track with straight vertical segments connected by semicircles or any other shape above and below the measurement chamber 80 wall 108 transitions into rigid walls 110 or is connected to such walls the mounting 93 for the second element 90 that is formed by wall 108 thus is moved by drive 106 continuously in the direction of arrow 102 toward the first element 88 and back in addition the said mounting 93 is moved for purposes of conveying the sample through the measurement chamber 80 in the flow direction 130 which breaks up the sample or avoids bridging and or jamming of the sample in the measurement chamber 80 the movement of wall 108 can serve to trigger a measurement by the transmission spectrometer when the elements 88 and 90 are at a distance from each other that is suitably matched to the wavelength of the light through the sample via the control device 104 which knows the position of the drive 106 and thus the cam 94 through an appropriate detector and or the control means of the drive 106 which is designed as a step motor or servomotor if the amplitude of the movement of the second element 90 is not sufficient for this the control device 104 can cause an additional drive 112 if necessary to move the entire curved track 96 and with it cams 94 and thus also wall 108 in the direction of arrow 102 by an additional drive 112 of course the first element 88 could also be mounted on wall 108 while the second element 90 then is affixed to the solid wall.