The diagram shows (a) the patient's eye; (b) the position of the observer; (c) and (l) lenses for giving an almost parallel beam of light. The optical arrangement ensured that the parallel beam of light illuminated the retina and was not cut off by the observer.

Pen and ink drawing.

Source: Hermann von Helmholtz. Handbuch der Physiologischen Optik. L Voss, Leipzig 1867.