The Accommodation Reflex and its Stimulus

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Abstract
There is a mechanism for the adjustment of accommodation quite apart from the accommodation-convergence synergic reflex. The change in accommodation was measured by the movement of the 2 halves of the retinal image using a Coincidence Optometer with a test object at a distance of 4 meters. Addition of a positive lens caused no accommodative change, but a negative lens produced an effort of accommodation which was relaxed on withdrawing the lens. The eye can, therefore, distinguish between a blurred image due to converging rays, and one due to diverging rays. Chromatic aberration of the eye, by providing a color fringe on the retina which varies according to the refractive state, is a factor in this reflex adjustment of accommodation, but 40% of observers were able to carry out the reflex even when monochromatic light was used. Quick scanning movements of the eye preceded any accommodative change. This may indicate that the Stiles-Crawford effect is the means whereby the visual mechanism interprets a difference of vergence into terms of a difference of light stimulus.

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