To observe any object in the sense world we need an appropriate medium of observation. For ordinary things, light provides this. In the sense in which light is understood to-day, this is possible because the spatial extension of the single light impulses, their so-called wavelength, is immeasurably smaller than the average magnitude of all microscopically visible objects. This ensures that they can be observed clearly by the human eye. Much smaller objects, however, will require a correspondingly shorter wave-length in the medium of observation. Now shorter wave-lengths than those of visible light have been found in ultra-violet light and in X-rays; and these, accordingly, are now often used for minute physical research.