_Miscellaneous Optical Researches of Newton—His Experiments on
Refraction—His Conjecture respecting the Inflammability of the
Diamond—His Law of Double Refraction—His Observations on the
Polarization of Light—Newton’s Theory of Light—His “Optics.”_
Before concluding our account of Newton’s optical discoveries, it is
necessary to notice some of his minor researches, which, though of
inferior importance in the science of light, have either exercised an
influence over the progress of discovery, or been associated with the
history of other branches of knowledge.
One of the most curious of these inquiries related to the connexion
between the refractive powers and the chymical composition of bodies.
Having measured the refractive powers and the densities of twenty-two
substances, he found that the forces which reflect and refract light
are very nearly proportional to the densities of the same bodies. In
this law, however, he noticed a remarkable exception in the case of
unctuous and sulphureous bodies, such as camphire, olive oil, linseed
oil, spirit of turpentine, and _diamond_, which have their refractive
powers two or three times greater in respect of their densities than
the other substances in the table, while among themselves their
refractive powers are proportional to their densities, without any
considerable variation. Hence he concluded that diamond “is an unctuous
substance coagulated,”—a sagacious prediction, which has been verified
in the discoveries of modern chymistry. The connexion between a high
degree of inflammability and a great refracting force has been still
more strongly established by the high refractive power which I detected
in phosphorus, and which was discovered in hydrogen by MM. Biot and
Arago.
There is no part of the optical labours of Newton which is less
satisfactory than that which relates to the double refraction of
light. In 1690, Huygens, published his admirable treatise on light,
in which he has given the law of double refraction in calcareous
spar, as deduced from his theory of light, and as confirmed by direct
experiment. Viewing it probably as a theoretical deduction, Newton
seems to have regarded it as incorrect, and though he has given Huygens
the credit of describing the phenomena more exactly than Bartholinus,
yet, without assigning any reason, he rejected the law of the Dutch
philosopher, and substituted another in its place. These observations
of our author form the subject of the twenty-fifth and twenty-sixth
queries at the end of his Optics, which was published fourteen years
after the appearance of Huygens’s work. The law adopted by Newton
is not accompanied with any of the experiments from which it was
deduced; and though he has given it without expressing any doubt of its
accuracy, it is, nevertheless, entirely incompatible with observation,
and has been rejected by all succeeding philosophers.
In his speculations respecting the successive disappearance and
reappearance of two of the four images which are formed when a
luminous object is viewed through two rhombs of calcareous spar, one
of which is made to revolve upon the other, Newton has been more
successful. He concluded from these phenomena that every ray of light
has two opposite sides originally endued with the property on which
the unusual refraction depends, and other two opposite sides not
endued with that property; and he suggested it as a subject for future
inquiry, whether there are not more properties of light by which the
sides of the rays differ, and are distinguished from one another. This
is the first occasion on which the idea of a _polarity_ in the rays of
light has been suggested.[32]
From the various optical inquiries in which Newton was engaged, he
was strongly impressed with the belief that light consists of small
material particles emitted from shining substances, and that these
particles could be again recombined into solid matter, so that “gross
bodies and light were convertible into one another.” He conceived also
that the particles of solid bodies and of light exerted a mutual action
upon each other, the former being agitated and heated by the latter,
and the latter being attracted and repelled by the former, with forces
depending on the inertia of the luminous particles. These forces he
regarded as insensible at all measurable distances, and he conceived
that the distances between the particles of bodies were very small when
compared with the extent of their sphere of attraction and repulsion.
With the exception of Hooke, Huygens, and Euler, almost all the
contemporaries and successors of Newton maintained the doctrine of the
materiality of light. It was first successfully assailed by Dr. Thomas
Young, and since that time it has been shaken to its foundation by
those great discoveries which have illustrated the commencement of the
present century. The undulatory theory, which has thus triumphed in
its turn, is still subject to grave difficulties, and we fear another
century must elapse before a final decision can be pronounced on this
long-agitated question.
The most important of the optical discoveries of Newton, of which
we have given a general history, were communicated to the Royal
Society in detached papers; but the disputes in which they had
involved their author made him hesitate about the publication of his
other discoveries. Although he had drawn up a connected view of his
labours under the title of “Opticks, or a Treatise on the Reflexions,
Refractions, Inflexions, and Colours of Light,” yet he resolved not to
publish this work during the life of Hooke, by whose rival jealousy
his tranquillity had been so frequently interrupted. Hooke, however,
died in 1702, and the Optics of Newton appeared in English in 1704. Dr.
Samuel Clark proposed a Latin edition of it, which appeared in 1706,
and he was generously presented by Sir Isaac with 500_l._ (or 100_l._
for each of his five children), as a token of the approbation and
gratitude of the author. Both the English and the Latin editions have
been frequently reprinted both in England and on the Continent,[33]
and there perhaps never was a work of profound science so widely
circulated.