A method of bonding together two optical surfaces, such as a scintillator plate and a plate which is formed by the ends of a bundle of optical fibres. An assembly of this kind can be used in an electron microscope in order to convert the electron image into an image of visible light which is transported, for example to a detector. It is specified that the two surfaces to be bonded must be bonded rigidly to one another without an intermediate layer and without a noticeable intermediate space( i.e. a space much smaller than the wavelength of the visible light). In accordance with the invention, this is achieved by optically polishing and cleaning the surfaces to be bonded, after which they are arranged one against the other without an intermediate material, the contacting surfaces then being subjected to a thermal treatment at a temperature beyond the fusion temperature but below the softening temperature of the material of each of the surfaces.