Physicists now tally more than 100 subatomic particles, and it sometimes seems as if a new one is added to the list every week. The number is embarrassingly high, and it raises in acute form the question of what is elementary. Some theorists, who prefer a world in which there are not so many particles with equal claim to being elementary, have worked out theories in which there is a simpler fundamental level underlying the present particles. All the present particles would be constructed in various ways out of a few subentities. The long-sought quark is one kind of proposed subunit. The experimental question is whether or not actual particles exhibit the kind of structure proposed in these theories. Is a proton, for example, built up out of identifiable subunits or is it a single undifferentiated body? Recent experiments in which other particles have been scattered off protons can be interpreted both ways. One reading indicates that protons may be composed of subunits and that the probe particles tend to bounce off one of these constituents. Whether the constituents correspond to any of the theoretically proposed subentities is not clear, and Richard P. Feynman has given them the neutral name partons. Another interpretation favors models in which the proton's innards are not granular. In this case the proton acts on a beam of incoming particles in a way analogous to the diffraction of a light beam by a block of glass: these models therefore go by the term diffractive. Two experiments now nearly ready to run at the Stanford Linear Accelerator are designed to find out more about what happens in high-energy collisions of other particles and protons. One approaches the question from the parton side and one from the diffractive-model side. They may find that partons do exist and discover something about their properties, or they may not. The evidence for the parton side goes back particularly to experiments done at SLAC and first reported in 1969 (SN: 8/30/69, p. 164). In these experiments, called deep inelastic scattering, in which the electrons excited the target nucleon to high energies and in which a large amount of momentum was transferred from the electrons to the targets, appeared a simplification of the results that has since become famous as scaling. In these results at high levels of momentum transfer the mathematical expression that represents the structure of the proton no longer depends separately on the energy transferred to the proton from the electrons and on the momentum transferred, but rather on a simpler combined quantity, their ratio. One possible explanation of this scaling is that the proton has a grainy structure so that the incoming particle interacts with one part (hence the term parton) rather than with the whole proton. Since 1969 experiments using other kinds of projectiles-pi mesons, K mesons and gamma rays-have shown possibly related scaling phenomena. At the same time other phenomena more compatible with diffractive models were coming to light in experiments in which high-energy photons were struck against protons. In these cases the photon, as it approaches the proton, appears to turn itself into one of three vector mesons, rho, phi or omega, and it is the transformed particle that interacts with the proton. The theory that describes this transformation is called vector meson dominance or rho dominance (about 75 percent of the time the change is to a rho), and it is one of the diffractive models. One of the new SLAC experiments, which will use mu mesons as projectiles, continues the procession of the scaling-law or possible-parton experiments. (It is being done by a collaboration of three SLAC groups: Elliot D. Bloom, R. L. Cottrell, H. DeStabler, L. Gershwin, M. Mestayer, C. Prescott, S. Stein of Group A; J. Ballam, T. Carrol, G. Chadwick, M. DeLaNegra, K. Moffet of Group B; and L. Keller of Experimental Facilities group.) The other experiment is more concerned with the possibility of meson dominance. It will use a beam of electrons as projectiles. (The experimenters are J. Dakin, B. Dieterle, G. Feldman, W. Lakin, F. Martin, E. Petraske, M. L. Perl and William T. Toner.) What both experiments hope to learn is what happens when a proton and a virtual photon meet, a situation that will occur in both cases. A virtual photon can have very different properties from a real one. A real photon is one that is flying free and can be detected, in a light beam or an X-ray beam, for instance. A virtual photon is one that is emitted and absorbed so quickly that its existence cannot be detected. In both these experiments it is virtual photons that will carry energy and momentum between SLAC ELECTRON BEAM AT 20.2 GeV/C