It is only this process, or its result, the physically or technically isolated ray of particles, which I describe as a 'physical selection' in contradistinction to a merely 'mental' or 'imagined' selection, such as we make when speaking of the class of all those particles which have passed, or will pass, through the range ∆p; that is, of a class within a wider class of particles from which it has not been physically screened off.