Images taken with large telescopes show that no two galaxies are alike and that the variety of galactic forms is endlessly fascinating. Indeed, photographs from Mount Wilson's great reflectors inspired Edwin Hubble to order galaxies along the lines of his now-famous “tuning-fork” sequence of galaxy types. The visual observer, however, is at the mercy of the eye's limited angular resolution (20 or 30 arcminutes) at low light levels. Thus only the broadest categories of galaxies can be differentiated. Nevertheless, by noticing all you can about a galaxy's structure, you can make a fairly reliable guess about its classification, or Hubble type. The origin and physical significance of galaxy types remains an area of active research, and they have yet to be completely accounted for in most professionals' eyes.