In most general terms the techniques of experimental high energy physics are intermediate between two extremes: those methods in which essentially all events initiated in a primary of secondary target are registered in the apparatus, and those in which selectivity is involved before primary data are recorded. Approxlsating the first category are the track chambers which will form the subject of another talk at this conference. As to the items in the second category I shall restrict myself to two: (a) magnetic spectrometers, and (b) selection of photon energies. It is perhaps appropriate that these two topics be emphasized-. in a conference devoted to high-energy electromagnetic interactions. In such interactions usually only a very small fraction of the events occur-ring are "of interest " in the experiment; the great bulk of events are low-momentum transfer, purely electromagnetic events which, at least in principle, do not contain new information. Hence "pre-selection " is of greater importance in this area than in the other branches of high-energy physics
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