Photographs were taken of cosmic-ray showers arising in lead plates of various thickness placed in turn inside a Geiger counter controlled cloud chamber. These showers are classified according to the number of particles in the shower and according to the manner of production, whether by an ionizing particle or by nonionizing radiation. The average size of showers was found to increase with the plate thickness. Studies of the frequencies of showers of different sizes as a function of plate thickness show that electron-produced and photon-produced showers occur in approximately equal numbers and are of the same character. These facts give support to the theories of Carlson and Oppenheimer and of Bhabha and Heitler. They consider the shower to be the result of repeated subdivision of incident radiation through pair production by gamma-rays and through radiative losses of high energy electrons giving rise to other gamma-rays. Eight percent of the electrons striking lead plates near seven millimeters thick gave rise to showers. About an equal number of showers from gamma-rays were present.