 Bridgman thus emphasized that our concepts did not automatically extend beyond the domains in which they were originally defined. He warned that concepts in far-out domains could easily become meaningless for lack of applicable measurement operations. The case of length in the very small scale makes that danger clear. Beyond the resolution of the eye, the ruler has to be given up in favor of various micrometers and microscopes. When we get to the realm of atoms and elementary particles, it is not clear what operations could be used to measure length, and not even clear what “length” means any more.