An in-line hologram is recorded without an explicit spatially separate reference beam. This technique, which is simple and capable of very high resolution, requires that the objects be small (in practice less than one millimetre) and that a large fraction of the area of the beam be not obscured by objects. Replay is by placing the developed hologram in the conjugate beam. In the in-line hologram the real and virtual images are formed simultaneously along the depth axis.