The two most important existing theories of the mechanism of the accommodation of the eye are directly opposed in that they assign differently the effect of the ciliary muscle in the production of the change in the form of the crystalline lens. The theory of Helmholtz, which is the more universally accepted, states that the lens swells and increases in convexity during accommodation because the tension upon it is relaxed when the ciliary muscle contracts.

In order to explain the change of the anterior surface of the lens to a hyperbolic form in accommodation, Tscherning has propounded a theory according to which the tension of the lens is maintained when the muscle contracts, and the forms of the surfaces are altered by pressure exerted by the vitreous humour upon the periphery of the posterior lens surface.

This paper describes the measurement of the radii of curvature of both anterior and posterior surfaces of the lens, and also the movement which the apices of these surfaces make in accommodation. In two selected cases of men of the same age and having the same refractive error, considerable differences in the behaviour of the lens in accommodation were found. The results show that for a given amount of accommodation, whereas the lens surfaces in one case are more increased in curvature than in the other, their apices suffer less movement. It was also found that the surface which was most altered in curvature showed the most pronounced hyperbolic form in both relaxed and accommodated conditions. The elastic property of the lens capsule and its variations in thickness are described. The changes in the form of the lens surfaces and the differences which occur in the lenses of different individuals in accommodation are explained upon the assumption of the Helmholtz theory, by taking into account the properties of the lens capsule.