When Prof. Yandell Henderson of Yale—one of the many people whose names are well known, and liked, on either side of the Atlantic ocean—came to England five years ago, he defined the objects of all scientific investigation in a peculiarly happy and paradoxical way, such as always lingers in the memory—"to understand, to predict, to control." Of these, the third is the last to be attained and the greatest, for unless one has first gained an understanding of the nature of basic phenomena, prediction is inaccurate and control impossible. In ophthalmology, so far as the basic facts of the metabolism of the eye are concerned, the first stage has not even been reached ; there is a long way to go before one can say that one understands ; it is not even possible to begin to prophesy or to direct events in the way one would choose with any