Besides the hearing and sound there is another mode by which we become sensible of the impressions of external objects, namely, sight and light. This subject also offers some observations bearing on our present purpose. It has been declared by writers on Natural Theology, that the human eye exhibits such evidence of design and skill in its construction, that no one, who considers it attentively, can resist this impression: nor does this appear to be saying too much. It must, at the same time, be obvious that this construction of the eye could not answer its purposes, except the constitution of light corresponded to it. Light is an element of the most peculiar kind and properties, and such an element can hardly be conceived to have been placed in the universe without a regard to its operation and functions. As the eye is made for light, so light must have been made, at least among other ends, for the eye. 1. We must expect to comprehend imperfectly only the mechanism of the elements. Still, we have endeavoured to show that in some instances the arrangements by which their purposes are effected are, to a certain extent, intelligible. In order to explain, however, in what manner light answers those ends which appear to us its principal ones, we must know something of the nature of light. There have, hitherto, been, among men of science, two prevailing opinions upon this subject: some considering light as consisting in the emission of luminous particles; others accounting for its phenomena by the propagation of vibrations through a highly subtle and elastic _ether_. The former opinion has, till lately, been most generally entertained in this country, having been the hypothesis on which Newton made his calculations; the latter is the one to which most of those persons have been led, who, in recent times, have endeavoured to deduce general conclusions from the newly discovered phenomena of light. Among these persons, the _theory of undulations_ is conceived to be established in nearly the same manner, and almost as certainly, as the doctrine of universal gravitation; namely, by a series of laws inferred from numerous facts, which, proceeding from different sets of phenomena, are found to converge to one common view; and by calculations founded upon the theory, which, indicating new and untried facts, are found to agree exactly with experiment. We cannot here introduce a sketch of the progress by which the phenomena have thus led to the acceptance of the theory of undulations. But this theory appears to have such claims to our assent, that the views which we have to offer with regard to the design exercised in the adaptation, of light to its purposes, will depend on the undulatory theory, so far as they depend on theory at all.[15]