“In the case of a work which is a mere exhibition of skill in conventional art, there may be some excuse for the delusion that the longer the artist works on it the nearer he will bring it to perfection. Yet even the victims of this delusion must see that there is an age limit to the process, and that though a man of forty-five may improve the workmanship of a man of thirty-five, it does not follow that a man of fifty-five can do the same.”