explain'd by supposing that they arise from new Modifications of the
Rays; which is an erroneous Supposition.
If Light consisted only in Pression propagated without actual Motion, it
would not be able to agitate and heat the Bodies which refract and
reflect it. If it consisted in Motion propagated to all distances in an
instant, it would require an infinite force every moment, in every
shining Particle, to generate that Motion. And if it consisted in
Pression or Motion, propagated either in an instant or in time, it would
bend into the Shadow. For Pression or Motion cannot be propagated in a
Fluid in right Lines, beyond an Obstacle which stops part of the Motion,
but will bend and spread every way into the quiescent Medium which lies
beyond the Obstacle. Gravity tends downwards, but the Pressure of Water
arising from Gravity tends every way with equal Force, and is propagated
as readily, and with as much force sideways as downwards, and through
crooked passages as through strait ones. The Waves on the Surface of
stagnating Water, passing by the sides of a broad Obstacle which stops
part of them, bend afterwards and dilate themselves gradually into the
quiet Water behind the Obstacle. The Waves, Pulses or Vibrations of the
Air, wherein Sounds consist, bend manifestly, though not so much as the
Waves of Water. For a Bell or a Cannon may be heard beyond a Hill which
intercepts the sight of the sounding Body, and Sounds are propagated as
readily through crooked Pipes as through streight ones. But Light is
never known to follow crooked Passages nor to bend into the Shadow. For
the fix'd Stars by the Interposition of any of the Planets cease to be
seen. And so do the Parts of the Sun by the Interposition of the Moon,
_Mercury_ or _Venus_. The Rays which pass very near to the edges of any
Body, are bent a little by the action of the Body, as we shew'd above;
but this bending is not towards but from the Shadow, and is perform'd
only in the passage of the Ray by the Body, and at a very small distance
from it. So soon as the Ray is past the Body, it goes right on.
[Sidenote: _Mais pour dire comment cela se fait, je n'ay rien trove
jusqu' ici qui me satisfasse._ C. H. de la lumiere, c. 5, p. 91.]
To explain the unusual Refraction of Island Crystal by Pression or
Motion propagated, has not hitherto been attempted (to my knowledge)
except by _Huygens_, who for that end supposed two several vibrating
Mediums within that Crystal. But when he tried the Refractions in two
successive pieces of that Crystal, and found them such as is mention'd
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