PHYSICS
61
PHYSICS
tions of the degree of temperature given since Amon-
tons's time, lie, at the first stroke, found the most
perfect. Equipped with instruments capable of
measuring pressure and registering temperature,
experimental physics could not but make rapid
progress, this being still further augmented by reason
of the interest shown by the learned societies that had
been recently founded. The Accademia del Cimento
was discontinued in 1667, but the Royal Society of
London had begun its sessions in 1663, and the
Academie des Sciences at Paris was founded or
rather organized by Colbert in 1666. These different
academies immediately became the enthusiastic
centres of scientific research in regard to natural
phenomena.
XXII. Undulatort Theory of Light. — It was to the Academie des Sciences of Paris that, in 1678, Christian Huygens (1629-9.5) presented his "Treatise on Light". According to the Cartesian system, light was instantly transmitted to any distance through the medium of incompressible subtile matter. Des- cartes did not hesitate to assure Ferniat that his entire philosophy would give way as soon as it should be demonstrated that light is propagated with a lim- ited velocity. In 1675 Ole Romer (1644-1710), the Danish astronomer, announced to the Academie des Sciences the extent of the considerable but finite velocity with which light traverses the space that separates the planets from one another, the study of the eclipses of Jupiter's satellites ha\'ing brought him to this conclusion. Descartes's optical theorj' was destroj'ed, and Huygens undertook to build up a new theory of light. He was constantly guided by the supposition that, in the midst of compressible ether, substituted for incompressible subtile matter, light is propagated by waves exacth' similar to those which transmit sound through a gaseous medium. This comparison led him to an explanation, which is still the standard one, of the laws of reflection and refraction. In this explanation the index of the refrac- tion of light passing from one medium to another equals the ratio of the velocity of propagation in the first medium to the velocity of propagation in the second. In 18.50 this fundamental law was confirmed by Foucault's experiments.
However, Huygens did not stop here. In 1669 Erasmus Berthelsen, known as Bartholinus (162.5- 98), discovered the double refraction of Iceland spar. By a generalization, as ingenious as it was daring, of the theory he had given for non-crj-stalhzed media, Huygens succeeded in tracing the form of the surface of a luminous wave inside of a crystal such as spar or quartz, and in defining the apparently complex laws of the double refraction of light in the interior of these crj'stals. At the same time, he called attention to the phenomena of polarization which accompany this double refraction; he was, however, unable to draw from his optical theorj' the ex-planation of these effects. The comparison between light and sound caused Malebranche (1638-1715) to make some very effective conjectures in 1699. He assumed that light is a vibrator}' motion analogous to that produced by sound; the greater or less amplitude of this motion, as the case may be, generates a greater or less inten- sity but, whilst in sound each period corresponds to a particular note, in light it corresponds to a particu- lar colour. Through this analogy Malebranche arrived at the idea of monochromatic light, which Newton was to deduce from admirably conducted experiments; moreover, he established between simple colour and the period of the vibration of Ught, the connexion that was to be preserved in the optics of Young and Fresnel.
XXIII. DEVELOPirENTS OF DYNAMICS. — Both Car- tesians and atomists maintained that impact was the only process by which bodies could put one another in motion; hence, to Cartesians and atomists, the
theory of impact seemed like the first chapter of
rational phj'sics. This theory had already enUsted
the attention of Gahleo, Marcus Marci (16.39), and
Descartes when, in 1668, the Royal Society of Lon-
don proposed it as the subject of a competition and,
of the three important memoirs submitted to the
criticism of this society by John Wallis, Christopher
Wren (1632-1723), and Huygens, the last is the only
one that we can consider. In his treati.se Huygens
adopted the follo^n-ing principle: if a material body,
subject merely to the action of gra\-ity, starts from a
certain position, with initial velocity equal to zero,
the centre of gra\-ity of this body can at no time rise
higher than it was at the outset of the motion. Huj--
gens justified this principle by observing that, if
it were false, perpetual motion would be possible.
To find the origin of this axiom it would be necessary
to go back to "De Subtilitate" by Cardano, who had
probably drawn it from the notes of Vinci; the propo-
sition on which TorricelU had based his statics was
a corollarj- from this postulate. By maintaining the
accuracy of this postulate, even in the case where
parts of the system clash; by combining it with the
law of the accelerated fall of bodies, taken from Gali-
leo's works, and with another postulate on the rela-
ti\ity of motion, Huygens arrived at the law of the
impact of hard bodies. He showed that the quantity
the value of which remains constant in spite of this
impact is not, as Descartes declared, the total
quantity of motion, but that which Leibniz called the
quantity of I'is viva (living force).
The axiom that had so happily served Huygens in the study of the impact of bodies he now extended to a body oscillating around a horizontal axis and his "Horologium oscillatorium", which appeared in 1673, solved in the most elegant and complete manner the problem of the centres of oscillation previously handled by Descartes and Roberval. That Huy- gens's axiom was the subversion of Cartesian dynamics was shown b)' Leibniz in 1686. If, like Descartes, we measure the efficiency of a force by the work that it does, and if, moreover, we admit Huygens's axiom and the law of falling bodies, we find that this effi- ciency is not measured by the increase in the quantity of motion of the moving body, but by the increase in half the product of the mass of the mo\-ing body and the square of its velocity. It was this product that Leibniz called vis viva. Huygens's "Horologium oscillatorium" not only gave the solution of the problem of the centre of oscillation but likewise a statement of the laws which, in circular motion, govern the magnitude of centrifugal force, and thus it was that the eminent physicist prepared the way for Newton, the lawgiver of djmamics.
XXIV. Newton's Work. — Most of the great dynamical truths had been discovered between the time of Gahleo and Descartes, and that of Huygens and Leibniz. The science of dynamics required a Euclid who would organize it as geometry had been organized, and this Euclid appeared in the person of Isaac Newton (1642-1727) who, in his Philosophise naturahs principia mathematica", pubUshed in 1687, succeeded in deducing the entire science of motion from three postulates: inertia; the independence of the effects of previously acquired forces and motions; and the equality of action and reaction. Had New- ton's "Principia" contained nothing more than this co-ordination of dynamics into a logical system, they would nevextheless have been one of the most im- portant works ever written; but, in addition, they gave the grandest possible application of this dynam- ics in utilizing it for the establishment of celestial mechanics. In fact, Newton succeeded in showing that the laws of bodies falling to the surface of the earth, the laws that preside over the motion of planets around the sun, and of satellites around the planets which they accompany, finally, the laws that govern
