ver 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, it
yet affects the earth by its attraction in an inappreciable 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 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 500
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
482 millions of millions of times; of yellowness, 542 millions of
millions of times; and of violet, 707 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.

_Extraordinary Property of Shadows_.

An eminent living geometer had proved by calculations, founded on strict
optical principles, that in the _centre of the shadow_ of a small
circular plate of metal, exposed in a dark room to a beam of light
emanating from a _very small brilliant point_, there ought to be no
darkness,--in fact, _no shadow_ at that place; but, on the contrary,
a degree of illumination precisely as bright as if the metal plate were
away. Strange and even impossible as this conclusion may seem, it has
been put to the trial, and fo