taken from the photo electric receiver 36 thereby increases and finally exceeds that of the photo electric receiver 24 so that at the output of the amplifier 40 depending on the voltage difference at the input a positive voltage is developed the latter influences the control valves of the arrangement 46 and causes an adjustment of the temperature to a value permitting an optimum attaching process the concentration in the dyeing vessel 10 is therefore reduced the voltage taken from the photo electric receiver 24 is increased the voltage difference at the inputs of the amplifier 40 changes in such a manner that the output pulse becomes less or even negative and the control valves will thus be throttled or closed an opening of the control valves and a temperature increase again takes place when the light ray admitted to the photo electric receiver 36 penetrates the vessel 28 through a smaller thickness so that its potential becomes higher than that of the photo electric receiver 24 by this continuous changing of the steam supply and the change and adjustment of temperature connected therewith the attaching process takes place under optimum conditions.