The optical property of fluorescence—the emission of visible light by certain resins while they are illuminated with ultraviolet light—can be used to advantage. Objects which do not fluoresce can be stained with fluorescent dyes. The spectral range of wavelengths which cause fluorescence (activation spectrum), as well as the color of the light emitted through fluorescence by the object (emission spectrum), are optical characteristics by means of which the resins may be studied. The optical principle of and instrumentation for fluorescence microscopy, as well as details of special techniques, are described.