Introduction

Ocham's Razor puts the astronomer into an uncomfortable quandry. It tells us ``if you can't detect something, then assume it's not there''. But with our limited technical capabilities the Universe may well be filled, even dominated, by objects we cannot presently see.

We live inside the spiral arm of a comparatively high surface brightness galaxy ourselves, so the sky is bound to be relatively bright. On a dark night at a good observing site only one percent of the light from the darkest parts of the sky is coming from beyond our Galaxy. All objects that are dimmer (i.e. lower in SB) than the terrestrial glare will be difficult to detect. And even when they are detected the accurate measurement of the true sizes and luminosities against the glare will be very uncertain. At the moment optical astronomers have only a very hazy idea about the true population of dim (Low SB) galaxies. The largest fair sample of spirals we could muster contained only 65 members! Too much dogmatism is therefore out of place. One can currently conjecture that the ``The great majority of galaxies, even in our neighbourhood, remain to be found.''

© Copyright Astronomical Society of Australia 1997