ABSTRACT
This chapter begins with the study of the ultrahigh energy cosmic ray composition using Cherenkov and fluorescence techniques. There are a number of experiments utilizing Cherenkov light techniques in the region of the "knee." These include the Samarkand, and Moscow State University-Yakutsk arrays; as well as the Buckland Park array near Adelaide, Australia. The Adelaide group measured the shape of the lateral distribution function of Cherenkov light using direct viewing phototubes. Adelaide group finds a rather rapid increase in Xmax across the region of the "knee". The "Fly's Eye" measures the shape of the longitudinal development on a shower by shower basis, and can determine both Xmax and the rate of rise and fall of the shower. Viewing showers in stereo from two eyes is critical for this application because it allows a check on how well the reconstruction of the shower shape is done.
