An apparatus for studying the diffraction of fast electrons by gaseous molecules is described. It is essentially conventional, but there are incorporated, both in the camera itself and its ancillary instruments, a variety of features which increase considerably the ease of manipulation, the quality of the diffraction patterns recorded, and the overall accuracy of the final results. Among the more important of these are: (i) a two-lens electron optical system, (ii) a device enabling direct measurement of the acclerating voltage to an absolute accuracy of 0.1%, (iii) a method of measuring to the nearest tenth of a millimetre the distance between the scattering material and the centre of the recording plate, and finally, (iv) a "quasi-hydrodynamic" jet of simple design and considerable temperature range mounted to achieve stability of position yet flexibility of adjustment.