References

Helmholtz, H. von (1856-1866). Handbuch der physiologischen optik. Leipzig: Voss.

Stiles, W. S. (1946). A modified Helmholtz line-element in brightness-colour space. Proceedings of the Physics Society of London, 58, 41-65. Stiles, W. S. (1959). Color vision: The approach through increment-threshold sensitivity. Proceedings of the National Academy of Sciences, 45, 100-114.

STILES-CRAWFORD EFFECT. In 1933, the English physicist Walter Stanley Stiles (1901-1985) and the English physiologist Brian Hewson Crawford (1908-1963) showed that light falling on different parts of the pupil of the eye is not equally effective in producing a sensory result, even though the light may reach the same point on the retina. In particular, the Stiles-Crawford effect is a demonstration that light rays passing through the edge of the pupil stimulate the retina less than those rays passing through the center of the pupil because edge rays and center rays do not meet the same conditions along their paths going to a given point on the retina. The majority of the Stiles-Crawford effect is due to the properties of the retina itself and is related to foveal cone vision in the light-adapted eye. The effect is found, also, in any part of the retina when deep red illumination is used as the light source, indicating again that the phenomenon is obtained predominantly as a response of the cones in the retina. The exact origin of the Stiles-Crawford effect is not certain, but best guesses ascribe it either to the shape of the cones or to the direction in which they point, and where the total internal reflection within the conical point of the cones may be responsible for a concentration of the light in the peripheral areas. The fact that the Stiles-Crawford effect is absent in pure rod vision may indicate that the phenomenon is due to differences between rods and cones in shape, in refractive index, and in the distribution of the photoreceptive pigments. The quantitative characteristics of the Stiles-Crawford effect have been calculated in detail by P. Moon and

D. Spencer. See also ABNEY'S LAW; STILES' COLOR VISION THEORY. REFERENCES

Stiles, W. S., & Crawford, B. H. (1933). The luminous efficiency of rays entering the eye pupil at different points. Proceedings of the Royal Society of London, 112B, 428-450. Stiles, W. S., & Crawford, B. H. (1934). The liminal brightness increment for white light for different conditions of the foveal and parafoveal retina. Proceedings of the Royal Society of London, 116B, 55-102. Moon, P., & Spencer, D. (1944). On the Stiles-Crawford effect. Journal of the Optical Society of America, 34, 319-329.

O'Brien, B. (1946). Theory of the Stiles-Crawford effect. Journal of the Optical Society of America, 36, 506509.

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