IN connection with a theoretical study of the lighting of rooms, we found it necessary to compare visual data obtained by various investigators. Some of the researches had been made with the natural pupil and others with an artificial pupil, and a method was required for translating results from one of these conditions to the other.

Formerly it was assumed that the visual effect of reducing the pupil size could be exactly compensated by an increase in the helios of the test field, the product of helios and pupil area being the same in both cases. The discovery of the Stiles-Crawford effect in 1933, however, changed this simple picture and complicated the transition from one set of data to another. Rays entering near the edge of the pupil were found to be much less effective visually than rays near the center of the pupil. Though numerous data were obtained, no standard average was suggested and no satisfactory empirical formula for the effect was advocated. Thus, although previous comparisons of visual data have been made with the Stiles-Crawford effect included, the process up to this time has always been a numerical or graphical one with no comprehensive analytic treatment. The following presentation consists of an examination of the available data, first on pupil size and then on the Stiles-Crawford effect. Average curves are obtained and analytic expressions are developed. The method is presented in the hope that it will expedite the work of others who may be confronted by problems involving pupil size and the Stiles-Crawford effect.