Fig. 21. The origin of the on-off phenomenon at neutrality. The far point of the eye is situated at the peephole of the retinoscope. Either all or none of the rays will pass through the peephole with the slightest shift in the subject's eye or the retinoscope or the retinoscopist's eye, causing the retinoscopist to see the contents of the pupil as either filled with light or black.