Documenta Ophthalmologica.

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This volume is dedicated to Karl Wessely, who recently celebrated his seventy-fifth birthday. It also records the untimely death of F. B. Fischer, of Utrecht, one of the editorial staff, whose work, like that of Wessely, is well known in this country, both men having made many notable contributions to the physiology and physical chemistry of the eye.

The original articles are as follows:

• The Two-Quanta Explanation

  1. The Experiments of van der Velden and the Theoretical Foundation of the Two-Quanta Explanation

  2. Theoretical and Experimental Observations on the Two-Quanta Explanation

• Deviations from the Two-Quanta Explanation for Threshold-Values When the Flash-Time and the Visual-Angle of the Test Spot are Large

  1. Our Confirmation of the Two-Quanta Theory When t < τ and d < D

  2. Discussion of the Deviations

• The Two-Quanta Hypothesis as a General Explanation for the Behaviour of Threshold-Values for the Several Receptors of