An interference pattern is obtained with a Michelson interferometer; the intensity distribution in the pattern is determined by counting the rate of photons by means of a photomultiplier. It is shown that the pattern obtained for very low intensities of light does not differ outside the margin of experimental error from the pattern obtained for normal intensities. At low intensities about 10 photons enter the interferometer per second thus for these low intensities in average at any time much less than one photon is contained inside the interferometer.