ABSTRACT
The term accommodation refers to the change in the refractive power of the eye that allows images of near objects to be focused on the retina. The most widely accepted theory to account for the mechanism of accommodation was proposed by Hermann von Helmholtz in 1855 in his Treatise on Physiological Optics. He observed that accommodation involves pupillary constriction and anterior movement of the iris. Helmholtz carefully observed the Purkinje images of the crystalline lens during accommodation using crossed glass plates placed between the subject’s eye and the observer viewing the eye with a telescope. He observed an increase in curvature of the anterior and posterior surfaces of the lens, although the anterior surface became more convex than the posterior surface. He noted that the sagittal thickness of the lens increased and hypothesized that the equatorial diameter of the lens decreased during accommodation. He proposed that these events occurred through contraction of the ciliary muscle. This anterior and axial movement of the muscle results in relaxation of zonular tension, which allows the lens (which is surrounded by its elastic capsule) to round up and increase in optical power, with the lens equator moving away from the sclera (1,2).
