The entopic phenomenon called “Haidinger's brushes” (HB) is used both as a diagnostic testing procedure and as a feedback mechanism in visual training. A recent explanation of HB incorporating corneal birefringence is presented. This new theory also explains a clinical phenomenon found by this author. By using a “heat-treated” glass prism, a HB pattern can be enhanced, stopped, or reversed along a continuum of rotation. Clinical application of this phenomenon in testing macular integrity as well as in visual training is discussed.