attachment of the window 20 as a viewing window 20 for instance a so called viewing cell may be used such as the sitec 740 01 viewing cell sitec sieber engineering ag aschbach switzerland which has a colorless sapphire as the window element viewing cells of this kind satisfy the requirements of the invention with respect to tightness and pressure and temperature resistance of the viewing window with a width of optical view of 6 to 28 mm alternatively a block of transparent industrial diamond may be used as a viewing window for example the diamond block may be soldered to a pressure tight threaded connection for this the diamond material can be brazed under a vacuum which will produce a reliable connection such a unit comprising a screw connection and diamond block can be used as a viewing window 20 according to the invention in place of a transparent diamond any high strength material can be used that will satisfy the requirements with respect to pressure tightness and transparency in the light region that is used such as diamond sapphire etc it should be noted that in contrast to the viewing window 20 and the light passage 21 the remaining structure of the chamber 11 is light impermeable since it is constituted by components made of steel or aluminum for example and any plastic parts that may be used e g seals will be made of light impermeable plastic or rubber as well this prevents any undesirable influence on the sample from disruptive light following photostimulation fig4 shows the sample cartridge 30 which is held in the end piece 16 of the holder 13 fig5 shows an exploded view of the cartridge 30 the end piece 16 is made of.