OPTICAL AND ELECTRICAL MEASURING DEVICES, NAMELY, PHOTOMETRIC, SPECTROSCOPIC AND LASER APPARATUS FOR MEASURING CONCENTRATIONS, COLOR AND TURBIDITY IN FLUIDS OR GASSES; ANALYTICAL DEVICES, NAMELY, DEVICES FOR ON-LINE CONCENTRATION MONITORING OF A CONTENT IN A FLUID OR GAS AND FOR USE IN COLOR AND TURBIDITY MEASUREMENTS; PROBES FOR USE IN PHOTOMETRIC, SPECTROSCOPIC OR LASER APPARATUS FOR MEASURING CONCENTRATIONS, COLOR AND TURBIDITY IN FLUIDS OR GASES; OPTICAL GLASS AND LENSES FOR USE IN THE AF...