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.

• From the Phisick collection