The author's attention was called to this subject in consequence of having had an attack of incipient cataract forty years ago. The laminæ of the lens had separated so much as to exhibit rays of light, and the prismatic colours, round every source of light. At the end of eight months the laminæ came into optical contact, the rays and colours disappeared, and the eye became as perfect as the other, —a proof that cataract, in its incipient stage, may be discovered optically, and may be cured. During a series of experiments on the changes in the crystalline lenses of animals after death, the author observed that the capsule of the lens transmitted water into the lens till it burst; and hence he concluded that cataract might arise from an unhealthy state of the aqueous humour,—hard cataract from too much albumen in the aqueous humour, and soft cataract from too much water.