Transcriber’s notes:
This e-book comprises the text of a lecture delivered on November 12,
1801 and subsequently published in the Philosophical Transactions of
the Royal Society, Issue 92 (1802), pp. 12–48.
The transcription has mostly been preserved in its original form,
including minor punctuation inconsistencies. However, near the end
of the book (preceding a table) there are four expressions that have
had to be displayed as pseudo fractions because of the limitations
of a plain text transcription. In the original, the 'numerators'
and 'denominators' are positioned one above the other but without a
separating line (vinculum).
Following the table there is a quotation derived from Hooke’s
_Micrographia_. It is not a true verbatim quotation, however, but
rather a selection of phrases and sentences extracted from Hooke’s
text, and it contains several seemingly random quotation marks that
were not present in the latter; they have been omitted from this
transcription.
II. _The Bakerian Lecture. On the Theory of Light and Colours.
By_ Thomas Young, _M.D. F.R.S. Professor of Natural Philosophy in the
Royal Institution._
Read November 12, 1801.
Although the invention of plausible hypotheses, independent of any
connection with experimental observations, can be of very little use
in the promotion of natural knowledge; yet the discovery of simple and
uniform principles, by which a great number of apparently heterogeneous
phenomena are reduced to coherent and universal laws, must ever be
allowed to be of considerable importance towards the improvement of the
human intellect.
The object of the present dissertation is not so much to propose any
opinions which are absolutely new, as to refer some theories, which
have been already advanced, to their original inventors, to support
them by additional evidence, and to apply them to a great number of
diversified facts, which have hitherto been buried in obscurity.
Nor is it absolutely necessary in this instance to produce a single
new experiment; for of experiments there is already an ample store,
which are so much the more unexceptionable, as they must have been
conducted without the least partiality for the system by which they
will be explained; yet some facts, hitherto unobserved, will be brought
forwards, in order to show the perfect agreement of that system with
the multifarious phenomena of nature.
The optical observations of NEWTON are yet unrivalled; and, excepting
some casual inaccuracies, they only rise in our estimation, as we
compare them with later attempts to improve on them. A further
consideration of the colours of thin plates, as they are described in
the second book of NEWTON’s optics, has converted that prepossession
which I before entertained for the undulatory system of light, into a
very strong conviction of its truth and sufficiency; a conviction which
has been since most strikingly confirmed, by an analysis of the colours
of striated substances. The phenomena of thin plates are indeed so
singular, that their general complexion is not without great difficulty
reconcileable to any theory, however complicated, that has hitherto
been applied to them; and some of the principal circumstances have
never been explained by the most gratuitous assumptions; but it will
appear, that the minutest particulars of these phenomena, are not only
perfectly consistent with the theory which will now be detailed, but
that they are all the necessary consequences of that theory, without
any auxiliary suppositions; and this by inferences so simple, that
they become particular corollaries, which scarcely require a distinct
enumeration.
A more extensive examination of NEWTON’s various writings has shown
me, that he was in reality the first that suggested such a theory as
I shall endeavour to maintain; that his own opinions varied less from
this theory than is now almost universally supposed; and that a variety
of arguments have been advanced, as if to confute him, which may be
found nearly in a similar form in his own works; and this by no less a
mathematician than LEONARD EULER, whose system of light, as far as it
is worthy of notice, either was, or might have been, wholly borrowed
from NEWTON, HOOKE, HUYGENS, and MALEBRANCHE.
Those who are attached, as they may be with the greatest justice, to
every doctrine which is stamped with the NEWTONIAN approbation, will
probably be disposed to bestow on these considerations so much the
more of their attention, as they appear to coincide more nearly with
NEWTON’s own opinions. For this reason, after having briefly stated
each particular position of my theory, I shall collect, from NEWTON’s
various writings, such passages as seem to be the most favourable to
its admission; and, although I shall quote some papers which may be
thought to have been partly retracted at the publication of the optics,
yet I shall borrow nothing from them that can be supposed to militate
against his maturer judgment.
HYPOTHESIS I.
_A luminiferous Ether pervades the Universe, rare and elastic in a high
degree._
_Passages from_ NEWTON.
“The hypothesis certainly has a much greater affinity with his own,”
that is, Dr. HOOKE’s, “hypothesis, than he seems to be aware of; the
vibrations of the ether being as useful and necessary in this, as in
his.” (Phil. Trans. Vol. VII. p. 5087. Abr. Vol. I. p. 145. Nov. 1672.)
“To proceed to the hypothesis: first, it is to be supposed therein,
that there is an ethereal medium, much of the same constitution with
air, but far rarer, subtler, and more strongly elastic.—It is not to
be supposed, that this medium is one uniform matter, but compounded,
partly of the main phlegmatic body of ether, partly of other various
ethereal spirits, much after the manner that air is compounded of
the phlegmatic body of air, intermixed with various vapours and
exhalations: for the electric and magnetic effluvia, and gravitating
principle, seem to argue such variety.” (BIRCH. Hist. of R. S. Vol.
III. p. 249. Dec. 1675.)
“Is not the heat (of the warm room) conveyed through the vacuum by the
vibrations of a much subtiler medium than air?—And is not this medium
the same with that medium by which light is refracted and reflected,
and by whose vibrations light communicates heat to bodies, and is put
into fits of easy reflection, and easy transmission? And do not the
vibrations of this medium in hot bodies, contribute to the intenseness
and duration of their heat? And do not hot bodies communicate their
heat to contiguous cold ones, by the vibrations of this medium
propagated from them into the cold ones? And is not this medium
exceedingly more rare and subtile than the air, and exceedingly more
elastic and active? And doth it not readily pervade all bodies? And is
it not, by its elastic force, expanded through all the heavens?—May
not planets and comets, and all gross bodies, perform their motions in
this ethereal medium?—And may not its resistance be so small, as to be
inconsiderable? For instance, if this ether (for so I will call it)
should be supposed 700,000 times more elastic than our air, and above
700,000 times more rare, its resistance would be about 600,000000 times
less than that of water. And so small a resistance would scarce make
any sensible alteration in the motions of the planets, in ten thousand
years. If any one would ask how a medium can be so rare, let him tell
me—how an electric body can by friction emit an exhalation so rare
and subtile, and yet so potent?—And how the effluvia of a magnet can
pass through a plate of glass, without out resistance, and yet turn a
magnetic needle beyond the glass?” (Optics, Qu. 18, 22.)