In an excited atom, we have observed, the phenomenon of light emission; and by analyzing the spectrum of the light emitted., we learned. how the atomic electrons behave around the atomic nucleus. Similarly, in the case of an 'excited nucleus', we also observed that there are radiations emitted. Yet, they consist of various different kinds of particles besides photons, e. g.: alpha-particles, deuterons, protons, neutrons, electrons, mesons, etc.. Usually the energy involved is much greater than that occurs in the case of atomic radiation. It is obvious that if we want to form any picture of nuclear structure, certainly first we have to analyze the nuclear spectrum in the same way as we did in studying atomic structure, i. e, to measure the energy values of the individual radiations respectively and to find the relations between them.