he had marked out for himself; but the very steadiness and strength of purpose with which he pursues it indicate the degree to which his mind had lost its wonted elasticity. In 1784 and 1785 he was traversing a portion of the same road. But then he was in the prime of his powers, and accordingly we recognize a versatility which enabled him to test and reject the methods of research which presented themselves to his mind. It was in those years that he invented his famous method of star-gauging, which our text-books of astronomy preposterously adopt as if it were an established and recognized method of scientific research. But Herschel himself, after trying it, and satisfying himself that it was unsound in principle, abandoned it altogether. In 1817 he adopted a method of research equally requiring to be tested, and, in my conviction, equally incapable of standing the test; but he now worked upon the plan he had devised, without subjecting it to any test. Nay, results which only a few years before he would certainly have rejected—for he did then actually reject results which were open to the same objection—passed muster in 1817 and 1818, and are recorded in his papers of those dates without comment. We may recognize another illustration of the loss of elasticity with advancing years, in the obstinacy, one may even say the perversity, with which Sir Isaac Newton, in the latter years of his life, adhered to opinions on certain points where, as has since been shown, he was unquestionably wrong, and where, had he possessed his former mental versatility, he must have perceived as much. Compare this with his conduct in earlier years, when for nineteen years he freely abandoned his theory of gravitation—though he had fully recognized its surpassing importance—simply because certain minute details were not satisfactorily accounted for. Many other instances might be cited, were it worth while, to show how the mind commonly changes when approaching an advanced age, in a manner corresponding to that bodily change—that stiffness and want of elasticity, without any marked loss of power—which comes on with advancing years. That old age does not necessarily involve any loss of power for routine work, has been clearly shown in the lives of many eminent men of our own era. The present Astronomer Royal for England affords a remarkable illustration of the fact, as also of the associated fact that new work is not easily achieved, nor an old mistake readily admitted or corrected at an advanced age.
It is well pointed out by Dr. Beard, in the lecture to which I have already referred, that "we must not expect to find at one age the mental qualifications due to another age—we must not look for experience and caution in youth, or for suppleness and versatility in age. We ought also to apportion to the various ages of a man the kind of work most suitable to them. Positions which require mainly enthusiasm and original work should be filled by the young and middle-aged; positions that require mainly experience and routine work, should be