WHAT assertion will make one believe that in one second of time—one beat of the pendulum of the clock—a ray of light travels over 155,000 miles, and would, therefore, perform the tour of the world in about the same time it requires to wink with our eyelids, and in much less time than a swift runner occupies in taking a single stride? What mortal can be made to believe, without demonstration, that the sun is over a million times larger than the earth, and so far from us, that a cannon ball shot directly toward it, and maintaining its full speed, would be twenty years in reaching it; yet the sun affects the earth appreciably, by its attraction, in an instant of time? Who would not ask for demonstration, when told that a gnat's wing, in its ordinary flight, beats many hundred times in a second? Or that there exist animated and regularly-organized beings, many thousands of whose bodies, laid together, would not cover the space of an inch? But what are these, to the astonishing truths which modern optical inquiries have disclosed, and which teach that every point of a medium through which a ray of light passes, is affected with a succession of periodical movements, regularly recurring at equal intervals, no less than five hundred million of millions of times in a single second? That it is by such movements, communicated to the nerves of the eye, that we are enabled to see; nay, more, that it is the difference in the frequency of these movements alone which causes the diversity of color? That, for instance, in acquiring the sensation of redness, our eyes are affected four hundred and eighty-two million of millions of times; of yellowness, five hundred and forty-two millions of times; and of violets, seven hundred and seven million of millions of times per second? Do not such things sound more like the ravings of madmen, than sober conclusions of people in their waking senses? They are, nevertheless, conclusions to which any one may certainly arrive, who will only be at the trouble of examining the chain of reasoning by which they have been obtained. It is worthy of examination.