The question of the maximum size of aperture that can be usefully employed to photograph an object through atmospheric turbulence constitutes one of the classic questions of atmospheric optics. The answer to this question lies in the determination of the spatial-correlation function of the electric field at the aperture of the camera. Although measurements of the correlation function of the irradiance furnish some relevant information, a satisfactory answer cannot be given by such measurements alone because they describe only the statistics of the amplitude of the radiation but not of the phase. This paper describes a new interferometer with which the spatial correlation of the electric field can be measured directly. The result of the measurement is in the form of an interferogram that displays the degree of coherence of the light over the aperture of the camera in terms of the visibility of a nominal fringe pattern. The interferometer requires no high-quality optical components. The results of a set of preliminary experiments to measure the spatial coherence of laser light after propagation through the atmosphere over horizontal paths are described.