Illumination is by means of a vertical illuminator placed above the objective.
The amplitudes and waveforms, as well as precision of timing, of the signal components are made visible. A virtual image has no real existence in space as does a real image from a lens. It does have a definite location, however, caused by the angles of divergence of the rays received by the eye. This can be shown by the common school experiment of placing a pin coincident with its mirror image behind a sheet of glass acting as a partial mirror. Its location can also be placed in design by extrapolating backwards to a focus.