  As the largest gravitationally collapsed objects, and as objects with a
relatively low space density, clusters of galaxies offer one of the best sets
of standard candles for trying to measure basic cosmological parameters such as
the injectivity diameter 2r_{inj} (the shortest distance between two
topologically lensed images of any object, e.g. cluster) and the out-diameter
2r_{+} (the maximum `size' of the Universe). Present constraints indicate that
either of these may be smaller or larger than the horizon diameter.
