  The existence of surface nanobubbles has been previously suggested using
various experimental techniques, including attenuated total reflection
spectroscopy, quartz crystal microbalance, neutron reflectometry, and x-ray
reflectivity, but all of these techniques provide a sole number to quantify the
existence of gas over (usually) hundreds of square microns. Thus `nanobubbles'
are indistinguishable from a `uniform gassy layer' between surface and liquid.
Atomic force microscopy, on the other hand, does show the existence of surface
nanobubbles, but the highly intrusive nature of the technique means that a
uniform gassy layer could break down into nanobubbles \textit{due to} the
motion of the microscope's probe. Here we demonstrate \textit{optical}
visualisation of surface nanobubbles, thus validating their individual
existence non-intrusively.
