This rather hypnotic looking device
tests for astigmatism by measuring the regularity of the cornea. It
is particularly useful in diagnosing keratoconus - a thinning of the
cornea which causes it to protrude resulting in irregular
astigmatism which can only be corrected with rigid contact lenses
and sometimes needs a corneal transplant. The examiner looks through
the central lens with black side towards him/herself and the
concentric rings pointing towards the patient. The reflection of the
image from the back of the cornea eye causes the concentric lines to
deviate where there is irregularity of the corneal surface.
