Nothing is more startling, or more likely to be received with
incredulity by minds unprepared for their reception, than what are, in
common parlance, termed the Marvels of the Universe. The philosophical
writers of our day have strikingly illustrated this fact, which should
be taken into account in writing of the _impedimenta_ to the progress of
science even in our own day. Sir John Herschel has thus forcibly stated
the case:
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.”
Now, the measure of the moon’s distance involves no principle more
abstruse than the measure of the distance of a tree on the opposite bank
of a river; and the Professor shows that the methods used for measuring
astronomical distances are, in some applications, absolutely the same as
the methods of ordinary theodolite surveying, and are in other
applications equivalent to them; and that, in fact, there is nothing in
their principles which will present the smallest difficulty to a person
who has attempted the common practice of plotting from angular
measures.[125]
The habit of beholding the spectacle of the sun gradually sinking, to
disappear after a time below the level of the sea,—this habit, we say,
and our astronomical knowledge, have long since familiarised us with the
phenomenon which, undoubtedly, would appear inexplicable were we to
witness it for the first time, and without being prepared. Who has not
in childhood felt this wonder? The ancients were far from being able to
account for it: some Greek philosophers regarded the sun as an inflamed
mass, which plunged itself every night into the waters of the sea; and
they pretended to have heard a hissing noise! We have found the same
idea lingering among the credulous peasantry of Sussex. We remember our
first nurse, a native of Battle, used to relate that, from the cliffs at
Eastbourne, she had seen the comet of 1769 dip its tail into the sea,
and that she had distinctly heard the “hissing noise.” Such is the
persistence of certain impressions, which, monstrous as they are, can
only be explained away by reasoning.[126]
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Footnote 125:
See Prof. Airy’s _Six Lectures on Astronomy_.
Footnote 126:
See _Things not Generally Known_, First Series, p. 11.
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