What mere assertion will make any one believe that in one second of time, in one beat of the pendulum of a clock, a ray of light travels over 192,000 miles, and would therefore perform the tour of the world in about the same time that it requires to wink with our eyelids, and in much less than a swift runner occupies in taking a single stride? What mortal can be made to believe, without demonstration, that the sun is almost a million times larger than the earth? and that, although so remote from us that a cannon-ball shot directly towards it, and maintaining its full speed, would be twenty years in reaching it, yet it affects the earth by its attraction in an appreciable 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 exists animated and regularly-organised beings, many thousands of whose bodies laid close together would not extend an inch? But what are these to the astonishing truths which modern optical inquiries have disclosed, which teach us 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 millions of millions of times in a single second! That it is by such movements communicated to the nerves of our eyes that we see; nay, more, that it is the difference in the frequency of their recurrence which affects us with the sense of the diversity of colour. That, for instance, in acquiring the sensation of redness, our eyes are affected four hundred and eighty-two millions of millions of times; of yellowness, five hundred and forty-two millions of millions of times; and of violet, seven hundred and seven millions of millions of times per second. Do not such things sound more like the ravings of madmen than the sober conclusions of people in their waking senses? They are, nevertheless, conclusions to which any one may most certainly arrive who will only be at the trouble of examining the chain of reasoning by which they have been obtained. Professor Airy, however, considers this difficulty to be over-estimated. He observes, that “persons who take great interest in Astronomy appear to regard the determination of measures, like those of the distance of the sun and moon, as mysteries beyond ordinary comprehension, based perhaps upon principles which it is impossible to present to common minds with the smallest probability that they will be understood; if they accept these measures at all, they adopt them only upon loose personal credit; in any case, the impression which the statement makes on the mind is very different from that created by a record of the distance in miles between two towns, or the number of acres in a field.”