The Virgo Cluster has played a key role in the development of modern theories concerning the organiza- tion of the universe. In the late eighteenth and early nineteenth centuries, the English father and son astronomers William and John Herschel discovered in the direction of the Virgo Cluster an excess of what were then called “nebulae” but are now known to be galaxies. The significance of their discoveries was not recognized for a number of years, because the true nature of the nebulae was unclear. Some astronomers thought they were clouds of gas or stars in our own galaxy; others believed them to be “island universes” far beyond the Milky Way.
    In the 1920s American astronomer Edwin Hubble, using