ONE IS accustomed to think of a prism as a wedge-shaped piece of glass which bends a ray of light traversing it in the direction of its base. That this ray of light affects the position of the corneal light reflex in a paradoxic manner is not generally known.
Displacement of a corneal light reflex from a fixational position is known to indicate ocular deviation, and any artificial method which could restore such displacement to a fixational position can likewise serve to register the amount of such deviation. Such artificial restoration by binocular instruments employing flexibly moving lighted targets through a measured range is established practice. That a corrective prism could likewise restore a displaced corneal light reflex to a fixational position was noted by me in a previous report.1
Maddox,2 in his excellent treatise entitled "The Clinical Use of Prisms," devoted a chapter to the internal reflection
KRIMSKY E. EFFECT OF A PRISM ON THE CORNEAL LIGHT REFLEX. Arch Ophthalmol. 1948;39(3):351-370. doi:10.1001/archopht.1948.00900020358010