Note m — BOOK I.

By reflecting the Light of the heavenly Bodies to us, I mean that Whiteness or Lightness which is in the Air in the Day-time, caused by the Rays of Light striking upon the Particles of the Atmosphere, as well as upon the Clouds above, and the other Objects beneath upon the Earth. To the same Cause also we owe the Twilight, viz. to the Sun-beams touching the uppermost Particles of our Atmosphere, which they do when the Sun is about eighteen Degrees beneath the Horizon. And as the Beams reach more and more of the airy Particles, so Darkness goes off, and Day light comes on and encreaseth. For an Exemplification of this, the Experiment may serve of transmitting a few Rays of the Sun through a small Hole into a dark Room: By which means the Rays which meet with Dust, and other Particles flying in the Air, are render’d visible; or (which amounts to the same) those swimming small Bodies are rendered visible, by their reflecting the Light of the Sun-beams to the Eye, which, without such Reflection, would it self be invisible.

The Azure Colour of the Sky Sir Isaac Newton attributes to Vapours beginning to condense, and that are not able to reflect the other Colours. V. Optic. l. 2. Par. 3. Prop. 7.