The luminous efficiency of rays entering the eye pupil at different points

Abstract The effect to be described in this paper was discovered in attempting to develop an apparatus for measuring the area of the eye pupil, using a photometric principle. The idea involved is made clear by the diagram fig. 1. The subject applies his eye to the eye-ring E and fixes his eye on the aperture in the diaphragm D. He then sees a photometric field divided into two parts. The lower half is illuminated by light diffused from the opal O, which itself receives light from the lamp S. The rays from this half of the field form a diverging beam which completely fills the pupil of the subject’s eye. The upper half of the field is illuminated by the method of Maxwellian view. An image of the small aperture P is focussed by the lens L on to the middle point of the eye-ring E. This image is only about 1.5 mm. in diameter and all the light is collected by the subject’s eye, provided the pupil of the latter is concentric with the eye-ring and has a diameter exceeding 1.5 mm. A variable graduated neutral wedge W is inserted at the aperture P and this enables the two halves of the photometric field to be adjusted to equality of brightness by the subject.