Abstract: Receptors are employed to which one or more fluroescent molecules are bound. When the receptors are bound to the epitopic sites on the surface and light of predetermined wave length is directed toward the surface, the fluorescing molecules are activated and fluoresce.In carrying out an assay, receptor is combined with an unknown suspected of containing molecules having the same epitopic sites bound to the surface. The receptor will bind to these molecules reducing the number of receptor sites available for binding to the epitopic sites on the surface. When the assay medium is contacted with the surface, the amount of receptor which binds to the surface, will be a function of available binding sites and, therefore, to the number of the molecules present in the unknown. Upon irradiation of the surface, substantially only the fluoroescent molecules bound to the surface will fluoresce. By monitoring the fluorescence, one can determine the presence and number of molecules of interest present in the unknown.

Abstract: An improved spectrophotometer of the non-dispersive absorption type useful for detecting the presence of SO.sub.2 in a gaseous sample, despite the simultaneous presence of NO.sub.2. Ultraviolet light from a commercially-available source impinges upon a fixed phosphor screen which fluoresces to produce ultraviolet radiation at a first wavelength, at which both SO.sub.2 and NO.sub.2 absorb radiation; and at a second wavelength at which absorption by SO.sub.2 is minimal but NO.sub.2 absorption is approximately the same as at the first wavelength. The radiation is directed through a sample chamber having windows which are transversely slidable to effect cleaning without destroying the integrity of the chamber seal, then introduced into an envelope enclosing filters and a pair of photosensors. One photosensor receives light at the second wavelength and produces a signal which reflects the attenuation by NO.sub.2 at that wavelength.

Abstract: Apparatus is disclosed for providing a visual representation of the absorption or transmission coefficients of the elements of a two dimensional matrix of elements notionally defined in a cross-sectional plane through a body. The representation is in the form of an analogue display comprising superimposed lines of information scanned on the surface of a suitable screen, the brightness of each line being indicative of the absorption suffered by penetrating radiation on traversing a respective path through said plane of the body. The orientation of each scanned line depends on the orientation of the respective path with respect to the body.