Abstract: In order to measure the refractive index of a fluid use is made of a rod (1) which projects into the fluid and has a refractive index which is slightly larger than that of the fluid. At the end which is not immersed in the fluid, the rod is provided with a light source (3) radiating into the rod and with a light detector (5). That part of the rod which is to be immersed in the fluid is partially surrounded by a casing which permits only total reflection of the light, while at the site of the part or parts of the rod not surrounded by the casing refracted light can, depending on the refractive index of the fluid, leak into the fluid. A casing which is stable against the corrosive action of an aggressive fluid, such as concentrated sulphuric acid in a battery, consists, according to the invention, of a layer (10), trapped in a transparent tube (8), of a medium which has a lower refractive index than the said rod, in particular gas or vacuum.

Abstract: A method and an apparatus for detecting low concentrations of at least one (bio-) chemical component present in a test medium in a test cell, having a metal layer as sub wall with an external glass prism, using the surface plasmon resonance effect. A light ray is coupled in and, after attenuated total reflection, is coupled out and the intensity thereof is measured. The incidence angle position of the resonance curve is determined under the influence of the change, caused by the component, in the dielectric constant of the test medium near the metal layer. An adjustable selector is applied to the metal layer, in order to influence the incidence angle position of the resonance curve, through which the concentrations or concentration changes of one or more components in the test medium can be simultaneously determined through one or more differential measurements. A preferential association and therefor a higher concentration at the metal layer of one component above another is brought about.