PHYSICS
62
PHYSICS
the form of the Earth and of the other stars, as also
the high and low tides of the sea, are but so many
corollaries from this unique hypothesis: two bodies,
whatp\or their origin or nature, exert over each other
an attraction proportional to the product of their
masses and in inverse ratio to the square of the dis-
tance that separates them.
The dominating principle of ancient physics declared the essential distinction between the laws that directed the motions of the stars — beings exempt from generation, change, and death — and the laws presiding over the motions of sublunary bodies sub- ject to generation and corruption. From the birth of Christian physics and especially from the end of the thirteenth century, physicists had been endeav- ouring to destroy the authority of this principle and to render the celestial and sublunary worlds subject to the same laws, the doctrine of universal gravitation being the outcome of this prolonged effort. In pro- portion as the time approached, when Newton was to produce his system, attempts at cosmology were multiplied, so many forerunners, as it were, of this discovery. When in 1672 Guericke again took up Kepler's celestial mechanics, he made but one cor- rection therein, which unfortunately caused the dis- appearance of the only proposition by which this work led up to Newton's discoveries. Kepler had maintained that two material masses of any kind attract each other, but, in imitation of Copernicus, Gilbert, and Galileo, Guericke limited this mutual attraction to parts of the same star, so that, far from being attracted by the Earth, portions of the moon would be repelled by the Earth if placed upon its surface. But, in 1644, under the pseudonym of Aristarchus of Samos, Roberval published a system of celestial mechanics, in which the attraction was perhaps mutual between two masses of no matter what kind; in which, at all events, the Earth and Jupiter attracted their satellites with a power iden- tical with tlie gravity with which they endow their own fragments. In 1665, on the pretence of explain- ing the motions of Jupiter's satellites, Giovanni Alfonso Borelli (1608-79) tried to advance a theory which simultaneously comprised the motions of the planets around the sun and of the satellites around the planets. He was the first of modern scientists (Plutarch having preceded him) to hold the opinion that the attraction which causes a planet to tend towards the sun and a satellite to tend towards the star which it accompanies, is in equilibrium with the centrifugal force produced by the circular motion of the planet or satellite in question. In 1674 Robert Hooke (1635-1702) formulated the same idea with great precision. Having already supposed the attrac- tion of two masses to vary inversely as the square of their distance, he was in possession of the funda- mental hypotheses of the theory of universal gravi- tation, which hypotheses were held by Wren about the same time. However, neither of these scientists was able to deduce therefrom celestial mechanics, as both were still unacquainted with the laws of centrifugal force, published just at this time bv Huygens. In 16S4 Edmund Halley (1656-1742) strove to combine Huygens's theories with Hooke's hypotheses, but, before his work was finished, Newton presented his "Principia" to the Royal Society, having for twenty years silently pursued his medita- tions on the system of the world. Halley, who could not forestall Newton, had the glory of broadening the domain of universal gravitation by making it include comets (1705).
Not satisfied with creating celestial mechanics, Newton also idiitiihnted largely to the progress of optics. From ancient times the colouring of the spectrum, produced by the passage of white light through a glass prism, had elicited the wonder of observers and appealed to the acumen of physicists
without, however, being satisfactorily explained.
Finally, a complete e.xplanation was given by Newton
who, in creating a theory of colours, accomplished
what all the philosophers from Aristotle down had
laboured in vain to achieve. The theory ad\anced
by the English physicist agreed with that proposed
by Malebranche at the same time. However, Male-
branche's theory was nothing more than a hypothesis
suggested by the analogy between light and sound,
whereas Newton's ex-planation was drawn from ex-peri-
ments, as simple as they were ingenious, its exposition
by the author being one of the most beautiful ex-
amples of experimental induction. ITnfortunately
Newton disregarded this analogy between sound and
light that had furnished Huygens and Malebranche
with such fruitful discoveries. Newton's opinion
was to the effect that light is formed of infinitely
small projectiles thrown off with extreme velocity by
incandescent bodies. The particles of the medium
in which these projectiles move exert over them an
attraction similar to universal attraction; however,
this new attraction does not vary inversely as the
square of the distance but according to another
function of the distance, and in such a way that it
exercises a very great power between a material
particle and a luminous corpuscle that are contiguous.
Nevertheless this attraction becomes altogether
insensible as soon as the two masses between which
it operates are separated from each other by a per-
ceptible interval.
This action exerted by the particles of a medium on the luminous corpuscles pervading them changes the velocity with which these bodies move and the direction which they follow at the moment of passing from one medium to another; hence the phenomenon of refraction. The index of refraction is the ratio of the velocity of light in the medium which it enters, to the velocity it had in the medium which it leaves. Now, as the index of refraction so understood was precisely the reverse of that attributed to it by Huygens's theory, in 1850 Foucault submitted both to the test of experiment, with the result that New- ton's theory of emission was condemned. Newton explained the ex-perimental laws that govern the colouring of thin laminae, such as soap bubbles, and succeeded in compelling these colours, by suitable forms of these thin Iamina>, to assume the regular order known as "Newton's Rings". To ex-plain this phenomenon he conceived that luminous projectiles have a form that may, at the surface of contact of two media, either pass easily or be easily reflected, according to the manner of their presentation at the moment of passage; a rotary motion causes them to pass alternately by "fits of easy transmission or of easy reflection".
Newton thought that he had accounted for the principal optical phenomena by supposing that, besides this universal attraction, there existed an attraction, sensible only at a very short distance, exerted by the particles of bodies on luminous cor- puscles, and naturally he came to believe that these two kinds of attraction would suffice to explain all physical phenomena. Action extending to a con- siderable distance, such as electric and magnetic action, must follow laws analogous to those which govern universal gravity; on the other hand, the effects of capillarity and cohesion, chemical decom- position and reaction must depend on molecular attraction extending only to extremely small dis- tances and similar to that exerted over luminous corpuscles. This comprehensive hypothesis proposed by Newton in a "question" placed at the end of the second edition of his "Optics" (1717) gave a sort of outline of the programme which eighteenth-century physics w:is to attempt to carry out.
XXV. Progress of General and Celestial Mechanics in the Eighteenth Century. — This
