Stars are the natural domain of nuclear physics. In them elements are manufactured by fusing light nuclei into heavy ones. Thus is produced the energy that stars give off, and in the various interactions the elusive particles called neutrinos are formed. The existence of the neutrino was postulated to balance the energy equations of nuclear beta decay. Physicists had seen that a number of atomic nuclei decayed radioactively by changing a neutron to a proton and emitting an electron in the process. But there seemed to be some energy lost, and according to a most basic law of physics, conservation of energy, that should not happen. The neutrino was postulated to make up that deficiency. It had to be without electric charge sinlce charges balance in beta decay, and it had to have very little interaction with other matter since no direct evidence of its existence was then available. Twenty-five years after it was postulated, the neutrino was finally directly observed in 1953. The sun is our nearest and most representative star and the only one from which there is a reasonable hope of recording many neutrinos. An experiment to detect solar neutrinos has been under way for some time in the Homestake gold mine at Lead, S.D., by a group of physicists from Brookhaven National Laboratory. Recent results, described by Dr. Raymond Davis Jr. at the meeting of the American Physical Society in Seattle in late August, show fewer solar neutrinos than there ought to be. This, he says, is a mystery that at present no one can explain to the satisfaction of all the different kinds of specialists involved. Because neutrinos are so unlikely to interact with other matter, a very large detector is necessary. The one in the Homestake mine is a tank containing 100,000 gallons of the dry cleaning fluid perchloroethylene (C2C14). The fluid was chosen for its high chlorine content and relatively low cost. The arrival of a neutrino in the detector may convert a nucleus of chlorine to one of radioactive argon 37. In any 50 days of observation the experimenters expected about 25 argon 37 atoms to be produced in the Homestake tank. Argon is chemically inert, and therefore when it is formed, it is not chemically bound as its parent chlorine is. It is merely in solution in the perchloroethylene. It can be removed by circulating helium through the liquid. The argon 37 is then absorbed by a charcoal trap, and the trap is placed in a proportional counter. An argon 37 nucleus decays by capturing an electron from its own atom. This produces a readjustment of electrons that results in emission of an electron with 2.8 kilo-electron-volts energy. This is what the counter looks for. The tank is placed 4,850 feet underground to minimize the effects of particles other than neutrinos. Still it must contend with argon 37 produced by mu mesons from cosmic rays and by slow neutrons from radioactivity in the surrounding rock. One of the gases produced by underground nuclear explosions is argon 37, so the experimenters must also take account of possible increases in atmospheric argon 37 accidentally released by underground tests. One-quarter cubic centimeter of atmospheric argon somehow gets into the tank per 100,000 gallons of liquid. Mu-meson production of argon 37 is measured by placing small tank cars of perchloroethylene at various levels in the mine and comparing the amounts of argon 37 in them. Slow-neutron production is measured by placing a known neutron source at the large tank. Lately a water shield has been built to filter out the slow neutrons. Atmospheric argon 37 is more difficult to estimate since few systematic observations of it have been made (argon comprises about one percent of the atmosphere, and most of that is non-