Flexible light guide

An instrument for use with a source of infra red radiation for applying radiation within a living body for medical purposes comprising a flexible light guide which comprises a column of a liquid having a predetermined refractive index and selected to absorb as little as possible of the wavelength range of said radiation, a flexible tube, surrounding and containing the liquid, of plastics material which has a refractive index somewhat smaller than that of the liquid in the wavelength range to be transmitted and is provided with a liquid-tight window at one end fashioned in a manner suitable for insertion into a living body and another liquid-tight window at the other end, said other end being fashioned in a manner suitable for receiving radiation from a radiation source outside the living body, the liquid in said tube being at rest, and a liquid supply container which is located near the radiation receiving end of said tube and is connected via a liquid duct with the interior of the flexible tube.