The use of fine glass fibres for transmitting light signals. The fibres used, which are normally less than 1 millimetre in diameter, are made of glass of a high refractive index, coated with a material of lower refractive index. Light signals fed into the fibre undergo successive internal reflections within the core fibre, which thus acts as a light guide. The method works even when the fibre is considerably curved. Images can be transmitted using bundles of very fine fibres in a fixed array. Such techniques have a variety of applications in astronomical instrumentation.