A single proton from a 19 GeV pencil focused beam leaves a track at the right of the picture as it enter through the beam window of the 1 m long Ecvole Polytechnique HLBC and penetrates a copper target of the same lenght and diamter as the one used in the magnetic horn. In its interaction with one of copper nuclei, the proton produces the two shorter tracks which are also protons, two longer tracks which are pions, and gamma-rays which have materialized in the bottom left corner of the chamber as electron-positron pairs. From about 100 000 trios of such stereo pictures, the input spectrum of pions and kaons for the magnetic horn can be determined experimentally, and subsequently by calculation, the neutrino spectrum