Page:Popular Science Monthly Volume 22.djvu/493

Rh  should take a peep through a certain key-hole. The allegory reveals the great arcanum of nature, the secret, namely, that the human shape divine has been evolved from the form of a fish."



T has become an almost consecrated custom for the President of this Association, instead of relating the progress which has been made in all the sciences that are the objects of your investigations, to treat more particularly of a single one of them, and to present a summary of the progress it has made. This custom appeal's an excellent one to me. By it we gain in precision and authority what we lose in extensiveness; and we owe to it many masterly efforts, the impression of which has not yet been effaced from our minds, and which cause in me a just apprehension that I may fall far short of their standard.

I shall endeavor to present to you a picture, sketched in broad outline, of the progress and influence of a branch of research which has played a considerable part in contemporary scientific movements, and the discoveries of which have not only revolutionized our astronomical knowledge, but have also opened new and unexpected horizons to philosophy—I mean physical astronomy.

Physical astronomy is a wholly modern—yes, as to its best parts, contemporary—science; and it can be regarded as old only as concerning its object. From the earliest times, in fact, that men began to look toward the sky, and the first reflections on nature were born of these first observations, man has asked what is that sun whose immense and beneficent function caused it to be designated at that early period as the soul of nature. He has asked himself what is the cause which lends to the moon the sweet and mysterious light that gives to the nights so poetical a charm; and afterward questions arose concerning those brilliant points with which the celestial vault is strewed. All these problems appertain to our science, but how little was man then in a condition to deal with them! Long ages of observations and labor were necessary before even the corner of the veil could be raised.

This was because physical astronomy presupposes a very clear knowledge of the properties of light, both as to itself and as to its relations with bodies, and great perfection in the mechanical arts to 